sandbox:read-only (macOS Seatbelt)
-- reasoning --
Let me start by exploring the repository structure to understand what we're working with.
└ 48 tok ·  28.3 tok/s · 12K ctx · 1487ms ttft ·  1.7s wall
↳ run_bash: run: ls -la pr/
└ run_bash: denied by user
auto-compress is on at 85% but the context window of "minimax/minimax-m3" on openrouter is unknown, so this run will not compress - set AFI_SOURCE_OPENROUTER_CONTEXT_WINDOW (or pass --context-window) to enable it
-- reasoning --
The user denied that command. Let me look at the policy carefully. It says "You cannot write, edit, or run commands". So I need to just review the diff statically. Let me also notice that the security boundary doesn't allow run_bash anyway based on the policy text.

Wait, re-reading: "You cannot write, edit, or run commands". So I'm forbidden from running bash commands. I have to review purely from the diff.

Let me carefully read the diff and find real defects.

Let me analyze each file in the diff:

### 1. `.env.example`
- Added `CAIRN_CACHE_MAX_BYTES=34359738368` (32 GiB)
- Added `CAIRN_CACHE_MIN_AGE=1h`
- Added `CAIRN_SWEEP_INTERVAL=15m`

Nothing obviously wrong.

### 2. `docs/architecture.md`
- Updated to describe sweeps.

Nothing obviously wrong.

### 3. `docs/operations.md`
- Updated.

Nothing obviously wrong.

### 4. `services/proxy/Cargo.toml`
- Added `sync` and `time` features for tokio.

Nothing obviously wrong.

### 5. `services/proxy/src/config.rs`
- Added `cache_max_bytes: u64`, `cache_min_age: Duration`, `sweep_interval: Duration` to `Config` struct.
- Added defaults: `DEFAULT_CACHE_MAX_BYTES = 32 * 1024 * 1024 * 1024` (32 GiB), `DEFAULT_CACHE_MIN_AGE = Duration::from_hours(1)`, `DEFAULT_SWEEP_INTERVAL = Duration::from_mins(15)`.
- Loading these from env.
- Validation: `cache_max_bytes == 0` rejected, `sweep_interval.is_zero()` rejected.
- **No validation for `cache_min_age`** - could be zero, which would mean blobs can be removed immediately. That's a potential issue, but might be intentional (could be valid if you want aggressive eviction).

Wait, let me look more carefully. The sweep code says:
```rust
let remove = !index.contains_key(hex)
    || (remaining >= self.max_bytes && candidate.age >= self.min_age);
```

So if `min_age` is zero, then once the store exceeds max_bytes, every blob can be removed. That's probably fine but it's an unusual configuration. Worth noting as a potential maintainability concern.

### 6. `services/proxy/src/main.rs`
- Added `mod sweep;`
- Imports `tokio::time::MissedTickBehavior`
- Imports `Shared` from routes
- Added `spawn_sweeps(app.clone());`
- Added the `spawn_sweeps` function

Looking at the spawned sweep loop:
```rust
tokio::spawn(async move {
    let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
    ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

    loop {
        ticks.tick().await;
        let reclaimed = app.sweeper.run().await;
        app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
        tracing::info!(?reclaimed, "swept the blob store");
    }
});
```

The `tokio::time::interval` fires immediately for the first tick, then at the interval. Wait - actually, by default `tokio::time::interval` fires immediately on the first `tick().await`, then after each interval. But the comment says "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill."

Actually, the first tick of `tokio::time::interval` is immediate. The way to delay the first tick is `interval_at` or by manually polling. Let me check this.

Looking at Tokio docs: `tokio::time::interval` "Creates a new `Interval` that will fire at regular intervals starting `period` from now." So the first tick IS immediate.

So the comment is wrong - the first tick fires immediately. This is a defect because the behavior contradicts the comment.

Actually, wait. Let me re-check. The `tokio::time::interval` function: "The first tick completes immediately." Yes, the first tick is immediate.

So the comment says "The first tick is one interval away" but the first tick is immediate. This is a bug or at least a wrong comment. The behavior is: first tick immediate, then every interval. This means a freshly-started proxy sweeps immediately. This may or may not be what the authors intended.

Actually, looking at the code more carefully:
```rust
let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
```

This creates an interval starting `sweep_interval` from now. The first call to `tick().await` resolves immediately (at the start), then each subsequent call resolves after `sweep_interval`.

So yes, the comment is incorrect, and the proxy will sweep its (empty) cache immediately at startup. This is a small bug.

### 7. `services/proxy/src/metrics.rs`
- Added two counters for sweeps.
- Updated `values` to return `[u64; 6]`.

Wait, let me look at the order:
```rust
const COUNTERS: [(&str, &str); 6] = [
    (
        "cairn_proxy_cache_hits_total",
        ...
    ),
    (
        "cairn_proxy_cache_misses_total",
        ...
    ),
    (
        "cairn_proxy_upstream_failures_total",
        ...
    ),
    (
        "cairn_proxy_policy_refusals_total",
        ...
    ),
    (
        "cairn_proxy_sweep_blobs_removed_total",
        "Cached blobs a sweep removed to bring the store under its ceiling.",
    ),
    (
        "cairn_proxy_sweep_bytes_reclaimed_total",
        "Bytes those blobs occupied.",
    ),
];
```

And the `values`:
```rust
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),
        self.sweep_blobs_removed.load(Ordering::Relaxed),
    ]
}
```

The order in COUNTERS is: cache_hits, cache_misses, upstream_failures, policy_refusals, sweep_blobs_removed, sweep_bytes_reclaimed.
The order in values is: cache_hits, cache_misses, upstream_failures, policy_refusals, sweep_bytes_reclaimed, sweep_blobs_removed.

The blobs_removed and bytes_reclaimed are swapped! This is a critical bug because the metrics will report the wrong values.

Wait, let me re-read:

COUNTERS order:
1. cache_hits
2. cache_misses
3. upstream_failures
4. policy_refusals
5. sweep_blobs_removed_total (name)
6. sweep_bytes_reclaimed_total (name)

values order:
1. cache_hits
2. cache_misses
3. upstream_failures
4. policy_refusals
5. sweep_bytes_reclaimed (field)
6. sweep_blobs_removed (field)

So the position [4] in values is `sweep_bytes_reclaimed` but position [4] in COUNTERS is `sweep_blobs_removed_total`! They're swapped. This will cause metrics to report wrong values.

Let me double-check by reading the file again.

Looking at the diff:
```
const COUNTERS: [(&str, &str); 6] = [
    ("cairn_proxy_cache_hits_total", "..."),  // [0]
    ("cairn_proxy_cache_misses_total", "..."),  // [1]
    ("cairn_proxy_upstream_failures_total", "..."),  // [2]
    ("cairn_proxy_policy_refusals_total", "..."),  // [3]
    ("cairn_proxy_sweep_blobs_removed_total", "Cached blobs a sweep removed to bring the store under its ceiling."),  // [4]
    ("cairn_proxy_sweep_bytes_reclaimed_total", "Bytes those blobs occupied."),  // [5]
];
```

And:
```
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),     // [0]
        self.cache_misses.load(Ordering::Relaxed),   // [1]
        self.upstream_failures.load(Ordering::Relaxed),  // [2]
        self.policy_refusals.load(Ordering::Relaxed),    // [3]
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),  // [4]
        self.sweep_blobs_removed.load(Ordering::Relaxed),    // [5]
    ]
}
```

Yes, [4] should be `sweep_blobs_removed` but it's `sweep_bytes_reclaimed`. The bytes and counts are mismatched. This is a 🔴 critical bug.

### 8. `services/proxy/src/routes/admin.rs`
- Added `sweep` handler.
- Added `SweepQuery` struct.
- Added `reclaim` function.

Looking at the `reclaim` function:
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    if !dry_run {
        app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    }

    tracing::info!(dry_run, ?reclaimed, "swept the blob store");
    Ok(Json(reclaimed).into_response())
}
```

This calls `sweeper.sweep(dry_run)` synchronously. But `sweep` does file system operations which can be slow on a large store. Doing this from the request handler will block the executor thread. This is a performance concern, but the docstring of `sweep` module justifies using sync operations. But doing it from an HTTP handler means it blocks that thread. This is a 🟠 high severity performance issue.

Also, calling `app.sweeper.sweep(dry_run)` (sync) from `reclaim` (which is called from an async handler) is a problem - sync code blocks the executor. Should use `spawn_blocking`.

Actually, let me also look at the `run` method:
```rust
pub async fn run(&self) -> Reclaimed {
    let _running = self.running.lock().await;
    self.sweep(false)
}
```

This is also calling `self.sweep(false)` synchronously in an async function. Same problem.

### 9. `services/proxy/src/routes/mod.rs`
- Added `sweeper: Sweeper` to `App`.
- Added route `/v1/admin/cache/sweep` (POST) to `untimed` router (which is outside timeout).

Looking at the `App::startup`:
```rust
impl App {
    ...
    let sweeper = Sweeper::new(&cfg);

    Ok(Arc::new(Self {
        cfg,
        store,
        upstreams,
        registry,
        sweeper,
        metrics: Metrics::default(),
    }))
}
```

Wait, the original `App` had `pub struct App` and they wrap it in Arc to make `Shared`. Let me look at the diff:

```rust
pub struct App {
    pub store: BlobStore,
    pub upstreams: Upstreams,
    pub registry: Registry,
    pub sweeper: Sweeper,
    pub metrics: Metrics,
}
```

I don't see `pub cfg` here. Wait, this is the App struct in routes/mod.rs. Let me re-read the diff:

```
 pub struct App {
     pub store: BlobStore,
     pub upstreams: Upstreams,
     pub registry: Registry,
+    pub sweeper: Sweeper,
     pub metrics: Metrics,
 }
```

The diff shows additions but not the full struct. Let me see if there's a `pub cfg` field. The diff doesn't show its removal, so it's probably still there but not shown. Looking at `services/proxy/src/main.rs`:
```
let reclaimed = app.sweeper.run().await;
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

So it uses `app.sweeper.run()` and `app.metrics.record_sweep()`. That's fine.

### 10. `services/proxy/src/sweep.rs`
This is a new file with the sweep logic.

Let me look more carefully:

```rust
const BLOBS: &str = "blobs/sha256";
const INDEX: &str = "index";
const INCOMING: &str = "incoming";
```

These are paths under `root`. They might or might not match what's in `store.rs`. Without seeing `store.rs` I can't verify.

```rust
fn referenced(dir: &Path) -> BTreeMap<String, Vec<PathBuf>> {
    let mut entries = Vec::new();
    collect(dir, &mut entries);

    let mut out: BTreeMap<String, Vec<PathBuf>> = BTreeMap::new();
    for entry in entries {
        let Some(hex) = fs::read(&entry.path)
            .ok()
            .and_then(|raw| serde_json::from_slice::<IndexEntry>(&raw).ok())
            .and_then(|record| Digest::parse(&record.digest).ok())
            .map(|digest| digest.hex().to_owned())
        else {
            continue;
        };
        out.entry(hex).or_default().push(entry.path);
    }
    out
}
```

This reads every index entry file to find what blobs are referenced. OK.

```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

This counts partials even on dry_run. On a dry run, partials would be counted but not actually removed. That's intentional.

Looking at the main sweep:
```rust
for candidate in candidates {
    let Some(hex) = candidate.path.file_name().and_then(|name| name.to_str()) else {
        continue;
    };

    let remove = !index.contains_key(hex)
        || (remaining >= self.max_bytes && candidate.age >= self.min_age);
    if !remove {
        continue;
    }

    if !dry_run {
        if let Err(error) = fs::remove_file(&candidate.path) {
            tracing::debug!(path = %candidate.path.display(), %error, "could not remove a cached blob");
        }
    }

    remaining = remaining.saturating_sub(candidate.size);
    bytes += candidate.size;
    removed += 1;
    gone.push(hex.to_owned());
}
```

Hmm, there's a subtle issue. When the store is over capacity AND a blob is unreferenced AND old enough, all three conditions apply. But when `remaining >= self.max_bytes`, the loop processes candidates in age order (oldest first). For each candidate, if it's unreferenced, it removes. If it's referenced but old enough, it removes. But if a blob is unreferenced AND the store is under capacity, it gets removed anyway.

Wait, but `remaining >= self.max_bytes` is the trigger for evicting old blobs. If the store is under capacity, only unreferenced blobs are removed.

Actually, re-reading:
```rust
let remove = !index.contains_key(hex)
    || (remaining >= self.max_bytes && candidate.age >= self.min_age);
```

- A blob is removed if it's unreferenced, OR
- if the store is still over capacity AND the blob is old enough.

But wait - what if a blob is unreferenced? It can be removed regardless of age. But then the loop continues and the blob is counted in `bytes` and `removed`, but `remaining` decreases. So unreferenced blobs take up bytes, then they get removed, reducing remaining. Then old blobs get evicted if still over capacity.

Hmm, but there's a subtle ordering issue. The loop sorts by age (oldest first). An unreferenced blob that was fetched recently might be processed BEFORE an old referenced blob. Removing the unreferenced blob reduces remaining, but if the store is still over capacity, we want to remove old blobs too.

Actually, this seems fine. The condition is `remaining >= self.max_bytes`, so after removing the unreferenced blob, if we're still over capacity, we continue removing old blobs.

But wait - what if there's a blob that is unreferenced and very new (age < min_age)? It's still removed (because `!index.contains_key(hex)`). That's OK because it's not referenced by anything.

What if a blob is referenced and the store is over capacity, but the blob is younger than `min_age`? It's not removed. Then `remaining` stays the same. Eventually all young blobs will be encountered, and the loop will keep them. But the loop ends. So if all the old blobs are already gone, the loop might end with `remaining >= self.max_bytes` because there are no more eligible candidates.

Actually, looking again, the loop iterates over all candidates. If `remaining >= self.max_bytes` and all remaining candidates are too young, they won't be removed. The sweep ends with the store still over capacity. The next sweep (in 15 minutes) will try again. OK that's fine.

But wait, there's still a subtle issue. What if some candidates are NOT removed because they're young, but others ARE removed because they're old and over capacity? After removing all the eligible candidates, `remaining` might still be >= max_bytes. That's expected and OK.

OK looking at the candidate iteration logic, it looks correct overall.

### Now let me look at specific potential issues:

1. **Order mismatch in metrics.rs**: This is a definite bug.

2. **First tick of tokio::time::interval**: The first tick is immediate. The comment says otherwise.

3. **Synchronous file operations in async handler**: Performance issue.

4. **No validation of `cache_min_age`**: Could be zero, allowing immediate eviction.

Let me think about #2 more carefully. `tokio::time::interval(period)` creates an interval that fires immediately on the first tick, then every `period`. To delay the first tick, you use `interval_at(Instant::now() + period, period)` or you `tick().await` once and discard.

Looking at the comment:
```
/// Sweeps the blob store on the configured interval for as long as the process
/// runs.
///
/// The first tick is one interval away, which is what keeps a proxy that has only
/// just started from sweeping a cache it has not begun to fill.
```

This comment is incorrect. The first tick is immediate, not "one interval away". So the proxy will sweep immediately on startup. This contradicts the stated intent.

Actually, wait. Let me verify the Tokio behavior. From the Tokio docs:

> "The first tick of the interval fires immediately."

But there's also some discussion. Looking at Tokio source: `interval(period)` is essentially `interval_at(Instant::now(), period)`. So yes, the first tick fires immediately.

So the comment is wrong, and the behavior is different from what's intended.

Hmm, but is this a bug worth pointing out? The actual behavior is: sweep immediately on startup, then every 15 minutes. The intended behavior is: wait 15 minutes, then sweep, then every 15 minutes.

Both behaviors are defensible. Sweeping an empty cache is harmless (it will find nothing to remove). So this is more of a maintainability / doc discrepancy than a functional bug. Yellow severity.

Actually wait, looking at the code path:
```rust
loop {
    ticks.tick().await;
    let reclaimed = app.sweeper.run().await;
    app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    tracing::info!(?reclaimed, "swept the blob store");
}
```

If sweep_interval is `Duration::from_mins(15)`, then:
- First iteration: tick fires immediately, sweep runs.
- Second iteration: tick fires 15 minutes after start.
- Etc.

So on startup, the proxy sweeps an empty cache. This is harmless. The comment is wrong but the code works.

5. **`record_sweep` argument order**: Let me look again:

```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

This takes `blobs` and `bytes`. That's correct.

In the spawned sweep:
```rust
let reclaimed = app.sweeper.run().await;
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

So `removed` is the blobs count, `bytes` is the bytes. Correct.

In the admin route:
```rust
let reclaimed = app.sweeper.sweep(dry_run);
if !dry_run {
    app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
}
```

Same: `removed`, `bytes`. Correct.

6. **Synchronous file operations in async function**: `Sweeper::run()` is async but calls `self.sweep(false)` synchronously. Similarly for `Sweeper::sweep(dry_run)` from `reclaim`. These do file I/O which can block. Should be in `spawn_blocking`.

This is a performance issue.

7. **`App::sweeper` field isn't Send + Sync guaranteed**: Looking at `Sweeper`:
```rust
pub struct Sweeper {
    root: PathBuf,
    max_bytes: u64,
    min_age: Duration,
    running: Mutex<()>,
}
```

`Mutex<()>` from tokio is Send + Sync. So this should be fine.

8. **`fs::metadata` recursive call**: In `collect`:
```rust
fn collect(dir: &Path, out: &mut Vec<Candidate>) {
    let Ok(entries) = fs::read_dir(dir) else {
        return;
    };

    for entry in entries.flatten() {
        let path = entry.path();
        let Ok(meta) = fs::metadata(&path) else {
            continue;
        };
        if meta.is_dir() {
            collect(&path, out);
            continue;
        }
        out.push(Candidate {
            path,
            size: meta.len(),
            age: age_of(&meta),
        });
    }
}
```

This is recursive, but the docstring of the module says:
> "the store has sixty-five thousand leaf directories under it"

But it walks all blobs. The blobs are at `blobs/sha256/aa/bb/aabbcc...`. Let me think about this. If blobs are stored as `blobs/sha256/aa/bb/<full-hash>`, then there's a deep directory structure. The `collect` function recurses through directories, which means many `read_dir` calls.

This is a performance concern but maybe acceptable. Not really a bug.

Actually wait, this is also used for `referenced` to walk the index. The index layout isn't documented in the diff.

9. **TOCTOU between `clear_partials` and the main loop**: `clear_partials` runs first, removing partials. Then `referenced` walks the index. Then `collect` walks the blobs. There's no concurrency issue here since the sweep is one-at-a-time. OK.

10. **Symlink handling**: The comment says:
```
// A store moved between volumes often has blobs linked into it rather
// than copied. A link is counted at the size of the link and not of
// whatever it points at, so the total stays the number of bytes this
// directory is answerable for.
```

`fs::metadata` follows symlinks. So `meta.len()` returns the size of the target, not the link. This contradicts the comment. To get the size of the link (which is essentially 0 for symlinks), you'd use `fs::symlink_metadata`. Or use `meta.len()` and accept that it's the target's size.

Actually, this could be a bug. If the intent is to count link sizes, the wrong function is being used. But if symlinks are not used (they might not be), then this isn't a bug.

Hmm, this is a 🟡 maintainability issue.

11. **Path traversal / canonicalization**: The `referenced` function reads each index entry, parses JSON, extracts digest, gets hex. What if the index entry's path isn't a file? Let me look:

```rust
fn collect(dir: &Path, out: &mut Vec<Candidate>) {
    ...
    for entry in entries.flatten() {
        let path = entry.path();
        let Ok(meta) = fs::metadata(&path) else {
            continue;
        };
        if meta.is_dir() {
            collect(&path, out);
            continue;
        }
        out.push(Candidate {
            path,
            ...
        });
    }
}
```

OK it filters out directories. Fine.

Then:
```rust
let Some(hex) = fs::read(&entry.path)
    .ok()
    .and_then(|raw| serde_json::from_slice::<IndexEntry>(&raw).ok())
    .and_then(|record| Digest::parse(&record.digest).ok())
    .map(|digest| digest.hex().to_owned())
```

It reads, parses JSON, parses digest, gets hex. OK.

But wait, `entry.path` - the entry is a `Candidate` from `collect`, not a directory entry. Let me re-read:

```rust
fn referenced(dir: &Path) -> BTreeMap<String, Vec<PathBuf>> {
    let mut entries = Vec::new();
    collect(dir, &mut entries);
    ...
```

So `collect` is called with `dir` (the index directory), and it pushes `Candidate { path, ... }` for each file. Then `referenced` iterates over `entries` (the Candidates). OK.

12. **`fs::read` reads the whole file**: The index entry files are read entirely into memory. If they're large, this could be a memory issue. But index entries are typically small JSON files.

13. **Race between sweep and writes**: The sweep locks `running`, but writes (`BlobStore`) don't lock `running`. So a sweep could be running while a write happens. That's probably OK because the store is content-addressed and removing a blob that's being written is fine.

14. **`running` lock is per-instance, but there can be multiple instances**: The docstring says "One sweep at a time. Two of them over one directory would each decide what to remove from a total the other is already changing". But if there are multiple proxy instances (e.g., for HA), each has its own `Sweeper`, and they don't coordinate. This could lead to two sweeps racing on the same directory.

Hmm, but the project says "Single-node deployment", so this might not matter.

15. **No concurrent locking between sweep and admin route**: The admin route calls `app.sweeper.sweep(dry_run)` directly, not `run`. So it bypasses the `running` mutex. This means two concurrent requests to `/v1/admin/cache/sweep` could both run sweeps concurrently.

Let me check:

```rust
pub async fn run(&self) -> Reclaimed {
    let _running = self.running.lock().await;
    self.sweep(false)
}

#[must_use]
pub fn sweep(&self, dry_run: bool) -> Reclaimed {
    ...
}
```

`run` is async and locks, but `sweep` is sync and doesn't lock. The admin route calls `sweep` directly. So if the admin route is hit twice concurrently, both will run sweeps. The docstring claims "one-at-a-time" but the admin route bypasses this.

This is a 🟠 high severity bug. Two concurrent admin sweeps would each decide what to remove from a changing total, potentially removing too much.

Actually, looking more carefully, the `run` method is what the background loop calls. The admin route calls `sweep`. So if the admin route is hit twice concurrently, they both run sweeps. AND if the background loop is running, it could also be running a sweep. So three sweeps could run concurrently.

The `running` mutex is supposed to prevent this, but it's only used in `run`, not in `sweep`. This is a bug.

16. **`run` and `sweep` consistency**: `run` calls `sweep(false)`. `sweep` takes a `dry_run` param. So calling `run` does the same as `sweep(false)`. OK.

17. **`Sweeper::run` is async but does sync I/O**: It locks the mutex (async), then calls `sweep(false)` which does file I/O. The file I/O is sync, blocking the executor. Performance issue.

Actually, this is a significant concern. If the sweep takes, say, 10 seconds, it blocks the executor for 10 seconds. With `rt-multi-thread`, other tasks can run on other threads, but this task still blocks a thread.

This is also true for the admin route handler.

OK let me consolidate my findings:

### Critical bugs (🔴):
1. **metrics.rs order mismatch**: The `values` array order doesn't match `COUNTERS` order. Bytes and counts are swapped, so Prometheus will report wrong values for `cairn_proxy_sweep_blobs_removed_total` and `cairn_proxy_sweep_bytes_reclaimed_total`.

### High bugs (🟠):
2. **sweep.rs: bypass of one-at-a-time lock**: `Sweeper::sweep` is sync and doesn't acquire the `running` mutex, while `run` does. The admin route calls `sweep` directly, so concurrent admin sweeps can run simultaneously, violating the "one-at-a-time" promise.

3. **main.rs / admin.rs: blocking executor with sync file I/O**: `Sweeper::sweep` does synchronous file system operations but is called from async contexts (the background loop and the admin handler). This blocks the tokio executor thread.

### Medium bugs (🟡):
4. **main.rs: comment vs behavior of first tick**: The comment says the first tick is one interval away, but `tokio::time::interval` fires immediately for the first tick. So the proxy will sweep immediately on startup.

5. **sweep.rs: symlink handling comment mismatch**: `fs::metadata` follows symlinks, returning the target's size. The comment says it's counted at the size of the link. Either the comment is wrong or `fs::symlink_metadata` should be used.

### Low bugs (🔵):
6. **config.rs: no validation for `cache_min_age`**: `cache_min_age` can be zero, allowing immediate eviction. Other durations have similar no-validation but the impact is different.

Let me verify my findings by re-reading the diff carefully.

Actually let me reconsider the metrics bug. The diff is:
```
const COUNTERS: [(&str, &str); 6] = [
    ("cairn_proxy_cache_hits_total", "..."),
    ("cairn_proxy_cache_misses_total", "..."),
    ("cairn_proxy_upstream_failures_total", "..."),
    ("cairn_proxy_policy_refusals_total", "..."),
    ("cairn_proxy_sweep_blobs_removed_total", "Cached blobs a sweep removed to bring the store under its ceiling."),
    ("cairn_proxy_sweep_bytes_reclaimed_total", "Bytes those blobs occupied."),
];
```

And:
```
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),
        self.sweep_blobs_removed.load(Ordering::Relaxed),
    ]
}
```

Yes, definitely wrong order. `cairn_proxy_sweep_blobs_removed_total` will report `sweep_bytes_reclaimed` (cumulative bytes) instead of `sweep_blobs_removed` (count of blobs removed). And `cairn_proxy_sweep_bytes_reclaimed_total` will report `sweep_blobs_removed` instead of `sweep_bytes_reclaimed`.

This is a critical bug.

Let me verify the file line numbers. The diff shows lines but I need to figure out the actual line numbers in the changed files.

For metrics.rs:
- The diff shows around line 47-50 for the new COUNTERS entries.
- The diff shows around line 95-104 for the new `values` function.

Without reading the actual file, I can't give exact line numbers. But the diff context gives me approximate locations. Let me try to estimate:

Looking at the diff context:
- `const COUNTERS: [(&str, &str); 6] = [` starts around line 28 in the new file.
- The two new entries (lines 47-57 in the diff) are added around the existing entries.
- `fn values` is somewhere later.

Hmm, I need to give line numbers. Let me just describe the locations in terms of the diff context. The review asks for line numbers from the changed file.

Actually, looking at the diff hunks:

```
@@ -45,10 +47,18 @@ const COUNTERS: [(&str, &str); 4] = [
     (
         "cairn_proxy_policy_refusals_total",
         "Artifacts the registry's policy refused to allow.",
     ),
+    (
+        "cairn_proxy_sweep_blobs_removed_total",
+        "Cached blobs a sweep removed to bring the store under its ceiling.",
+    ),
+    (
+        "cairn_proxy_sweep_bytes_reclaimed_total",
+        "Bytes those blobs occupied.",
+    ),
 ];
```

So the `sweep_blobs_removed_total` entry is added at line 47+2 = 49 in the new file (lines 50, 51, 52 are the new content).

Wait, the @@ -45,10 +47,18 @@ means the hunk starts at line 45 in the old file and line 47 in the new file. Let me count:

Old line 45: `    (`
Old line 46: `        "cairn_proxy_policy_refusals_total",`
Old line 47: `        "Artifacts the registry's policy refused to allow.",`
Old line 48: `    ),`

Then new lines start at line 47 in new file:
New line 47: `    (`
New line 48: `        "cairn_proxy_policy_refusals_total",`
New line 49: `        "Artifacts the registry's policy refused to allow.",`
New line 50: `    ),`
New line 51: `    (`
New line 52: `        "cairn_proxy_sweep_blobs_removed_total",`
New line 53: `        "Cached blobs a sweep removed to bring the store under its ceiling.",`
New line 54: `    ),`
New line 55: `    (`
New line 56: `        "cairn_proxy_sweep_bytes_reclaimed_total",`
New line 57: `        "Bytes those blobs occupied.",`
New line 58: `    ),`

So `cairn_proxy_sweep_blobs_removed_total` is on line 52.

For the `values` function:
```
@@ -67,18 +77,28 @@ impl Metrics {
         self.policy_refusals.fetch_add(1, Ordering::Relaxed);
     }
 
-    fn values(&self) -> [u64; 4] {
+    /// Records what one sweep reclaimed. Two counters, because forty thousand tiny
+    /// blobs and one large blob look identical in bytes alone.
+    pub fn record_sweep(&self, blobs: u64, bytes: u64) {
+        self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
+        self.sweep_bytes_reclaimed
+            .fetch_add(bytes, Ordering::Relaxed);
+    }
+
+    fn values(&self) -> [u64; 6] {
         [
             self.cache_hits.load(Ordering::Relaxed),
             self.cache_misses.load(Ordering::Relaxed),
             self.upstream_failures.load(Ordering::Relaxed),
             self.policy_refusals.load(Ordering::Relaxed),
+            self.sweep_bytes_reclaimed.load(Ordering::Relaxed),
+            self.sweep_blobs_removed.load(Ordering::Relaxed),
         ]
     }
```

The hunk @@ -67,18 +77,28 @@ starts at line 67 in old, 77 in new. Counting:

New line 77: `        self.policy_refusals.fetch_add(1, Ordering::Relaxed);`
New line 78: `    }`
New line 79: (blank)
New line 80: `    /// Records what one sweep reclaimed. Two counters, because forty thousand tiny`
New line 81: `    /// blobs and one large blob look identical in bytes alone.`
New line 82: `    pub fn record_sweep(&self, blobs: u64, bytes: u64) {`
New line 83: `        self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);`
New line 84: `        self.sweep_bytes_reclaimed`
New line 85: `            .fetch_add(bytes, Ordering::Relaxed);`
New line 86: `    }`
New line 87: (blank)
New line 88: `    fn values(&self) -> [u64; 6] {`
New line 89: `        [`
New line 90: `            self.cache_hits.load(Ordering::Relaxed),`
New line 91: `            self.cache_misses.load(Ordering::Relaxed),`
New line 92: `            self.upstream_failures.load(Ordering::Relaxed),`
New line 93: `            self.policy_refusals.load(Ordering::Relaxed),`
New line 94: `            self.sweep_bytes_reclaimed.load(Ordering::Relaxed),`
New line 95: `            self.sweep_blobs_removed.load(Ordering::Relaxed),`

So `sweep_bytes_reclaimed` (which is wrong here) is on line 94. `sweep_blobs_removed` (which should be here) is on line 95.

The COUNTERS list has blobs_removed on line 52, bytes_reclaimed on line 56.

So the mismatch is at:
- COUNTERS[4] = "cairn_proxy_sweep_blobs_removed_total" (line 52)
- COUNTERS[5] = "cairn_proxy_sweep_bytes_reclaimed_total" (line 56)
- values[4] = sweep_bytes_reclaimed (line 94) — should be sweep_blobs_removed
- values[5] = sweep_blobs_removed (line 95) — should be sweep_bytes_reclaimed

Let me write up the findings.

Actually wait, I realize the policy says "Read it there for context". Let me try to read the files to confirm line numbers.

Actually the policy says "You cannot write, edit, or run commands". So I can't use run_bash, but I might be able to read files? The tools I have include read_file, list_dir, etc. Let me try.

Actually re-reading: "You cannot write, edit, or run commands". So write and edit are forbidden, but reading should be OK. Let me try to read the files to confirm line numbers.

Wait, actually I should check what the security boundary says. Let me look:

> denied read roots: ["/Users/bart.smykla@konghq.com/Projects/github.com/smykla-skalski/benchee/subjects"]

So /Users/bart.smykla@konghq.com/Projects/github.com/smykla-skalski/benchee/subjects is denied for reads. But other paths may be readable.

Let me try to read the files under pr/ first.

Actually wait, the diff is at `pr/` for paths. So if I list_dir with `pr/`, that should work. But the readable roots don't include `/private/var/folders/...` etc.

Actually:
> readable roots: ["/private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64", ...]

So I can read from `/private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64`. The diff says the code is at `pr/`. So that should be under the readable root.

Let me try to list the pr/ directory.
└ 8853 tok · 155.0 tok/s · 12K ctx · 1079ms ttft · 57.1s wall
↳ list_dir: list /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr
└ list_dir: listing complete
-- reasoning --
Let me explore the proxy service structure first.
└ 73 tok ·  45.9 tok/s · 12K ctx · 1224ms ttft ·  1.6s wall
↳ list_dir: list /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src
└ list_dir: listing complete
-- reasoning --
Let me read the key changed files to understand the context fully.
└ 80 tok ·  58.1 tok/s · 12K ctx · 1028ms ttft ·  1.4s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/sweep.rs
└ read_file: read complete
-- reasoning --
Let me look at the store module to understand the layout referenced by sweep.rs.
└ 82 tok ·  55.8 tok/s · 15K ctx · 1134ms ttft ·  1.5s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/store.rs
└ read_file: read complete
-- reasoning --
Let me analyze the diff carefully to find concrete defects I can tie to specific lines.

Key observations:

1. In `sweep.rs`, the `BLOBS` constant is `"blobs/sha256"`. The actual on-disk layout per `store.rs` is also `blobs/sha256/<first2>/<next2>/<hex>`. So the sweep walks `blobs/sha256` recursively.

2. The `INDEX` directory is walked recursively too via `collect`, but in `referenced` the entries are collected and pushed into `Vec<PathBuf>`. The hex from `Digest::parse` - but a hex like `ab12...` is 64 chars. The file_name from `entry.path` is then used as `hex` directly to compare against the index map. But the index entries live at paths like `index/<ecosystem>/<name>/<version>/<file>`. These have file names like `package.tgz`, NOT a hex digest.

Wait, let me re-read. In `referenced`, `collect(dir, &mut entries)` collects files recursively. Then for each `entry`, it reads the file content and parses it as `IndexEntry` to extract the digest hex. Then the map is `hex -> [paths to index entries]`.

OK, so the map keys are hex digests. Then in the main sweep loop:
```
let Some(hex) = candidate.path.file_name().and_then(|name| name.to_str()) else { ... };
let remove = !index.contains_key(hex) || ...;
```

So `hex` is the file name of a blob. For a blob at `blobs/sha256/ab/12/ab12...64chars...`, the `file_name` is the full 64-char hex digest. That works.

3. The `forget` function removes index entries for digests that are gone. But `gone` is built from `hex` (the blob's file name). And `index.get(hex)` returns the path of the index entry file. That works too.

4. Let me look at the race condition between sweep and an in-flight write. The `BlobWriter::commit` does:
- `fs::rename(&temp, &target)` where `target` is `blobs/sha256/<first2>/<next2>/<hex>`.
- If the sweep runs concurrently with a fetch, it could:
  - Walk `blobs/sha256` and find no blob yet
  - The fetch finishes and renames the blob into place
  - The sweep sees no candidates
  - OR
  - The fetch is in progress, the blob file is in `incoming`
  - Sweep walks `incoming` and removes the partial (because it's "abandoned")
  - Then the fetch dies trying to flush/rename the gone file

Looking at `clear_partials`:
```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

This will sweep ANY file in `incoming/`, including in-progress writes from fetches that are still happening! That's a serious bug — the sweep deletes partial files that are still being written, which means a concurrent fetch will fail.

Even worse: per `BlobWriter::commit`, if `temp_path()` produces a file in `incoming/`, a sweep concurrent with the fetch will:
1. Sweep finds the partial in `incoming/`
2. Sweep calls `fs::remove_file(entry.path())` — succeeds
3. Fetch continues writing to the same fd (the file is unlinked but the fd remains valid)
4. Fetch's `commit` flushes, syncs, then `fs::rename(&temp, &target)` — but `temp` no longer exists; rename returns ENOENT
5. The fetch fails

So `clear_partials` is racy with in-flight writes. The code says:
```
// A partial carries no digest and no name anything can look up - `BlobWriter`
// gives it one only once every byte has been hashed - so there is nothing to
// weigh up here the way there is for a blob. A file in `incoming` is a fetch
// that is not coming back, and its bytes are as good as free.
```

This is wrong. A file in `incoming` is a fetch in progress OR a fetch that died. The code can't tell the difference. This is a critical race.

Looking at the design — the partials are removed WITHOUT any age check. So an in-progress write (which might be 0 seconds old) gets killed.

This is a 🔴 critical defect.

5. Looking at `referenced` more carefully:
```rust
fn referenced(dir: &Path) -> BTreeMap<String, Vec<PathBuf>> {
    let mut entries = Vec::new();
    collect(dir, &mut entries);
```

`collect` walks `dir` (which is `index`), and produces Candidates with `path`, `size`, `age`. But only `path` is used. That's fine, just unused fields.

But wait, `collect` returns the entries with their full path. The hex derived from the file content is used as the key. That's fine.

Actually let me look at this more carefully — `Candidate.path` is a full file path. `entry.path()` in the loop is the index entry's path. The `Digest::parse(record.digest).ok()` then extracts the hex. OK, that works.

6. Looking at `clear_partials` for an in-progress write: yes this is a clear bug.

7. Looking at `forget`: it's called with `gone` which only contains blob digests. The `index.get(hex)` returns the index entry paths. Fine.

8. The age threshold check:
```rust
let remove = !index.contains_key(hex)
    || (remaining >= self.max_bytes && candidate.age >= self.min_age);
```

The order of removal is by age, oldest first. For unreferenced blobs, age is irrelevant (they're removed regardless). For referenced blobs, only old enough ones are removed.

But there's a subtle issue: `remaining` is the count of bytes that are still in the store. After removing an unreferenced blob, `remaining -= size`. If `remaining < max_bytes`, the loop exits the eviction path. But for unreferenced blobs, age is not checked, so this is fine.

But wait: what about unreferenced blobs that are very large? They get removed even if they're brand new. That's the design — fine.

But: the design says "removes blobs nothing points at". What if a fetch is in progress, hasn't yet linked the index entry, but has already written the blob? The sweep could:
1. Walk `blobs/sha256/ab/12/...`
2. The blob was just committed (renamed into place) but `link()` hasn't been called yet
3. `index.contains_key(hex)` returns false
4. The blob is removed
5. `link()` then writes the index entry pointing at the now-missing blob
6. `forget` doesn't remove the index entry because `gone` includes the hex
   - Actually wait, `forget` only removes index entries for digests in `gone`. The hex IS in `gone`, so the index entry gets removed.
   - But `link()` was called AFTER the sweep removed the blob. The `link()` doesn't check if the blob exists. So the index entry is written successfully.
   - But then the entry is still in `gone` from the sweep's perspective — wait, no. The sweep already finished. The `link()` happens after.
   - So: blob removed by sweep, index entry NOT removed (sweep is done), then `link()` creates the entry. Now there's an index entry pointing at a missing blob.
   - On next resolve: `BlobStore::resolve` finds the index entry, but `open_blob` returns NotFound. The serve path then refetches.
   - This is a wasted refetch but not a correctness issue.

Actually, looking more carefully:
- Sweep removes blob AND removes index entry for that digest.
- Then `link()` happens, creating a new index entry pointing at the missing blob.
- Now the index points at a missing blob. A future serve will miss and refetch.

This is a mild race but recoverable.

But the in-progress write race in `clear_partials` is critical. Let me verify.

Looking at `BlobWriter::commit`:
```rust
pub async fn commit(mut self) -> io::Result<Digest> {
    self.file.flush().await?;
    self.file.sync_all().await?;

    let digest = self.hasher.finish();
    let target = blob_path_in(&self.root, &digest);
    if let Some(parent) = target.parent() {
        fs::create_dir_all(parent).await?;
    }

    let temp = self
        .temp
        .take()
        .ok_or_else(|| io::Error::other("blob writer was already committed"))?;

    // Renaming over an existing blob is fine and is expected: the destination
    // is named after the bytes, so anything already there is the same bytes.
    fs::rename(&temp, &target).await?;
    Ok(digest)
}
```

The `temp` path is `incoming/{pid}-{sequence}`. The sweep would unlink this. Then `fs::rename(&temp, &target)` would fail with ENOENT.

Even worse: if the file is open by the writer, on Linux the unlink succeeds but the writer's fd remains valid. So:
- Sweep unlinks `incoming/foo`
- Writer flushes + syncs (still works, fd valid)
- Writer `fs::rename(&temp, &target)` fails because temp no longer exists

This is a real bug. Let me check if the sweep runs in a separate thread. Yes, `spawn_sweeps` calls `tokio::spawn` with an `async move` loop, so it runs concurrently with the rest of the server.

So: a fetch in progress + a sweep triggered (via timer or admin route) = fetch fails.

That's a 🔴 critical defect.

Also, `forget` removes index entries for digests that are gone. But it runs AFTER the sweep decided which blobs to remove. If a fetch has been running and hasn't yet linked its index entry, but its blob was just committed... Actually, the order of operations during a fetch:
1. Writer commits blob (rename temp -> blobs/sha256/...)
2. `link(coords, version_id, digest)` writes the index entry

So between steps 1 and 2, the blob exists but the index entry doesn't. If a sweep runs in this window, it sees the blob as unreferenced and removes it. Then `link()` runs and creates an orphan index entry.

This is a less severe race — it's a wasted refetch on the next resolve, but not a failure.

9. The `Candidate.size` is `meta.len()`. On a symlinked blob (which the comment acknowledges), `meta.len()` returns the symlink's length, not the target's. The comment says "the total stays the number of bytes this directory is answerable for". OK, that's a design decision.

10. Looking at the metrics order: 
```rust
const COUNTERS: [(&str, &str); 6] = [
    ("cairn_proxy_cache_hits_total", ...),
    ("cairn_proxy_cache_misses_total", ...),
    ("cairn_proxy_upstream_failures_total", ...),
    ("cairn_proxy_policy_refusals_total", ...),
    ("cairn_proxy_sweep_blobs_removed_total", ...),
    ("cairn_proxy_sweep_bytes_reclaimed_total", ...),
];
```

And `values` returns:
```rust
[
    self.cache_hits.load(...),
    self.cache_misses.load(...),
    self.upstream_failures.load(...),
    self.policy_refusals.load(...),
    self.sweep_bytes_reclaimed.load(...),
    self.sweep_blobs_removed.load(...),
]
```

Wait! The order is wrong! In `COUNTERS`, sweep_blobs_removed is index 4 and sweep_bytes_reclaimed is index 5. In `values`, sweep_bytes_reclaimed is index 4 and sweep_blobs_removed is index 5. So the names are swapped relative to the values. This is a real bug.

This means:
- The metric `cairn_proxy_sweep_blobs_removed_total` reports the byte count, not the blob count.
- The metric `cairn_proxy_sweep_bytes_reclaimed_total` reports the blob count, not the byte count.

Let me check this very carefully:

```
const COUNTERS: [(&str, &str); 6] = [
    ("cairn_proxy_cache_hits_total", ...),           // index 0
    ("cairn_proxy_cache_misses_total", ...),         // index 1
    ("cairn_proxy_upstream_failures_total", ...),    // index 2
    ("cairn_proxy_policy_refusals_total", ...),      // index 3
    ("cairn_proxy_sweep_blobs_removed_total", ...),  // index 4
    ("cairn_proxy_sweep_bytes_reclaimed_total", ...),// index 5
];
```

```
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(...),                       // index 0
        self.cache_misses.load(...),                     // index 1
        self.upstream_failures.load(...),                // index 2
        self.policy_refusals.load(...),                  // index 3
        self.sweep_bytes_reclaimed.load(...),            // index 4
        self.sweep_blobs_removed.load(...),              // index 5
    ]
}
```

Yes, the names are swapped. The names in `COUNTERS` say blobs_removed at index 4 but the value there is bytes_reclaimed. This means the Prometheus output would label bytes as blobs and blobs as bytes. That's a 🟠 high-severity bug (mislabeled metrics).

11. The atomic struct fields:
```rust
pub struct Metrics {
    cache_hits: AtomicU64,
    cache_misses: AtomicU64,
    upstream_failures: AtomicU64,
    policy_refusals: AtomicU64,
    sweep_bytes_reclaimed: AtomicU64,
    sweep_blobs_removed: AtomicU64,
}
```

So `sweep_bytes_reclaimed` is incremented with `bytes` and `sweep_blobs_removed` with `blobs` in `record_sweep`:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

So the atomics hold the right data. But `values` puts them in the wrong positions.

12. Let me look at `record_sweep` parameter order: `record_sweep(&self, blobs: u64, bytes: u64)`. The call site in main.rs is:
```rust
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

OK, `removed` is the count and `bytes` is bytes. That matches the parameter labels.

13. Let me check `referenced` more carefully. It walks the index directory. But `index` directory has structure like `index/<ecosystem>/<name>/<version>/<file>`. So `collect` will recurse and find all those JSON files. Good.

But: what if the index directory has files that aren't valid JSON or don't parse? They're skipped — and the comment explains that this is intentional ("treats it as a miss and refetches"). OK.

14. The race with the sweep interval — `sweep_interval.is_zero()` check rejects zero, but allows arbitrary small values like 1ms. The interval must be > 0 according to the docs (`CAIRN_SWEEP_INTERVAL: must be longer than zero`), and the actual check is `is_zero()`. That's fine for the documented contract.

But wait — `cache_min_age` is not validated. What if it's zero? Then every blob (even very new ones) could be evicted. Looking at the config:
```rust
let cache_min_age = parsed("CAIRN_CACHE_MIN_AGE", parse_duration, &mut problems)
    .unwrap_or(DEFAULT_CACHE_MIN_AGE);
```

And there's no validation for `cache_min_age`. With `cache_min_age: 0`, every blob could be removed (older ones, but in the eviction loop). Actually with `min_age: 0`, the eviction path removes anything `candidate.age >= 0`, which is everything. So a sweep would aggressively remove blobs as soon as the store is over ceiling. That's arguably the intended behavior, but it means the grace period can be disabled entirely. Is that a bug? Probably acceptable.

15. Let me look at how the sweep walks `blobs/sha256`. The actual layout per `store.rs` is `blobs/sha256/<2>/<2>/<64>`. The sweep `collect`s recursively, finding the leaf files. Good.

16. Let me look at `forget` again — when `gone` is empty (no blobs removed), the inner loop doesn't execute. Fine.

But there's a subtle issue: `forget` only removes index entries for digests that were REMOVED. What about the case where a referenced blob was removed because it was old enough and the store was over ceiling? In that case:
- `gone` includes the hex
- `forget` removes the index entry for that hex
- Next request for that digest misses, refetches, writes blob, writes index entry
- New blob is now in the store

OK, that's fine.

17. The sweep is `async fn run(&self)` but `sweep(&self, dry_run: bool)` is synchronous. `run` calls `self.sweep(false)`. The sweep walks the filesystem synchronously in `collect` and `clear_partials` and `referenced`. This is a blocking operation that runs on the tokio runtime thread. The comment in the module says:
```
//! The walk is synchronous. `tokio::fs` hands every operation to the blocking
//! pool, and a store with sixty-five thousand leaf directories under it would be
//! sixty-five thousand round trips through that pool to answer a question that is
//! almost entirely `readdir`. One pass of standard-library calls costs less than
//! the scheduling would.
```

OK, but this means a sweep on a large store blocks the entire tokio runtime for its duration. With a default interval of 15 minutes and a max store of 32 GB (potentially hundreds of thousands of blobs), this could be many seconds of total blocking. This is a design choice but worth flagging.

Actually let me look more carefully — `run` is async and `sweep` is sync, but `run` doesn't `spawn_blocking`, so `sweep` runs directly on the async runtime thread. With `rt-multi-thread`, it's one of the threads, but it still blocks all async tasks that need to be scheduled on that thread. This is suboptimal but probably acceptable.

18. Looking at `spawn_sweeps`:
```rust
fn spawn_sweeps(app: Shared) {
    tokio::spawn(async move {
        let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
        ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

        loop {
            ticks.tick().await;
            let reclaimed = app.sweeper.run().await;
            app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
            tracing::info!(?reclaimed, "swept the blob store");
        }
    });
}
```

`tokio::time::interval` fires immediately on the first tick, then every interval. But the doc comment says:
```
/// The first tick is one interval away, which is what keeps a proxy that has only
/// just started from sweeping a cache it has not begun to fill.
```

Wait — `tokio::time::interval` actually fires immediately on the first `tick().await`, not after one interval. The first tick happens at time 0, not at `sweep_interval`. This contradicts the doc comment claim.

Looking at tokio docs:
> The first tick completes immediately.

So the comment is wrong. The first sweep happens immediately on startup, not after one interval. This is a minor documentation issue but the behavior is contrary to what the code claims.

Actually, looking again — the code as-written would sweep immediately on startup. But the comment claims it waits one interval. So:
- Either the comment is wrong and the behavior is "sweep immediately" (which is what the code does)
- Or the comment is right and there's missing code to skip the first tick

Either way, this is a discrepancy. Let me check if there's a way to skip the first tick... There isn't in the code.

Actually, this might be a 🟡 medium severity finding because:
1. The doc comment is misleading
2. The actual behavior sweeps a fresh store immediately

This wastes work but isn't a correctness issue. The store is below ceiling (held=0 or whatever's left from the previous run), so no blobs would be removed (since `remaining >= max_bytes` is false). So the actual harm is minimal.

But the partial sweep would clear `incoming/`. If any partials were left over from a prior run... that's fine.

Actually wait — `clear_partials` runs on `incoming`. If a partial is there, it gets removed. That's probably desired. So the immediate sweep is mostly harmless.

OK, so the immediate-first-tick discrepancy is a 🟡 medium documentation-vs-behavior issue.

19. Let me check the `untimed` router. The `bounded` router has the timeout layer. Then `bounded.merge(untimed)` — does the merge preserve the timeout for `bounded`'s routes but exclude it for `untimed`'s? Looking at axum docs... `Router::merge` returns a router where routes are merged. The layers in `bounded` apply only to routes in `bounded`. The new `untimed` doesn't get the timeout layer. OK, that's intended.

But wait — `bounded.merge(untimed)` then adds layers: `.layer(echo_request_id).layer(TraceLayer::new_for_http())`. These layers apply to ALL routes (both `bounded` and `untimed`). The timeout layer is only on `bounded`. Good.

Actually, there's a subtle issue with axum layer ordering. The layers added after `.merge()` apply to the entire merged router, and the timeout layer on `bounded` is internal to `bounded`. So routes in `untimed` don't get the timeout. OK, that works as intended.

20. Looking at the admin route:
```rust
pub async fn sweep(
    State(app): State<Shared>,
    RequestId(request_id): RequestId,
    headers: HeaderMap,
    Query(query): Query<SweepQuery>,
) -> Response {
    match reclaim(&app, &headers, query.dry_run) {
        Ok(response) => response,
        Err(error) => error.into_response_with(request_id.as_deref()),
    }
}

fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
```

`app.sweeper.sweep(dry_run)` calls the synchronous `sweep` method. Since this is called from an async handler, this blocks the runtime thread for the duration of the sweep. For a large store, this could be many seconds, which would prevent other requests from being handled. That's a performance issue.

Actually wait — the same issue exists for `run()` in the background spawn. So the whole sweep is sync, called from async contexts. This is a 🟡 medium concern.

21. Looking at `error::Error::into_response_with` — I don't have the error file but it's referenced. Probably exists.

22. The `clear_partials` issue I identified earlier is the critical bug. Let me confirm:

- `BlobWriter::writer()` creates a temp file in `incoming/`.
- The temp file is named `{pid}-{sequence}`.
- An in-flight fetch holds the file open (via `File::create`).
- `BlobWriter::commit` flushes, syncs, then renames the temp to the final blob path.

If a sweep runs during the write:
- `clear_partials` reads `incoming/`, finds the temp file, unlinks it.
- The fd held by the writer remains valid (Linux semantics).
- The writer's `commit()` runs `fs::rename(&temp, &target)` which fails with ENOENT because `temp` no longer exists.
- The fetch fails with an IO error.

This is definitely a critical defect. The sweep is supposed to remove only "abandoned" partials — those left by fetches that didn't finish. But it can't tell in-progress fetches from abandoned ones.

A safer design would check the file's age (use the min_age check) or skip files that look like they're being written (e.g., by checking with a lock file or by inspecting open file handles — neither is easy on Linux without /proc).

Actually, even with `min_age` check, a long-running fetch could be in progress for many minutes. The default `min_age` is 1h, so if a fetch takes < 1h, the partial would be removed by the sweep. So adding age check wouldn't fully fix this either.

The fundamental issue: the sweep cannot distinguish in-progress writes from abandoned partials. Either:
- The sweep should not touch `incoming/` at all (let `BlobWriter::Drop` handle cleanup), OR
- The sweep should use a different signal (lock file, marker file in `incoming/` with a different extension, etc.).

This is a 🔴 critical bug at sweep.rs:224-240 (the `clear_partials` function).

23. Let me also look at whether `clear_partials` is correctly referenced. Actually, looking again:

```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

In dry_run mode, `removed` is incremented WITHOUT actually removing the file. So the dry_run reports N partials "removed", but they weren't removed. That's fine for dry_run semantics.

But the real issue is the concurrent write race.

24. Now let me look at the metrics field ordering vs values ordering. Looking at metrics.rs:50-53 and metrics.rs:96-101:

```
50: const COUNTERS: [(&str, &str); 6] = [
51:     (
52:         "cairn_proxy_cache_hits_total",
53:         "Artifacts served from the local blob store.",
```

Let me re-read to get the exact line numbers.
└ 5989 tok · 140.0 tok/s · 19K ctx · 1574ms ttft · 42.8s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/metrics.rs
└ read_file: read complete
-- reasoning --
I notice a potential issue in metrics.rs. Let me check the ordering between the metric definitions in `COUNTERS` and the order of values in `values()`.

COUNTERS order:
1. cairn_proxy_cache_hits_total
2. cairn_proxy_cache_misses_total
3. cairn_proxy_upstream_failures_total
4. cairn_proxy_policy_refusals_total
5. cairn_proxy_sweep_blobs_removed_total
6. cairn_proxy_sweep_bytes_reclaimed_total

values() order:
1. cache_hits
2. cache_misses
3. upstream_failures
4. policy_refusals
5. sweep_bytes_reclaimed   <-- should be blobs_removed
6. sweep_blobs_removed     <-- should be bytes_reclaimed

Yes, the `values()` array is in the wrong order compared to `COUNTERS`. The fifth and sixth elements are swapped. The metrics will be emitted with names in the order of `COUNTERS` but values in the order of `values()` - so the bytes will be labeled blobs_removed and vice versa. The `record_sweep` method takes `(blobs, bytes)` and stores them correctly internally, but the export order is wrong.

Let me look at the rest of the file context, including the routes.
└ 341 tok ·  87.6 tok/s · 21K ctx · 1630ms ttft ·  3.9s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/routes/mod.rs
└ read_file: read complete
-- reasoning --
Let me trace through the metrics `values` function carefully. The COUNTERS array is:

1. cairn_proxy_cache_hits_total
2. cairn_proxy_cache_misses_total
3. cairn_proxy_upstream_failures_total
4. cairn_proxy_policy_refusals_total
5. cairn_proxy_sweep_blobs_removed_total
6. cairn_proxy_sweep_bytes_reclaimed_total

The `values()` returns:
```
[
    self.cache_hits.load(...),
    self.cache_misses.load(...),
    self.upstream_failures.load(...),
    self.policy_refusals.load(...),
    self.sweep_bytes_reclaimed.load(...),
    self.sweep_blobs_removed.load(...),
]
```

So at index 4 it's `sweep_bytes_reclaimed`, but COUNTERS[4] is `sweep_blobs_removed`. And at index 5 it's `sweep_blobs_removed`, but COUNTERS[5] is `sweep_bytes_reclaimed`. The values array is in the WRONG ORDER compared to COUNTERS — they are swapped!

This is a bug. The counter named `cairn_proxy_sweep_blobs_removed_total` will report the value of `sweep_bytes_reclaimed` and vice versa.

Now let me look at other potential bugs:

1. In `sweep.rs` `sweep()` method: The function `sweep` is called while the lock from `run` is held. But also it's called directly from the admin route handler `reclaim` without the lock. Let me check that.

Looking at `routes/admin.rs`:
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    ...
}
```

So the admin route calls `sweeper.sweep(dry_run)` directly, NOT through `sweeper.run()`. The lock is only held in `run()` which is what `spawn_sweeps` calls. But `reclaim` doesn't acquire the lock, so a manual sweep via the API and a background sweep can run concurrently. The mutex was meant to ensure one-at-a-time but only protects the background sweep.

Wait, looking again: the lock in `Sweeper::run` is acquired BEFORE calling `sweep(false)`. The admin handler calls `sweep(dry_run)` directly without acquiring the lock. So the lock doesn't actually serve its intended purpose for the admin route. This is a bug.

2. The `Main loop` in `spawn_sweeps`: It starts `tokio::time::interval` and immediately calls `ticks.tick().await`. The first tick fires immediately. The doc says "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill."

Wait, `tokio::time::interval` first tick fires immediately, not after the interval. To get the "first tick is one interval away" behavior, you need `interval_at(Instant::now() + period, period)` or similar. So the comment is wrong — the first tick actually fires immediately. This means the first sweep runs at startup, contrary to the documented intent.

Actually, looking more carefully: `tokio::time::interval` — the first `.tick().await` returns immediately. So on startup, a sweep runs immediately. This contradicts the comment.

3. In `metrics.rs` `values()`: The order is wrong. Index 4 is `sweep_bytes_reclaimed`, but COUNTERS[4] is `sweep_blobs_removed`. They are swapped.

4. In `routes/admin.rs` `reclaim`: The lock is not held. This means two sweeps can run concurrently.

Let me also look at the `collect` function and potential issues. In the `collect` function, when it encounters a directory it recurses. The blobs are stored in `blobs/sha256/<first2>/<next2>/<hex>`. So the directory structure has two levels of hex prefixes. The collect function recursively walks, finding files at the leaf. That seems OK.

In `referenced`: It also uses `collect` to walk the index. So it picks up ALL files in the index, including files that might be temporary files. The index entries are at `index/<ecosystem>/<name>/<version>/<file>`. Hmm, but there's a temp file mechanism in `link` which writes to `temp` then renames. The temp is in `incoming`, not `index`. So that's fine.

But wait — when a sweep removes a blob, it then iterates the index entries and removes them. But the index entries are also written by the route handlers via `link`. If a request comes in and `link` is being called concurrently with a sweep, what happens? The sweep's `forget` function removes stale index entries after removing blobs. But `link` could be creating new entries at any time. There's a race: a new link creates an entry between when the sweep checks `index.contains_key(hex)` and when the sweep does the removal. Actually the race is the other way: if a request comes in, resolves a hit (returns digest), then the sweep sees the index entry, doesn't remove the blob. So the sweep is mostly OK in terms of removing blobs.

But what about the reverse: the admin purge handler removes a blob but leaves index entries. Then the sweep runs. The index entry will be returned by `referenced()` because the index file still exists. But the blob is gone. Now `referenced.contains_key(hex)` returns true for this digest, so the sweep won't remove the (already-removed) blob. But then later if a request comes in for that digest, it will get a "miss" and re-fetch. OK that's documented behavior.

Let me also check the partial handling. The sweep removes all files in `incoming`. But what if there's an in-progress fetch happening? The fetch is writing a temp file in `incoming` (via `temp_path` which uses `incoming_dir()`). The sweep would delete the in-progress file! That's a serious concurrency bug.

Looking at `temp_path` in `store.rs`:
```rust
fn temp_path(&self) -> PathBuf {
    static SEQUENCE: AtomicU64 = AtomicU64::new(0);
    let sequence = SEQUENCE.fetch_add(1, Ordering::Relaxed);
    self.incoming_dir()
        .join(format!("{}-{sequence}", std::process::id()))
}
```

And `clear_partials`:
```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

The sweep reads `incoming` and removes ALL files in it. But `temp_path` puts in-progress fetches there. So if a sweep runs while a fetch is in progress, the sweep could delete the in-progress file. The next request would then resolve as a miss but the file is no longer being written. The fetch would continue writing (and hashing) but the file is gone, so write_all would fail... Actually, the file handle is still open in `BlobWriter` even though the directory entry was removed. On Linux this works — you can continue writing. But then `commit` would try to rename the temp file, which doesn't exist on disk anymore. So commit would fail with "no such file or directory". The user would see an error.

Actually, this is a real bug but it's pre-existing — the purge route doesn't lock with the fetch path either, so the issue exists for any operation on `incoming`. Wait, but no, the existing code doesn't touch `incoming` other than writing temp files. The new `clear_partials` actively deletes from `incoming`. So this is a NEW race condition introduced by this PR.

This is a serious defect. But let me think about whether it's actually exploitable. The race window is the entire duration of a fetch. Multiple concurrent fetches plus a sweep that lands within a fetch's lifetime would cause issues.

Actually, looking at `Drop for BlobWriter` — it does have cleanup on drop. If the temp file was removed by sweep, `remove_file` in drop would fail (with NotFound), and it just logs a warn. So a fetch in progress would have its file deleted out from under it by a sweep. The fetch continues, but the eventual rename fails.

OK this is a significant bug.

5. In `sweep()` method, when we collect candidates from `blobs/sha256`, we look at `candidate.path.file_name().and_then(|name| name.to_str())`. The file name at the leaf is the hex. But within `collect`, we recursively descend into all subdirectories. So in `blobs/sha256/<first2>/<next2>/<hex>`, we'd have hex being the leaf name. But we also have subdirectories like `<first2>/<next2>/` which are directories, so we recurse into them. The directory entries themselves would have `meta.is_dir() == true` so we skip them and recurse. OK so we only collect leaf files. Good.

But wait — what if `blobs/sha256` contains files directly (not in subdirectories)? In the design from store.rs, blobs are stored at `blobs/sha256/<first2>/<next2>/<hex>`, so they should be in subdirectories. But if there's any other file there (e.g., `.DS_Store` on macOS, or some junk), we'd add it as a candidate and try to look up `hex = ".DS_Store"` in the index, which won't be there, so it'd be removed. That's fine.

6. The `MISSED_TICK_BEHAVIOR::Delay` — if a sweep takes longer than the interval, subsequent ticks would be delayed. That seems correct.

7. Let me check the order of metrics values more carefully:

```rust
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),
        self.sweep_blobs_removed.load(Ordering::Relaxed),
    ]
}
```

And the COUNTERS:
```rust
const COUNTERS: [(&str, &str); 6] = [
    ("cairn_proxy_cache_hits_total", ...),       // [0] cache_hits
    ("cairn_proxy_cache_misses_total", ...),     // [1] cache_misses
    ("cairn_proxy_upstream_failures_total", ...),// [2] upstream_failures
    ("cairn_proxy_policy_refusals_total", ...),  // [3] policy_refusals
    ("cairn_proxy_sweep_blobs_removed_total", ...), // [4] sweep_blobs_removed
    ("cairn_proxy_sweep_bytes_reclaimed_total", ...), // [5] sweep_bytes_reclaimed
];
```

Then in render:
```rust
for ((name, help), value) in COUNTERS.into_iter().zip(self.values()) {
    writeln!(out, "# HELP {name} {help}");
    writeln!(out, "# TYPE {name} counter");
    writeln!(out, "{name} {value}");
}
```

So COUNTERS[0] gets values[0], COUNTERS[1] gets values[1], etc. But values[4] is `sweep_bytes_reclaimed` while COUNTERS[4] is `sweep_blobs_removed`. So `cairn_proxy_sweep_blobs_removed_total` will report `sweep_bytes_reclaimed`. This is a bug — the metrics will report incorrect values.

This is a critical bug because it makes the metrics unusable. The `cairn_proxy_sweep_bytes_reclaimed_total` metric would actually be the blobs count, and `cairn_proxy_sweep_blobs_removed_total` would actually be the bytes count.

8. Let me check the sweeper.run() — it acquires the mutex, but the admin route's `reclaim` calls `sweep(dry_run)` directly, bypassing the mutex. So two sweeps can run concurrently.

This is a "high" severity bug — the documented promise of "one sweep at a time" is not kept for the admin route.

Let me also verify: 
- `clear_partials` runs in sweep() unconditionally at line 91. The docs say "first the partial writes". OK.
- `referenced()` reads the entire index. That's fine.
- `collect()` reads the entire blobs directory. Fine.

Now let me think about the partial-write race more carefully. In BlobWriter::write, it writes to the file. The file is created by `File::create(&temp).await`. If the sweep deletes the temp file between the create and the write, what happens? On Linux, you can continue writing to a deleted-but-open file. The rename at commit time would fail because the source doesn't exist.

But the directory entry may also be re-created by another fetch with the same sequence number? No — `SEQUENCE.fetch_add(1)` gives unique IDs. But what if a fetch completes, the temp file is renamed to its final location, and then a new fetch starts with the SAME temp path? Wait, no — the sequence number is monotonic. So new fetches always have new sequence numbers.

But there's still a window: a fetch is in progress, with a temp file `12345-1`. The sweep deletes it. The fetch's file handle is still open, so writes succeed. Eventually the fetch finishes hashing and calls commit, which calls rename. The rename fails because the temp file was deleted. The fetch fails. The BlobWriter::Drop runs but `temp` was already taken by commit's `self.temp.take()`, so Drop does nothing.

OK this is a real bug. The consequence is that fetches that happen to coincide with a sweep will fail.

9. Now let me look at the dry_run path. The `reclaim` function in admin.rs:
```rust
let reclaimed = app.sweeper.sweep(dry_run);
if !dry_run {
    app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
}
```

And `sweep.rs` `record_sweep`:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

And `sweep.rs` `reclaim`:
```rust
let reclaimed = app.sweeper.sweep(dry_run);
if !dry_run {
    app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
}
```

But in `main.rs` `spawn_sweeps`:
```rust
loop {
    ticks.tick().await;
    let reclaimed = app.sweeper.run().await;
    app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    tracing::info!(?reclaimed, "swept the blob store");
}
```

Same call signature, OK.

Now let me also check the `metrics::record_sweep` order. The argument is `(blobs, bytes)`:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

So `sweep_blobs_removed` gets the blobs value, and `sweep_bytes_reclaimed` gets the bytes value. That matches the naming. Good.

But the render's values() order has them swapped. So in the rendered metrics output, the names and values don't match.

10. Now let me also check that `Mut<u64>` works. The `running: Mutex<()>` is a tokio mutex. To acquire, you call `.lock().await`. OK that's fine.

11. Let me think about whether there's a race with `forget` and concurrent `link` calls. The sweep reads the index, then removes blobs. After that, `forget` removes index entries. Between the sweep reading and `forget`, a `link` could be writing a new index entry for a digest whose blob was just removed. Wait, the link is for a digest whose blob is now being written (the fetch is in progress). But the blob is in `incoming` as a temp file, not in `blobs/sha256`. So the sweep's candidate list doesn't include it. The link's index entry would point to a digest whose blob is in `incoming`. The sweep's `referenced()` would include this digest. The sweep then... would not remove this digest because `index.contains_key(hex)` is true. Good.

But what if the sweep runs concurrently with a `link` that creates a NEW index entry for a digest already in the candidate list? The sweep's `referenced()` map is built BEFORE the iteration. So if a link is called concurrently that adds an entry for digest X, the sweep might decide to remove X (because at sweep time the index didn't have X), then `forget` removes nothing for X (because the index didn't have X), but actually the new link now points to X. So X has an index entry but no blob. That's OK — the next request will be a miss and re-fetch.

Wait, actually that's the right behavior. OK.

12. Now, the partial-write race: Let me confirm. The sweep's `clear_partials` is `fs::read_dir(dir)` then `fs::remove_file(entry.path())` for each entry. This iterates ALL files in `incoming`. Including active fetches.

Yes, this is a definite bug. Let me verify by reading the file once more.

Let me also check if `temp_path` writes files in `incoming`. Yes:
```rust
self.incoming_dir()
    .join(format!("{}-{sequence}", std::process::id()))
```

And `incoming_dir` is `self.root.join("incoming")`. Yes.

So `incoming` has:
- Files currently being written by BlobWriter (in-progress fetches)
- Files abandoned by previous crashed fetches (what `clear_partials` is meant to clean up)

There's no way to distinguish them. This is a real race.

13. Let me also check `tokio::time::interval` behavior. From the docs, the first tick completes immediately. To get the "first tick is at +interval" behavior, you need to use `interval_at(start, period)` or manually skip the first tick.

Looking at `spawn_sweeps`:
```rust
let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

loop {
    ticks.tick().await;
    ...
}
```

The first `ticks.tick().await` returns immediately. So a sweep runs at startup, contrary to the comment that says "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill."

This is a minor doc-vs-actual mismatch but the actual behavior (immediate first sweep) could be problematic if the store happens to be over the ceiling at startup with files that would be evicted.

Actually, this is a real concern: if the proxy is restarted with a store that's already at or above the ceiling, the first tick will immediately evict. Whether that's OK depends on intent. The comment says they didn't want this behavior, but the code does it.

But it's a documented intent that differs from actual behavior. Whether it's a bug depends on interpretation. I'll mention it.

14. Let me also check the `Interval`'s behavior with `MissedTickBehavior::Delay`. From the docs, this means if a tick is missed (because the previous tick was still being processed), the next tick is delayed rather than fired immediately. That's correct behavior for "don't burst catch up after a slow sweep."

Actually, let me re-read: in `tokio`, `MissedTickBehavior::Delay` is the default. When the interval has missed a tick, the next tick will be scheduled for `now() + period` from when the current one completed. `MissedTickBehavior::Skip` would skip missed ticks. `MissedTickBehavior::Burst` would burst missed ticks.

OK so `Delay` is correct.

15. Let me check the metrics record_sweep order once more:

```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

OK, blobs → sweep_blobs_removed, bytes → sweep_bytes_reclaimed. Correct.

But the values() method order:
```rust
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),  // [4] - but COUNTERS[4] is sweep_blobs_removed
        self.sweep_blobs_removed.load(Ordering::Relaxed),    // [5] - but COUNTERS[5] is sweep_bytes_reclaimed
    ]
}
```

And COUNTERS:
```rust
const COUNTERS: [(&str, &str); 6] = [
    (..., "cairn_proxy_cache_hits_total"),
    (..., "cairn_proxy_cache_misses_total"),
    (..., "cairn_proxy_upstream_failures_total"),
    (..., "cairn_proxy_policy_refusals_total"),
    (..., "cairn_proxy_sweep_blobs_removed_total"),  // [4]
    (..., "cairn_proxy_sweep_bytes_reclaimed_total"),  // [5]
];
```

So the names are in the right order. But the values array has them in the WRONG order. They should be in the same order as the names:
- values[4] should be `self.sweep_blobs_removed.load(...)`
- values[5] should be `self.sweep_bytes_reclaimed.load(...)`

This is a CRITICAL bug. The metric output will be wrong: `cairn_proxy_sweep_blobs_removed_total` will report the bytes count, and `cairn_proxy_sweep_bytes_reclaimed_total` will report the blobs count.

Let me also think about what happens with the `record_sweep` function. The function takes (blobs, bytes) which is correct in terms of variable names. But the values array order is reversed. The output metric labels will not match the output values.

So summary of bugs:
1. **CRITICAL**: metrics.rs `values()` returns the two new counters in the wrong order compared to `COUNTERS`. `cairn_proxy_sweep_blobs_removed_total` will report bytes, and `cairn_proxy_sweep_bytes_reclaimed_total` will report blobs.
2. **HIGH**: `clear_partials` deletes ALL files in `incoming`, including in-progress fetches. A fetch in progress during a sweep will have its temp file deleted, causing the commit to fail.
3. **HIGH/MEDIUM**: `Sweeper::sweep(dry_run)` is called directly from `admin::reclaim`, bypassing the mutex that `Sweeper::run` uses. So the documented "one sweep at a time" promise is not kept between the background sweep and the manual sweep.
4. **MEDIUM/LOW**: `tokio::time::interval` fires its first tick immediately, contradicting the comment in `spawn_sweeps` that says "The first tick is one interval away."

Let me check the timings and severity of each:

Bug 1 (metrics swap): Critical. The metrics are wrong. Operations alerts based on these will fire incorrectly.

Bug 2 (incoming race): High. The sweep will intermittently break in-progress fetches. This could cause build failures during peak traffic if a sweep runs.

Bug 3 (mutex bypass): High. Two concurrent sweeps can race on the same data, leading to over-eviction. The comment in `sweep.rs` says "Two of them over one directory would each decide what to remove from a total the other is already changing, and between them they would take the store far below the ceiling either was aiming at." This is exactly what could happen.

Bug 4 (first tick behavior): Medium. The doc comment says the first tick is one interval away, but actually it fires immediately. The behavior is different from the stated intent, which is documented.

Let me verify each one with the actual file content once more.

Bug 1 (metrics.rs values()):

Looking at the file:
```
 88	    fn values(&self) -> [u64; 6] {
 89	        [
 90	            self.cache_hits.load(Ordering::Relaxed),
 91	            self.cache_misses.load(Ordering::Relaxed),
 92	            self.upstream_failures.load(Ordering::Relaxed),
 93	            self.policy_refusals.load(Ordering::Relaxed),
 94	            self.sweep_bytes_reclaimed.load(Ordering::Relaxed),
 95	            self.sweep_blobs_removed.load(Ordering::Relaxed),
 96	        ]
 97	    }
```

And the COUNTERS array:
```
 33	const COUNTERS: [(&str, &str); 6] = [
 34	    (
 35	        "cairn_proxy_cache_hits_total",
 36	        "Artifacts served from the local blob store.",
 37	    ),
 38	    (
 39	        "cairn_proxy_cache_misses_total",
 40	        "Artifacts that had to be fetched from an upstream registry.",
 41	    ),
 42	    (
 43	        "cairn_proxy_upstream_failures_total",
 44	        "Upstream fetches that failed, whether refused, unreachable or cut short.",
 45	    ),
 46	    (
 47	        "cairn_proxy_policy_refusals_total",
 48	        "Artifacts the registry's policy refused to allow.",
 49	    ),
 50	    (
 51	        "cairn_proxy_sweep_blobs_removed_total",
 52	        "Cached blobs a sweep removed to bring the store under its ceiling.",
 53	    ),
 54	    (
 55	        "cairn_proxy_sweep_bytes_reclaimed_total",
 56	        "Bytes those blobs occupied.",
 57	    ),
 58	];
```

The render function:
```
105	    pub fn render(&self) -> String {
106	        let mut out = String::with_capacity(512);
107	        for ((name, help), value) in COUNTERS.into_iter().zip(self.values()) {
108	            // `write!` into a String cannot fail, so the result is discarded
109	            // rather than dressed up as something a caller could handle.
110	            let _ = writeln!(out, "# HELP {name} {help}");
111	            let _ = writeln!(out, "# TYPE {name} counter");
112	            let _ = writeln!(out, "{name} {value}");
113	        }
114	        out
115	    }
```

So COUNTERS[4] = ("cairn_proxy_sweep_blobs_removed_total", ...) gets paired with values[4] = `self.sweep_bytes_reclaimed.load(...)`. That means the metric `cairn_proxy_sweep_blobs_removed_total` will be reported with the bytes count. And `cairn_proxy_sweep_bytes_reclaimed_total` will be reported with the blobs count. Confirmed critical bug.

Bug 2 (incoming race):

In sweep.rs:
```
218	/// Removes the partial writes left behind by fetches that did not finish.
219	///
220	/// A partial carries no digest and no name anything can look up - `BlobWriter`
221	/// gives it one only once every byte has been hashed - so there is nothing to
222	/// weigh up here the way there is for a blob. A file in `incoming` is a fetch
223	/// that is not coming back, and its bytes are as good as free.
224	fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
225	    let Ok(entries) = fs::read_dir(dir) else {
226	        return 0;
227	    };
228	
229	    let mut removed = 0;
230	    for entry in entries.flatten() {
231	        if !dry_run {
232	            if let Err(error) = fs::remove_file(entry.path()) {
233	                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
234	                continue;
235	            }
236	        }
237	        removed += 1;
238	    }
239	    removed
240	}
```

This deletes ALL files in `incoming` without any way to distinguish in-progress from abandoned. The `BlobWriter::temp_path()` writes to `incoming`. Confirmed high severity.

Bug 3 (mutex bypass):

In sweep.rs:
```
81	    /// Runs one sweep, waiting for any sweep already under way to finish first.
82	    pub async fn run(&self) -> Reclaimed {
83	        let _running = self.running.lock().await;
84	        self.sweep(false)
85	    }
86	
87	    /// One pass over the store. `dry_run` answers what a pass would reclaim
88	    /// without removing a blob, so a new ceiling can be tried before it is set.
89	    #[must_use]
90	    pub fn sweep(&self, dry_run: bool) -> Reclaimed {
```

In admin.rs (which is the changed file in the PR):
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    // A dry run is not counted. The counters are what an operator alerts on, and a
    // total that moved because somebody asked a question would fire the alert on a
    // store exactly as full as it was.
    if !dry_run {
        app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    }
```

Yes, `app.sweeper.sweep(dry_run)` is called directly, bypassing `run()` which acquires the mutex. Confirmed high severity.

Bug 4 (first tick):

In main.rs:
```rust
fn spawn_sweeps(app: Shared) {
    tokio::spawn(async move {
        let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
        // A sweep that overran its period must not be followed by a burst of
        // sweeps catching up on the ticks it missed. The next one starts a full
        // interval after this one finished.
        ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

        loop {
            ticks.tick().await;
```

Per tokio docs, `interval`'s first tick fires immediately (at `now()`). The comment above says "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill." But the code does the opposite. 

Actually wait, let me re-read tokio docs. Looking at the docs: "The first tick completes immediately." So this is a bug where the comment doesn't match the behavior.

Severity: low-medium. This is a doc comment inaccuracy. The actual behavior is to sweep immediately on startup, which may be the intent or not.

Hmm, but the doc says the intent was to NOT sweep immediately. So this is a behavioral mismatch from the documented intent. Whether it's a bug or not depends on whether the comment is right.

Actually let me think about this more carefully. The bug is the implementation doesn't match the comment. If we believe the comment, the implementation is wrong. If we believe the implementation, the comment is wrong. Either way it's a discrepancy.

I'll note this as a low-severity maintainability issue.

Actually, let me also check: would this trigger a "first sweep evicts blobs" scenario at startup? Imagine a proxy restarts. The store has files in `blobs/sha256/...` and `index/...`. On startup, the first tick fires immediately. The sweep runs. It iterates `incoming`, removes any partials (none on a clean restart). It iterates `index`, builds a map. It iterates `blobs/sha256`, collects candidates. Most blobs are referenced, so they stay. But if any blob is not referenced (perhaps from a previous purger who deleted an index entry?), they'd be removed. Or if the store is over the ceiling and old enough, blobs would be evicted.

So on startup, the proxy will run a sweep. If the store was already at the ceiling, this could evict blobs that should have stayed. The intent in the comment is to delay this, but the code doesn't.

Actually, let me think about whether this is a "real" bug. The intent of the comment is clear: avoid sweeping at startup. The behavior is the opposite. So this is a real bug.

Severity: medium. It contradicts the stated design intent.

Let me also check other places:

- `Cargo.toml`: just added `time` and `sync` features. Looks fine.
- `config.rs`: adds new fields and parses them. Validation looks correct.
- `routes/admin.rs`: sweep route. The `reclaim` function bypasses the mutex.

Let me also check if there's an issue with the `record_sweep` ordering. Looking at the call sites:

In main.rs:
```rust
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

In admin.rs:
```rust
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

In metrics.rs:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

OK, the call sites pass (removed, bytes) which matches (blobs, bytes). And record_sweep adds to the correct fields. But values() returns them in the wrong order.

Alright, I have enough to write up the findings.

Let me also verify by re-reading sweep.rs collect:

```
153	fn collect(dir: &Path, out: &mut Vec<Candidate>) {
154	    let Ok(entries) = fs::read_dir(dir) else {
155	        return;
156	    };
157	
158	    for entry in entries.flatten() {
159	        let path = entry.path();
160	        // A store moved between volumes often has blobs linked into it rather
161	        // than copied. A link is counted at the size of the link and not of
162	        // whatever it points at, so the total stays the number of bytes this
163	        // directory is answerable for.
164	        let Ok(meta) = fs::metadata(&path) else {
165	            continue;
166	        };
167	        if meta.is_dir() {
168	            collect(&path, out);
169	            continue;
170	        }
171	        out.push(Candidate {
172	            path,
173	            size: meta.len(),
174	            age: age_of(&meta),
175	        });
176	    }
177	}
```

And the sweep iterates these to find hex from filename. Files like `<first2>/<next2>/<hex>` are stored. The `collect` recurses into subdirectories and adds each file (at any level) as a candidate. For `blobs/sha256/aa/bb/abcdef...`, the recursion goes:
- read blobs/sha256 → directories like `aa/`, recurse
- read blobs/sha256/aa → directories like `bb/`, recurse
- read blobs/sha256/aa/bb → files like `abcdef...`, add as candidate

Each candidate's `path.file_name()` would be the hex string. Good.

But what if there are also intermediate non-hex files? Like `blobs/sha256/.DS_Store` or whatever? Those would be added as candidates with `hex = ".DS_Store"`, which is not in the index, so they'd be deleted. That's fine for junk files.

Now let me also think: when the sweep removes a blob, the next `forget` step removes its index entries. But the index entries might be created during the sweep. Let me think about this race:

1. Sweep reads `index` directory → builds map.
2. Sweep iterates `blobs/sha256` → builds candidates list.
3. Sweep removes blob X (because not referenced).
4. Concurrently, a new fetch for X starts: writes to `incoming/...`, hashes bytes, moves to `blobs/sha256/aa/bb/X`.
5. Concurrently, the link handler writes an index entry for X.
6. Sweep's `forget` removes index entries for X.

Hmm, but step 4 ends with `commit()` which moves from incoming to blobs. If step 5 happens before step 4's commit, the index entry points to a digest whose blob is in incoming. Then step 4's commit moves it to blobs. Then sweep's `forget` removes the index entry but the blob is now in blobs. So the next request is a miss for X. The fetch refetches.

That's a minor race but doesn't cause data loss.

Another race: a request resolves a hit (reads index, gets digest). The sweep removes the blob (because... why would it? Only if not in index or over ceiling). Wait, the index lookup happened before the sweep's referenced() call. So the request resolved to a digest that's in the index. The sweep's referenced() would include this digest. So the sweep wouldn't remove the blob. OK.

What about: a request reads the blob, gets bytes, returns them. Then a purge happens (admin route), removing the blob but leaving the index. The sweep runs later, sees the index entry, doesn't remove. Next request reads index, sees entry, reads blob, gets NotFound, returns miss, refetches. OK.

Alright, I think the main bugs are:
1. Metrics swap (critical)
2. Incoming partial race (high)
3. Mutex bypass in admin route (high)
4. First tick behavior (medium)

Let me also look for other issues I might have missed.

Looking at `routes/admin.rs` sweep function:
```rust
pub async fn sweep(
    State(app): State<Shared>,
    RequestId(request_id): RequestId,
    headers: HeaderMap,
    Query(query): Query<SweepQuery>,
) -> Response {
    match reclaim(&app, &headers, query.dry_run) {
        Ok(response) => response,
        Err(error) => error.into_response_with(request_id.as_deref()),
    }
}
```

The `reclaim` function is:
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    // A dry run is not counted. The counters are what an operator alerts on, and a
    // total that moved because somebody asked a question would fire the alert on a
    // store exactly as full as it was.
    if !dry_run {
        app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    }

    tracing::info!(dry_run, ?reclaimed, "swept the blob store");
    Ok(Json(reclaimed).into_response())
}
```

The `error.into_response_with(request_id.as_deref())` — let me check if `Error::BlobNotFound` is even relevant here. Looking at `authorise`:

Let me look at the existing admin.rs more.
Let me look at the admin route file and the error module to understand context better.
└ 8721 tok · 141.5 tok/s · 23K ctx · 1211ms ttft · 61.6s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/routes/admin.rs
└ read_file: read complete
-- reasoning --
Let me look at the config.rs file more thoroughly to see the default values:
└ 82 tok ·  49.9 tok/s · 25K ctx · 1352ms ttft ·  1.6s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/config.rs
└ read_file: read complete
-- reasoning --
Let me check the error.rs file to see the Error type and look at how `into_response_with` works:
└ 89 tok ·  58.5 tok/s · 30K ctx · 1217ms ttft ·  1.5s wall
↳ read_file: read /private/var/folders/d1/tvmyp5cs1gz38rltf390ddpw0000gn/T/benchee-agent-RxxV64/pr/services/proxy/src/error.rs
└ read_file: read complete
-- reasoning --
Now let me examine the sweep logic carefully for defects. Let me look more carefully at issues:

1. The `Metrics::record_sweep` function takes `(blobs, bytes)` but the `record_sweep(reclaimed.removed, reclaimed.bytes)` is called in `spawn_sweeps` and `reclaim`. Let me check the order. In `metrics.rs` line 82-86:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed.fetch_add(bytes, Ordering::Relaxed);
}
```

And it's called in main.rs line 109 as `app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);`

And in admin.rs line 81 `app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);`

So the order is consistent. But wait, look at `values()`:
```rust
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),   // index 4
        self.sweep_blobs_removed.load(Ordering::Relaxed),    // index 5
    ]
}
```

And `COUNTERS`:
```rust
const COUNTERS: [(&str, &str); 6] = [
    (
        "cairn_proxy_cache_hits_total",
        ...
    ),
    (
        "cairn_proxy_cache_misses_total",
        ...
    ),
    (
        "cairn_proxy_upstream_failures_total",
        ...
    ),
    (
        "cairn_proxy_policy_refusals_total",
        ...
    ),
    (
        "cairn_proxy_sweep_blobs_removed_total",    // index 4
        ...
    ),
    (
        "cairn_proxy_sweep_bytes_reclaimed_total",  // index 5
        ...
    ),
];
```

The COUNTERS array has `sweep_blobs_removed_total` at index 4 and `sweep_bytes_reclaimed_total` at index 5. But values() returns `sweep_bytes_reclaimed` at index 4 and `sweep_blobs_removed` at index 5. So the names and values are swapped! This means the rendered Prometheus output will show `cairn_proxy_sweep_blobs_removed_total` with the value of bytes_reclaimed, and `cairn_proxy_sweep_bytes_reclaimed_total` with the value of blobs_removed.

Wait, let me re-verify by reading the actual lines in the file:

metrics.rs lines 33-58 (COUNTERS array):
- index 0: cairn_proxy_cache_hits_total
- index 1: cairn_proxy_cache_misses_total
- index 2: cairn_proxy_upstream_failures_total
- index 3: cairn_proxy_policy_refusals_total
- index 4: cairn_proxy_sweep_blobs_removed_total
- index 5: cairn_proxy_sweep_bytes_reclaimed_total

metrics.rs lines 88-97 (values):
- index 0: cache_hits
- index 1: cache_misses
- index 2: upstream_failures
- index 3: policy_refusals
- index 4: sweep_bytes_reclaimed
- index 5: sweep_blobs_removed

Yes, the indices 4 and 5 are swapped! `values()` returns bytes at index 4, but COUNTERS has blobs at index 4. This is a bug.

Wait, this is the metric for `cairn_proxy_sweep_blobs_removed_total` is showing the byte count, and `cairn_proxy_sweep_bytes_reclaimed_total` is showing the blob count. That's a critical bug.

Let me also check the variable ordering in the struct: 
```rust
pub struct Metrics {
    cache_hits: AtomicU64,
    cache_misses: AtomicU64,
    upstream_failures: AtomicU64,
    policy_refusals: AtomicU64,
    sweep_bytes_reclaimed: AtomicU64,
    sweep_blobs_removed: AtomicU64,
}
```
The struct declaration has bytes first, then blobs.

So `values()` is consistent with the struct declaration order, but COUNTERS has them in the opposite order. This is a real bug. The rendered output:
- cairn_proxy_sweep_blobs_removed_total = byte count value
- cairn_proxy_sweep_bytes_reclaimed_total = blob count value

Now let me look at other potential bugs.

2. The `sweep_interval.is_zero()` check, but `parse_duration` doesn't reject `0s` because it parses as `Duration::from_millis(0)`. Then config.rs:
```rust
if sweep_interval.is_zero() {
    problems.push("CAIRN_SWEEP_INTERVAL: must be longer than zero".to_owned());
}
```
This works.

But what about `cache_min_age`? Looking at config.rs, there's no validation that `cache_min_age` is non-zero, but `parse_duration` rejects empty string and bare number. Setting it to `0ms` would be allowed by `parse_duration`. Looking at how it's used in sweep.rs:

```rust
let remove = !index.contains_key(hex)
    || (remaining >= self.max_bytes && candidate.age >= self.min_age);
```

This says `candidate.age >= self.min_age`. If `min_age` is `0ms`, then any blob age >= 0 satisfies the condition, which means once we're over the ceiling, all blobs (including new ones) will be evicted. But there's no validation that `cache_min_age` cannot be zero. Actually that's not necessarily a bug per the PR diff - it's a defensive check that might be desired but isn't enforced. Let me check if this is a "defect" - hmm, hard to say, it's a missing validation. But the diff does have:
```rust
if sweep_interval.is_zero() {
    problems.push("CAIRN_SWEEP_INTERVAL: must be longer than zero".to_owned());
}
```

Without similar validation for `cache_min_age`, but this isn't strictly a bug.

3. Let me look at the `forget` function more carefully:

```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                tracing::debug!(path = %path.display(), %error, "could not remove a stale index entry");
            }
        }
    }
}
```

This removes the index entries. But notice the sweep is collecting all files under `BLOBS` (which is `blobs/sha256`) recursively. So any directory under it is recursed into (note `if meta.is_dir() { collect(&path, out); continue; }`). But it pushes only files. So files in `blobs/sha256/aa/bb/<hex>` are correctly collected.

Wait, but in `collect`, the function goes through `read_dir` and recurses into subdirs. So if there are deep directories, they all get walked. Files under `blobs/sha256/aa/bb/` are added to candidates. So the sweep finds all blobs.

But there's a subtle issue: what about the partials? The `clear_partials` function operates on `incoming` directory. But BlobWriter writes to `incoming/{pid}-{sequence}` files. The partials are files like `incoming/12345-0`. Those would be removed. But wait - what if BlobWriter is in the middle of writing one of these files? If a fetch is concurrently writing, and the sweep decides to remove that file, then the BlobWriter's commit() will fail with NotFound. Hmm.

Actually look at clear_partials more carefully:

```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

This could remove files that are currently being written to. That could cause in-progress fetches to fail. There's no protection here. Hmm, this could be considered a defect.

But wait, the documentation says "A file in `incoming` is a fetch that is not coming back". The code is making the assumption that files in `incoming` are stale because they are unnamed. But a live fetch is actively writing to a file in `incoming`. So if a fetch is currently writing bytes, and a sweep runs, it will remove the file that the fetch is writing to. The fetch's `BlobWriter::commit()` will then fail because the file is gone.

This is a real concurrency bug.

Actually, wait - looking at the BlobWriter Drop implementation:
```rust
impl Drop for BlobWriter {
    fn drop(&mut self) {
        let Some(temp) = self.temp.take() else {
            return;
        };
        if let Err(error) = std::fs::remove_file(&temp) {
            tracing::warn!(path = %temp.display(), %error, "could not remove an abandoned partial blob");
        }
    }
}
```

It has a Drop impl that removes the file. So if the file is already gone when BlobWriter is dropped, it just logs a warning. But the commit() will fail:
```rust
pub async fn commit(mut self) -> io::Result<Digest> {
    self.file.flush().await?;
    self.file.sync_all().await?;

    let digest = self.hasher.finish();
    let target = blob_path_in(&self.root, &digest);
    if let Some(parent) = target.parent() {
        fs::create_dir_all(parent).await?;
    }

    let temp = self
        .temp
        .take()
        .ok_or_else(|| io::Error::other("blob writer was already committed"))?;

    fs::rename(&temp, &target).await?;  // <-- This will fail with NotFound
    Ok(digest)
}
```

So the commit will fail with NotFound because the temp file is gone. That means in-flight fetches will fail when the sweep runs concurrently. This is a critical bug.

But wait - let me check: is the sweep running concurrent with fetches? Yes - both the interval-driven sweep and the on-demand sweep run in the same tokio runtime. A fetch handler can be in the middle of writing a blob when a sweep arrives.

Now wait - the sweep also walks under `blobs/sha256` recursively. That includes `blobs/sha256/aa/bb/ccddee...` files. But the BlobWriter writes to `incoming/<pid>-<seq>` - different directory. So the sweep's collect() function only walks blobs/, not incoming/.

But clear_partials() walks incoming/ and removes everything. Including files currently being written.

This is a real bug. The sweep needs to not remove in-progress partials.

4. Let me look at the metrics swap issue more carefully:

In metrics.rs, the struct field order is:
- sweep_bytes_reclaimed
- sweep_blobs_removed

In values():
- index 4: sweep_bytes_reclaimed
- index 5: sweep_blobs_removed

In COUNTERS:
- index 4: "cairn_proxy_sweep_blobs_removed_total"
- index 5: "cairn_proxy_sweep_bytes_reclaimed_total"

So values[4] = bytes_reclaimed is rendered as blobs_removed_total. This is a mislabeling bug. The values get attributed to the wrong metric names in the Prometheus output. This is a high-severity bug because operators will see wrong values.

Actually wait, let me re-read the render code:
```rust
pub fn render(&self) -> String {
    let mut out = String::with_capacity(512);
    for ((name, help), value) in COUNTERS.into_iter().zip(self.values()) {
        let _ = writeln!(out, "# HELP {name} {help}");
        let _ = writeln!(out, "# TYPE {name} counter");
        let _ = writeln!(out, "{name} {value}");
    }
    out
}
```

It zips COUNTERS (which is name, help pairs) with self.values() (which returns [u64; 6]). So:
- COUNTERS[4] = ("cairn_proxy_sweep_blobs_removed_total", help) zips with values()[4] = sweep_bytes_reclaimed
- COUNTERS[5] = ("cairn_proxy_sweep_bytes_reclaimed_total", help) zips with values()[5] = sweep_blobs_removed

So the rendered Prometheus output is:
```
# HELP cairn_proxy_sweep_blobs_removed_total Cached blobs a sweep removed to bring the store under its ceiling.
# TYPE cairn_proxy_sweep_blobs_removed_total counter
cairn_proxy_sweep_blobs_removed_total <BYTES_VALUE>
```

Yes, this is a swap bug. Critical.

5. Looking at `forget` more carefully:

```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                tracing::debug!(path = %path.display(), %error, "could not remove a stale index entry");
            }
        }
    }
}
```

This deletes index entries. But what about partial failures? If we successfully remove a blob but fail to remove the index entry (because of a permissions issue), the next time sweep runs:
- The blob is already gone
- The index entry still exists
- The collect() will not find the blob
- The candidate for that hex will not be added to candidates list
- The forget() will not be called with that hex (since it wasn't in `gone`)
- The index entry stays

That's not a critical bug, but it's a minor data hygiene issue.

Actually wait, the way `sweep` works:
```rust
for candidate in candidates {
    let Some(hex) = candidate.path.file_name().and_then(|name| name.to_str()) else {
        continue;
    };

    let remove = !index.contains_key(hex)
        || (remaining >= self.max_bytes && candidate.age >= self.min_age);
    if !remove {
        continue;
    }

    if !dry_run {
        if let Err(error) = fs::remove_file(&candidate.path) {
            tracing::debug!(...);
        }
    }

    remaining = remaining.saturating_sub(candidate.size);
    bytes += candidate.size;
    removed += 1;
    gone.push(hex.to_owned());
}

forget(&index, &gone);
```

If `fs::remove_file` fails for the blob, we still push to `gone` (line 137). Then forget() will try to remove index entries. That's slightly inconsistent - we might delete index entries even though the blob wasn't deleted. That's a minor bug but not a critical one.

Actually this is important - if the blob fails to be removed (perhaps due to permissions), we still add it to `gone`, which means we will remove the index entry for it. So now we have an index entry pointing to nothing. That's stale. Could be considered a defect.

Let me re-look at the issues to compile the report.

6. Look at the candidates collection. In `collect`:
```rust
fn collect(dir: &Path, out: &mut Vec<Candidate>) {
    let Ok(entries) = fs::read_dir(dir) else {
        return;
    };

    for entry in entries.flatten() {
        let path = entry.path();
        let Ok(meta) = fs::metadata(&path) else {
            continue;
        };
        if meta.is_dir() {
            collect(&path, out);
            continue;
        }
        out.push(Candidate {
            path,
            size: meta.len(),
            age: age_of(&meta),
        });
    }
}
```

This includes ALL files, including non-blob files. If there's a stale lockfile or other random file in the blob directory, it would be counted as a "candidate". Not necessarily a bug, but worth noting. Also, the metadata is read synchronously which could be slow on some filesystems.

The header comment mentions: "store moved between volumes often has blobs linked into it rather than copied. A link is counted at the size of the link and not of whatever it points at, so the total stays the number of bytes this directory is answerable for."

But `fs::metadata` follows symlinks, not `fs::symlink_metadata`. So if there's a symlink, `fs::metadata` returns the metadata of the target, not the link. This contradicts the comment. The actual behavior would be:
- Symlink to a 200MB file: meta.len() = 200MB (target's size), not the symlink's size
- This means the total is the bytes of the targets, not the bytes "answerable for"

Hmm, this is a contradiction between the comment and the code. Whether it's a bug depends on interpretation - the comment says they want the size of the link, but the code follows the link. Actually, `meta.len()` on a symlink metadata gives the length of the target name (not the target file size). But with `fs::metadata`, it follows the symlink to the target.

This is a code/comment mismatch. The comment describes intended behavior; the code does the opposite.

7. Looking at `collect()` and `referenced()` - they reuse the same `collect()` function for both purposes:

```rust
fn referenced(dir: &Path) -> BTreeMap<String, Vec<PathBuf>> {
    let mut entries = Vec::new();
    collect(dir, &mut entries);
    ...
}
```

This collects all entries under `index/`. For each entry, it reads the file, parses it as IndexEntry, extracts the digest hex, and adds to the map. That seems correct.

But `referenced` treats the entire file as a Candidate too (with `path`, `size`, `age`). The age and size are discarded. That's a bit wasteful but not a bug.

8. Looking at the `forget` function issue:
```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                tracing::debug!(path = %path.display(), %error, "could not remove a stale index entry");
            }
        }
    }
}
```

This iterates `index.get(hex).into_iter().flatten()` - if `index.get(hex)` is `None`, `into_iter()` returns an empty iterator. If it's `Some(vec)`, it iterates the vec. OK that's fine.

But `into_iter().flatten()` on `Option<Vec<PathBuf>>` returns an iterator of `&PathBuf`. Actually wait: `Option<T>::into_iter()` returns `IntoIter<T>` which yields 0 or 1 T. So flattening `Option<Vec<PathBuf>>` yields `Vec<PathBuf>`. And then iter()ing gives `&PathBuf`... Hmm let me check.

Actually, `Option<Vec<PathBuf>>::into_iter()` yields `Vec<PathBuf>` (0 or 1 times). Then `.flatten()` on that requires the inner type to be IntoIterator, and `Vec<PathBuf>` is IntoIterator. So `.flatten()` flattens to the elements of the Vec.

Yes, that's correct.

9. Looking at the sweep lock:

```rust
pub async fn run(&self) -> Reclaimed {
    let _running = self.running.lock().await;
    self.sweep(false)
}
```

The lock is held for the duration of `self.sweep(false)`, which is synchronous. This blocks the entire sweep from concurrent runs. But the synchronous sweep runs on the current thread, not the blocking pool. This is intentional per the module documentation.

But wait - the lock is a tokio Mutex, not a std Mutex. tokio Mutex is async-aware. But here we hold it while doing sync work. That means the executor thread is blocked for the duration of the sweep. That's OK if there's just one sweep thread, but it's blocking the tokio worker. Actually a tokio runtime with `rt-multi-thread` will run this on whatever thread happened to schedule it, blocking that thread. So we don't block all I/O, just one thread.

10. Let me look at the sweep.rs more carefully. The `clear_partials` is called concurrently with `BlobWriter::write`. If BlobWriter is in the middle of writing to a file in `incoming/`, and clear_partials removes it, then:
- The BlobWriter's `file.write_all(chunk)` will fail with NotFound or Broken pipe
- The fetch will return an error
- The Drop on BlobWriter will try to remove the file again, logging a warning

This is a real concurrency bug.

11. Also, what about a sweep happening concurrently with a fetch writing to incoming/? The fetch holds the File open. On Linux, you can unlink a file while it's open - the inode stays alive until the last reference is dropped. So the BlobWriter keeps writing to the inode, but the directory entry is gone. When BlobWriter tries to commit, it tries to rename the now-nonexistent path:
```rust
fs::rename(&temp, &target).await?;
```
This will fail with ENOENT.

OK so this is a real concurrency bug.

12. Now let me also check the "scan and remove" pattern. The sweep first does `collect()` and `referenced()` which can be expensive (readdir thousands of entries). Then it iterates and removes. Meanwhile, other fetches can be:
- Writing new files to `incoming/`
- Renaming files from `incoming/<pid>-<seq>` to `blobs/sha256/aa/bb/<hex>`

These concurrent operations can race with the sweep. But in this case, the race only affects which files the sweep sees, not correctness.

But what about a fetch that started before the sweep and commits during it? Let's say a fetch has been writing to `incoming/12345-5`. The sweep starts and clear_partials removes that file. The fetch's write fails, fetch fails. That's the bug.

13. Let me also check `forget`. After the sweep removes a blob (which was unreferenced), `forget` removes the index entries. But wait - what if the blob was removed because it's unreferenced (no index entry points at it)? Then there are no index entries to forget. So `forget` is essentially a no-op for those.

What if the blob was removed because of age/ceiling? Then there are index entries pointing at it, and forget correctly removes them.

But if `forget` fails for some reason (e.g., permission denied), the next sweep will see those index entries again. But the blob file is gone. The `referenced()` function will still include that digest in the map. The candidate for that hex (if there was one) would have been removed in the prior sweep, so `collect` won't see it. But there's no `gone` push, so `forget` won't be called for it. The stale index entry remains. That's a minor issue.

Let me focus on the most critical issues:

**Critical bugs:**
1. metrics.rs - The order of sweep metrics in `values()` and `COUNTERS` is swapped. `values()[4]` returns `sweep_bytes_reclaimed` but `COUNTERS[4]` is `sweep_blobs_removed_total`. This causes the rendered Prometheus output to have swapped metric names and values. Operations on these metrics will see wrong data.

2. sweep.rs - `clear_partials` removes files in `incoming/` without coordinating with in-flight fetches. A concurrent fetch writing to a partial file will lose that file, and its commit() will fail. This causes spurious fetch failures during sweeps.

Let me also verify the bug by reading the lines carefully one more time.

metrics.rs line 50-57 shows the COUNTERS array has `cairn_proxy_sweep_blobs_removed_total` at index 4 and `cairn_proxy_sweep_bytes_reclaimed_total` at index 5.

metrics.rs line 88-97 shows the values() returns `sweep_bytes_reclaimed` at index 4 and `sweep_blobs_removed` at index 5.

In render() (line 105-115), it iterates `COUNTERS.into_iter().zip(self.values())`. So the i-th element of COUNTERS is zipped with the i-th element of values(). So:
- Index 4: (cairn_proxy_sweep_blobs_removed_total, ...) with sweep_bytes_reclaimed value
- Index 5: (cairn_proxy_sweep_bytes_reclaimed_total, ...) with sweep_blobs_removed value

Yes, definitely a critical bug.

Let me also examine other files.

Let me check the Cargo.toml addition of sync and time features, and look at the docs changes.

Let me check the architecture.md change:

```
+The proxy sweeps the store on `CAIRN_SWEEP_INTERVAL`, removing blobs nothing points at and then the oldest blobs until the store is back within `CAIRN_CACHE_MAX_BYTES`. `POST /v1/admin/cache/sweep` runs one now, and `?dry_run=true` reports what one would reclaim without removing anything.
```

That's just docs.

Let me re-read the main.rs changes:

```rust
fn spawn_sweeps(app: Shared) {
    tokio::spawn(async move {
        let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
        ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

        loop {
            ticks.tick().await;
            let reclaimed = app.sweeper.run().await;
            app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
            tracing::info!(?reclaimed, "swept the blob store");
        }
    });
}
```

The first tick: `tokio::time::interval` fires immediately for the first tick, then waits for the period. Wait, actually, `interval` first tick fires immediately by default. Let me verify.

Looking at tokio docs: "The first tick completes immediately." So the first tick is immediate, then subsequent ticks are at the interval.

But the comment says:
```
/// The first tick is one interval away, which is what keeps a proxy that has only
/// just started from sweeping a cache it has not begun to fill.
```

This contradicts the actual behavior of `tokio::time::interval`. The first tick fires immediately. So the sweep runs immediately on startup. This contradicts the documentation comment.

Wait, actually, let me verify by reading the docs more carefully. From tokio docs:

> Each `Interval` is created with an initial delay, e.g. `Interval::new(Duration::from_secs(1), MissedTickBehavior::Skip)` will tick every second.
> The first tick completes immediately.

So the first tick is immediate. The code comment says "The first tick is one interval away" which is incorrect.

Actually, wait. There's `MissedTickBehavior::Delay`. From tokio docs:

> Delay: If ticks are missed (e.g. due to the executor being busy), the next tick is delayed rather than firing immediately.

This only applies after the first tick. The first tick always fires immediately.

Hmm, let me double-check. From tokio source code:
- `Interval::new` creates an interval.
- The first call to `tick()` returns immediately.
- Subsequent calls wait for the period.

So the comment in the code is wrong - the first tick is NOT one interval away; it's immediate.

This means a freshly-started proxy will run a sweep immediately on startup. That contradicts the comment.

Actually wait, the comment says "which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill." But that's the opposite of what `tokio::time::interval` does. The comment author wanted a delayed first tick but used `interval()` which fires immediately.

This is a documentation/behavior mismatch, but more importantly, the BEHAVIOR contradicts the documentation comment. A fresh proxy will sweep on startup, but the documentation says it won't.

Let me look at how to actually delay the first tick. Tokio's `interval_at` allows setting a first tick time. Or `tokio::time::sleep(period).await` followed by a `tokio::time::interval` after the first sleep.

OK so this is a bug. The comment describes intended behavior, but the code does the opposite.

Actually wait, this isn't a code defect in the sense of the task - the actual sweep on startup is just slightly different from intended, but it's not catastrophic. But the comment/code mismatch is real.

Hmm, let me also think about whether the first-tick-immediate behavior is a bug. The comment says the first tick is one interval away, but it's actually immediate. The expected behavior (delayed first tick) might be preferable for many reasons (e.g., not running a full sweep right at startup). So this is a defect.

Let me also examine the routes/mod.rs changes. Looking at:

```rust
let untimed = Router::new()
    .route(
        "/v1/cache/{ecosystem}/{name}/{version}/{file}",
        get(cache::serve),
    )
    .route("/v1/admin/cache/sweep", post(admin::sweep));
```

The sweep route has no timeout (TimeoutLayer only applies to `bounded`). That's intentional per the comment.

Wait, but the comment says "a sweep of a large store legitimately takes longer than ten seconds". The sweep could take a long time, blocking this HTTP handler. That could be a DoS vector if many concurrent sweep requests come in. But there's a Mutex on the sweeper, so they would queue up. Hmm.

Actually, the admin route handler is synchronous (the `reclaim` function). It calls `app.sweeper.sweep(dry_run)` which is synchronous. So it blocks the tokio task for the duration of the sweep. If the sweep takes 10 seconds, the HTTP request holds the connection open for 10 seconds. The 504 timeout layer doesn't apply, so the client waits.

This is by design per the comment. But it could be improved by spawning the sweep on the blocking pool.

Hmm, but this is by design and acceptable.

Let me check one more thing - what about the auth check in admin.rs?

```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    ...
}
```

`sweep(dry_run)` is called BEFORE the lock is acquired - wait, no, `sweep` is synchronous and doesn't acquire the lock. The lock is acquired in `run` (the async version). So in `reclaim`, the sweep runs without holding the async mutex.

But the interval-driven sweep uses `run`, which acquires the mutex. The interval sweep holds the mutex for the duration. The on-demand sweep doesn't acquire the mutex. So they can run concurrently!

That's a bug. The module-level comment says "One sweep at a time." But the on-demand route doesn't enforce that. Two concurrent sweeps could conflict.

Actually wait, `sweep` is `#[must_use]` and takes `&self`. The async `run` uses `self.running.lock().await`. The sync `sweep` doesn't. So calling `sweep` directly bypasses the mutex.

Hmm, but maybe that's OK because the on-demand sweep is held by the request handler thread, which is a different concern. Actually no, the module doc says one sweep at a time. The on-demand sweep can run concurrently with the interval sweep.

This is a violation of the module's contract.

Let me also examine if there's another issue with the on-demand sweep running concurrently. The interval sweep uses `run` which acquires the mutex. So if the interval sweep is running, the on-demand sweep can still run concurrently because it doesn't try to acquire the mutex.

Yes, this is a bug.

OK let me organize my findings:

1. **CRITICAL**: metrics.rs - The order of sweep metrics in `COUNTERS` array vs `values()` array is mismatched, causing Prometheus to render swapped names/values.

2. **HIGH**: sweep.rs - `clear_partials` can remove files being actively written by in-flight fetches, causing those fetches to fail at commit time.

3. **HIGH**: sweep.rs - The async `Sweeper::run` acquires the mutex, but the sync `Sweeper::sweep` does not. The on-demand sweep route bypasses the mutex, so two sweeps can run concurrently, violating the "one sweep at a time" invariant.

4. **MEDIUM**: main.rs - The `tokio::time::interval` first tick fires immediately, contradicting the code comment that says "The first tick is one interval away". This means a freshly-started proxy runs a full sweep on startup, contrary to the documented intent.

5. **LOW**: sweep.rs `collect` - The doc comment says symlinks are counted at "the size of the link and not of whatever it points at", but `fs::metadata` follows symlinks and returns the size of the target. This is a code/comment mismatch.

Actually, I should be careful. Let me re-read the code:

```rust
let Ok(meta) = fs::metadata(&path) else {
    continue;
};
if meta.is_dir() {
    collect(&path, out);
    continue;
}
```

Yes, `fs::metadata` follows symlinks. So if `path` is a symlink, `meta` is the target's metadata. `meta.is_dir()` returns true if the target is a directory, and `meta.len()` returns the size of the target file (not the symlink length).

So the comment is wrong, but the actual behavior is also questionable - the size might double-count if the same blob is linked into multiple subdirectories.

Actually wait, let me re-read the comment:

> A store moved between volumes often has blobs linked into it rather than copied. A link is counted at the size of the link and not of whatever it points at, so the total stays the number of bytes this directory is answerable for.

Hmm. The author is talking about hardlinks, not symlinks. The word "link" here refers to hardlinks. With hardlinks, `fs::metadata` returns the size of the file (not the link). And the comment seems to imply they want to count the link's contribution. But there's no easy way to do that with hardlinks.

Or maybe the comment is saying: if the file is a symlink, count its size (length of the path string), not the size of what it points to. In that case, `fs::symlink_metadata` should be used.

Either way, the comment says one thing and the code does another. This is a defect, though it's edge-case.

Actually, thinking more, hardlinks to the same file: `fs::metadata` returns the metadata of the file (size etc). All hardlinks to the same file have the same size. So if there are multiple hardlinks to the same file in different subdirectories, the sweep would count the size multiple times. That's a bug.

But this is an edge case. Let me think about whether to include this.

Actually, let me check the on-demand sweep again:

```rust
pub fn sweep(&self, dry_run: bool) -> Reclaimed {
```

This takes `&self`. The Mutex is `tokio::sync::Mutex<()>`. If you wanted to acquire it synchronously, you'd need `try_lock()` and fail. Or `blocking_lock()`. But neither is being used.

So yes, `sweep` is a synchronous method that doesn't acquire the mutex. The on-demand route calls it directly without the mutex.

This is the bug.

Let me also check whether this is a real bug by considering the path:
1. Interval sweep is running. It has acquired `running` lock and is in the middle of removing blobs.
2. On-demand sweep request arrives. It calls `app.sweeper.sweep(true)` (sync method). This bypasses the mutex and starts walking the directory.
3. Both sweeps walk the same directory concurrently. They each see different sizes because files are being removed. They each compute their own `remaining` based on their own view. They may decide to remove too many files (bringing store below ceiling).
4. Or one sweep sees the other's about-to-be-removed blob and tries to remove it (race).

Yes, real bug.

Let me also check the `forget` issue:

```rust
if !dry_run {
    if let Err(error) = fs::remove_file(&candidate.path) {
        tracing::debug!(path = %candidate.path.display(), %error, "could not remove a cached blob");
    }
}

remaining = remaining.saturating_sub(candidate.size);
bytes += candidate.size;
removed += 1;
gone.push(hex.to_owned());
```

If `fs::remove_file` fails (e.g., NotFound because the blob was already removed), we still push to `gone`. Then `forget` removes index entries. So the index entry is removed but the blob (was already gone) is fine.

But what if the blob is gone for other reasons? Then we have:
- Index entry gone (by `forget`)
- Blob file still exists

That's a contradiction. If the blob file exists but no index points to it, the next sweep will see it as unreferenced and remove it. So it's eventually consistent.

If the blob fails to be removed (e.g., permission denied), the file remains. But we've removed the index entry. So now:
- Blob file remains
- Index entry gone

Next sweep: blob is unreferenced (since no index entry points to it), and blob age >= min_age (likely). So it's removed next sweep.

That's not really a bug, just inconsistent intermediate state.

OK let me think about other issues.

Let me also look at the on-demand sweep route. The handler calls:

```rust
let reclaimed = app.sweeper.sweep(dry_run);
```

`sweep` is a synchronous method. It blocks the current tokio task for the duration of the sweep. The comment in routes/mod.rs says "a sweep of a large store legitimately takes longer than ten seconds". So this is acceptable, but it's blocking one tokio worker thread. With `rt-multi-thread`, this means one worker is blocked.

If multiple sweep requests come in concurrently, they each block a worker. If many workers are blocked, the runtime can be starved.

But this is by design and the mutex in `run` (async) doesn't apply here.

OK my findings are:

1. CRITICAL: metrics.rs - COUNTERS and values() are out of order for sweep metrics.
2. HIGH: sweep.rs - clear_partials can delete in-flight partials.
3. HIGH: sweep.rs - sweep(dry_run) bypasses the async mutex held by run().
4. MEDIUM: main.rs - tokio::time::interval fires immediately on first tick; comment says otherwise.

Let me also look at if `cache_min_age` validation is missing. Actually the diff doesn't add such a check, so it's not a new defect introduced.

Let me also check the docs and verify they match the code. I'll look at the docs/operations.md change:

> `POST /v1/admin/cache/sweep` runs one immediately, takes the same bearer token as the purge route

Looking at the admin.rs code, the route uses `authorise` which checks against `CAIRN_SERVICE_TOKEN`. The purge route does the same. So this is correct.

> and with `?dry_run=true` reports what a sweep would reclaim without removing anything — which is how to try a new ceiling before setting it.

The code uses `Query<SweepQuery>` with `dry_run: bool`. The default is false. So `?dry_run=true` enables dry run. This is correct.

Actually wait, let me look at the docs/operations.md text:

> `CAIRN_CACHE_MIN_AGE` is the grace period underneath that, so a package one job in a pipeline fetched is still there for the next.

Looking at the sweep logic:

```rust
let remove
REASONING-ONLY LIMIT - 36K chars; cutting
REASONING-ONLY STALL - 36K chars; nudging (1/3)
-- reasoning --
Now I have enough context. Let me think carefully about the diff.

## Sweep.rs analysis

### Bug 1: `forget` uses stale `gone` list

Looking at sweep.rs lines 134-137:
```rust
remaining = remaining.saturating_sub(candidate.size);
bytes += candidate.size;
removed += 1;
gone.push(hex.to_owned());
```

The code records `removed += 1` and `bytes += candidate.size` BEFORE actually removing the file (which happens earlier at line 126). That's actually fine because the metrics are counting what was decided, not what was successfully unlinked.

Wait, the issue is in `forget` at line 249-256:

```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                tracing::debug!(path = %path.display(), %error, "could not remove a stale index entry");
            }
        }
    }
}
```

This function removes the index entries for blobs that were removed. However, the index path passed here is the full path to the index file. So `fs::remove_file(path)` removes the file from the index directory. Good.

Wait, let me look more carefully at the `gone` variable and how it's used. The `gone` contains hex strings. Each hex maps to potentially multiple index entries in the BTreeMap. The forget function then walks those entries and removes them. This looks correct.

### Bug 2: `Sweeper::run` race - the lock is dropped after `sweep(false)` returns

In `run`:
```rust
pub async fn run(&self) -> Reclaimed {
    let _running = self.running.lock().await;
    self.sweep(false)
}
```

Wait, the lock is held during the synchronous sweep. The Mutex from tokio is async-aware, so holding it during a sync call is fine. The lock will be released when `_running` goes out of scope at the end of the function.

But the comment says "Held for the whole of a sweep" - that's true, the lock is held across the whole sweep, including all the synchronous I/O. That's fine, as the runtime just blocks this task while the I/O happens.

But wait - the lock is from `tokio::sync::Mutex`. When held across synchronous code, this works but prevents other async tasks from making progress while waiting. That's expected.

But actually, since the sweep is purely synchronous (uses std::fs), and the Mutex is acquired with `.lock().await`, and the lock is held during the synchronous sweep... this means the Mutex guards against overlapping sweeps correctly. Two concurrent sweep requests will serialize through this Mutex.

But `reclaim` in admin.rs at line 73 calls `app.sweeper.sweep(dry_run)` directly (NOT through `run`). This is a synchronous call. So the admin route bypasses the Mutex. Two admins hitting `POST /v1/admin/cache/sweep` at the same time will both run sweeps concurrently!

Let me re-check admin.rs:
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;
    let reclaimed = app.sweeper.sweep(dry_run);
    ...
}
```

Yes, this calls `sweep` directly, bypassing the Mutex in `run`. The module docs claim "One sweep at a time" but the admin route can violate that.

This is a real bug. The admin endpoint can run a sweep concurrently with the background sweep, defeating the one-at-a-time invariant the module documents.

Severity: Medium. The sweeps are idempotent (remove blobs based on filesystem state), so the worst case is removing more than intended.

Wait, let me think more carefully. Both sweeps compute `held`, `index`, `candidates`, then iterate candidates in age order. Each sweep iterates ALL candidates and decides which to remove. Both sweeps operate on the same set of candidates they independently collected. Both decide to remove the same orphaned/old blobs. Both try to unlink. One succeeds, the other gets ENOENT (logged at debug). No data loss, but both sweep past the ceiling more than necessary.

Actually that's not bad in terms of correctness. But the doc explicitly promises "One sweep at a time" and the design comment says the Mutex is "what keeps the type's promise rather than something every caller remembers". The admin handler is one such caller that forgot. This is the bug.

### Bug 3: `SweepQuery::dry_run` field unused warning / order

Looking at admin.rs line 53-56:
```rust
#[derive(Debug, Deserialize)]
pub struct SweepQuery {
    #[serde(default)]
    dry_run: bool,
}
```

`dry_run` is a field but `Debug` is implemented. Fine.

Actually let me think about whether `?dry_run=true` would work. With axum's Query, default is set to false. So if not specified, dry_run is false. If specified as `?dry_run=true`, it becomes true. That's fine.

### Bug 4: Metrics order mismatch

Looking at metrics.rs lines 33-58 vs 88-97:

COUNTERS:
1. cairn_proxy_cache_hits_total
2. cairn_proxy_cache_misses_total
3. cairn_proxy_upstream_failures_total
4. cairn_proxy_policy_refusals_total
5. cairn_proxy_sweep_blobs_removed_total
6. cairn_proxy_sweep_bytes_reclaimed_total

values():
1. cache_hits
2. cache_misses
3. upstream_failures
4. policy_refusals
5. sweep_bytes_reclaimed
6. sweep_blobs_removed

Wait! There's a mismatch! In COUNTERS, blobs_removed comes first (index 4), then bytes_reclaimed (index 5). But in `values()`, bytes_reclaimed is at index 4, blobs_removed at index 5.

When `render()` does `COUNTERS.into_iter().zip(self.values())`, it pairs them by position:
- COUNTERS[0] = cache_hits_total, values()[0] = cache_hits ✓
- COUNTERS[1] = cache_misses_total, values()[1] = cache_misses ✓
- COUNTERS[2] = upstream_failures_total, values()[2] = upstream_failures ✓
- COUNTERS[3] = policy_refusals_total, values()[3] = policy_refusals ✓
- COUNTERS[4] = sweep_blobs_removed_total, values()[4] = sweep_bytes_reclaimed ✗ (mismatch!)
- COUNTERS[5] = sweep_bytes_reclaimed_total, values()[5] = sweep_blobs_removed ✗ (mismatch!)

This is a clear bug. The metric name and value get mismatched. Operations alerts would be wrong.

Also `record_sweep(blobs, bytes)`:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

Argument order: blobs first, bytes second. The struct fields: blobs_removed, bytes_reclaimed. The fetches update the right fields.

But the values() return order has bytes first, blobs second. And the COUNTERS table has blobs name first, then bytes name.

So COUNTERS[4].name = "blobs_removed" gets paired with values()[4] = bytes_reclaimed value. That's wrong - it reports "blobs_removed N" where N is actually bytes count.

This is a real bug that breaks Prometheus scraping.

### Bug 5: spawn_sweeps & shutdown

Looking at main.rs:

```rust
fn spawn_sweeps(app: Shared) {
    tokio::spawn(async move {
        let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
        ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

        loop {
            ticks.tick().await;
            let reclaimed = app.sweeper.run().await;
            app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
            tracing::info!(?reclaimed, "swept the blob store");
        }
    });
}
```

The task is spawned but never aborted. When the runtime shuts down, this task will be aborted as part of the runtime dropping. But what about during graceful shutdown? The HTTP server stops, but the spawned task keeps running until the runtime is dropped.

Actually the comment says "Not part of the graceful shutdown either". This is a deliberate choice but does mean that during shutdown, a sweep may be in progress when the runtime drops. Since the sweep is purely unlinks with no half-state, this is acceptable.

But what about between `axum::serve` returning and the runtime dropping? There's `Ok(())` between them. Could be a sweep in progress. Fine, accepted.

### Bug 6: Interval first tick fires immediately

`tokio::time::interval` fires the first tick immediately. The comment says "The first tick is one interval away" but `tokio::time::interval` actually fires the first tick at time 0 (immediately). To defer the first tick, you need `interval_at(start, period)` or call `.tick().await` once and discard.

So the comment is wrong AND the behavior is wrong. The first sweep runs immediately, not after `sweep_interval`.

Wait, let me verify. According to tokio docs:
> The first tick completes immediately

Yes. So a proxy that just started up sweeps its empty cache immediately. That's not a correctness bug per se, but it's a wasted sweep. The comment says one thing, code does another.

This is a maintainability/correctness issue with documentation/code mismatch.

Severity: low. The first sweep is harmless (nothing to remove). But the comment is wrong.

### Bug 7: Sweep candidates use stale sort key after index change

Actually let me think about this. The candidates are collected, then sorted by age. Then iterated. During iteration, files are unlinked. But the candidates Vec holds PathBufs and metadata snapshots. The sort and iteration are over the snapshot, not the live filesystem. So no issue there.

### Bug 8: Sweep ignoring dry_run for `gone`

Looking at sweep.rs:
```rust
if !dry_run {
    if let Err(error) = fs::remove_file(&candidate.path) {
        ...
    }
}

remaining = remaining.saturating_sub(candidate.size);
bytes += candidate.size;
removed += 1;
gone.push(hex.to_owned());
```

In dry_run mode, the candidate is "logically removed" (counted), but the file isn't actually deleted. The hex is added to `gone` regardless. Then `forget(&index, &gone)` removes index entries for those hexes:

```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                ...
            }
        }
    }
}
```

WAIT. In dry_run mode, the `forget` function still deletes index entries! This is a bug. A dry run should not remove anything, but the code removes index entries.

Let me look more carefully... yes, `forget(&index, &gone)` is called unconditionally at line 140 after the loop. In dry_run mode, the index entries are still being deleted.

This is a clear bug. A dry run (`?dry_run=true`) intended to "report what a sweep would reclaim without removing anything" would actually delete the index entries. Subsequent cache lookups would miss and refetch, defeating the cache.

Severity: High.

Actually wait, let me re-read the dry_run logic carefully:

```rust
if !dry_run {
    if let Err(error) = fs::remove_file(&candidate.path) {
        ...
    }
}

remaining = remaining.saturating_sub(candidate.size);
bytes += candidate.size;
removed += 1;
gone.push(hex.to_owned());
```

So `bytes`, `removed`, `gone` are updated regardless of dry_run. The actual `remove_file` is gated. Then `forget` deletes the index entries based on `gone`, regardless of dry_run.

So yes, dry_run still removes the index entries. Confirmed bug.

### Bug 9: Same issue with partials - dry_run counted but removed

Looking at clear_partials:
```rust
fn clear_partials(dir: &Path, dry_run: bool) -> u64 {
    let Ok(entries) = fs::read_dir(dir) else {
        return 0;
    };

    let mut removed = 0;
    for entry in entries.flatten() {
        if !dry_run {
            if let Err(error) = fs::remove_file(entry.path()) {
                tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
                continue;
            }
        }
        removed += 1;
    }
    removed
}
```

This correctly removes only when not dry_run. So partials handling is correct. The bug is only in `forget`.

### Bug 10: `gone` should only contain successfully removed

The `gone` is added to unconditionally. If `remove_file` fails (debug-logged), the candidate is still added to `gone`. Then `forget` removes the index entry. Result: blob still on disk, index entry removed. Next cache lookup misses and refetches. Not catastrophic but suboptimal.

But more importantly, in dry_run mode, `gone` contains everything that would be removed, and `forget` actually deletes the index entries. That's the main bug.

### Bug 11: MissedTickBehavior::Delay is the wrong default

The comment says "A sweep that overran its period must not be followed by a burst of sweeps catching up on the ticks it missed. The next one starts a full interval after this one finished." This describes `Delay` correctly.

But the default for `tokio::time::interval` is `Burst`, which means if ticks are missed, they all fire rapidly. Setting `Delay` does mean the next tick is a full interval later. OK, this is correct.

### Bug 12: lock_during_blocking_io

The `run` function uses `tokio::sync::Mutex` and holds it across blocking I/O. If you have multiple concurrent sweep requests, they all queue up. But this is a different concern from the actual bugs.

### Bug 13: Sweeper::sweep ignores the Mutex

Already covered above. admin.rs calls `sweeper.sweep(dry_run)` directly, bypassing the Mutex that `Sweeper::run` uses.

Let me trace once more:
- `Sweeper::run` (async, used by background task) - acquires lock
- `Sweeper::sweep` (sync, used by admin handler) - does NOT acquire lock

So admin handler doesn't acquire the lock. Background and admin can run concurrently. Bug.

### Summary of confirmed defects

1. **Metrics name/value mismatch** at services/proxy/src/metrics.rs:88-95 (HIGH) - COUNTERS and values() order differs for the two new metrics, causing wrong values reported for wrong names.

2. **dry_run removes index entries anyway** at services/proxy/src/sweep.rs:140 (HIGH) - The `forget` function is called unconditionally, but the `gone` list includes all candidates regardless of dry_run. A dry run deletes index entries.

3. **Admin sweep bypasses mutex** at services/proxy/src/routes/admin.rs:76 (MEDIUM) - Sweeper::sweep called directly, bypassing the Mutex that Swipper::run uses. Module docs promise "One sweep at a time" but two callers now race.

4. **Interval first tick fires immediately** at services/proxy/src/main.rs:113 (LOW) - The comment says "The first tick is one interval away" but tokio::time::interval's first tick is immediate. The code does the opposite of what the comment promises.

Let me also double-check the metrics ordering more carefully:

In metrics.rs:
```
const COUNTERS: [(&str, &str); 6] = [
    (
        "cairn_proxy_cache_hits_total", ...
    ),
    (
        "cairn_proxy_cache_misses_total", ...
    ),
    (
        "cairn_proxy_upstream_failures_total", ...
    ),
    (
        "cairn_proxy_policy_refusals_total", ...
    ),
    (
        "cairn_proxy_sweep_blobs_removed_total", ...
    ),
    (
        "cairn_proxy_sweep_bytes_reclaimed_total", ...
    ),
];
```

So COUNTERS indices:
- 0: cache_hits
- 1: cache_misses
- 2: upstream_failures
- 3: policy_refusals
- 4: sweep_blobs_removed
- 5: sweep_bytes_reclaimed

values():
```
fn values(&self) -> [u64; 6] {
    [
        self.cache_hits.load(Ordering::Relaxed),
        self.cache_misses.load(Ordering::Relaxed),
        self.upstream_failures.load(Ordering::Relaxed),
        self.policy_refusals.load(Ordering::Relaxed),
        self.sweep_bytes_reclaimed.load(Ordering::Relaxed),  // index 4 = bytes
        self.sweep_blobs_removed.load(Ordering::Relaxed),    // index 5 = blobs
    ]
}
```

values indices:
- 0: cache_hits
- 1: cache_misses
- 2: upstream_failures
- 3: policy_refusals
- 4: sweep_bytes_reclaimed
- 5: sweep_blobs_removed

In render():
```rust
for ((name, help), value) in COUNTERS.into_iter().zip(self.values()) {
    let _ = writeln!(out, "# HELP {name} {help}");
    let _ = writeln!(out, "# TYPE {name} counter");
    let _ = writeln!(out, "{name} {value}");
}
```

So:
- index 4 emits "cairn_proxy_sweep_blobs_removed_total" with the value of `sweep_bytes_reclaimed`. WRONG.
- index 5 emits "cairn_proxy_sweep_bytes_reclaimed_total" with the value of `sweep_blobs_removed`. WRONG.

This is a definite bug. Severity: high (because Prometheus alerts would be on the wrong values).

Now let me think about the spawn_sweeps first tick more carefully.

```rust
fn spawn_sweeps(app: Shared) {
    tokio::spawn(async move {
        let mut ticks = tokio::time::interval(app.cfg.sweep_interval);
        ticks.set_missed_tick_behavior(MissedTickBehavior::Delay);

        loop {
            ticks.tick().await;
            let reclaimed = app.sweeper.run().await;
            ...
        }
    });
}
```

The doc says:
> The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill.

But `tokio::time::interval(d)` fires its first tick immediately. So a fresh proxy will sweep immediately. The comment is wrong AND the behavior is wrong.

To fix this you'd use `interval_at(Instant::now() + d, d)` or call `ticks.tick().await` once to consume the immediate tick and then enter the loop.

Severity: Low (operationally harmless but wasteful).

Wait, actually let me think about this. The "consume the first tick" pattern is to call `.tick().await` once outside the loop, then enter the loop. But the comment says "The first tick is one interval away". If the comment claims behavior but code does the opposite, it's an inconsistency.

The user-visible effect: a fresh proxy sweeps immediately on startup. A 32GB empty cache means this sweep does nothing. It's wasteful CPU/IO but not broken.

Severity: low.

Now let me look once more at the index_path approach in `forget`. Each `gone` hex might map to multiple index paths. The `forget` removes all of them. That seems correct.

But wait - the way candidates are collected includes traversing directories. The blob directory structure is `blobs/sha256/<first2>/<next2>/<hex>`. The index directory structure is `index/<ecosystem>/<name>/<version>/<file>` (a JSON file).

In `collect`, files are added to `candidates` with `meta.len()` as size. This works for blobs because each file is one blob. But for `referenced`, it walks the index directory and treats every file as an index entry. Fine.

But for `collect` called for the index dir... no, `collect` is only called once for blobs. `referenced` walks the index separately.

Hmm, wait. The `collect` function is generic - it walks any directory tree. It treats all files the same way. In `referenced`, `collect` is called with `&mut entries` to get all index files. Each is then parsed as IndexEntry to extract the digest hex. Good.

OK let me also look at the path:blob_path vs referenced path.

`blob_path` = `blobs/sha256/<2>/<2>/<hex>`. The filename is `<hex>`. The `candidate.path.file_name()` gives the hex. That hex is then checked against `index.contains_key(hex)`.

The `index` map is keyed by hex digest strings. Good.

Wait, the comment in `referenced` says "A map rather than a set because one digest is reachable through several sets of coordinates." True. One blob may have multiple index entries. Good.

OK let me also look at the Reclaimed struct serialization:

```rust
#[derive(Debug, Serialize)]
pub struct Reclaimed {
    pub held: u64,
    pub scanned: u64,
    pub removed: u64,
    pub bytes: u64,
    pub partials: u64,
}
```

The handler returns this as JSON. Good.

But: `app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes)` - this passes `removed` as blobs and `bytes` as bytes. The `record_sweep` signature is `pub fn record_sweep(&self, blobs: u64, bytes: u64)`. So blobs=removed, bytes=bytes. The struct fields are `sweep_blobs_removed` and `sweep_bytes_reclaimed`. So `removed` -> `sweep_blobs_removed` and `bytes` -> `sweep_bytes_reclaimed`. Consistent.

But in `values()` the order is reversed (bytes first, blobs second). This is the bug.

Alright let me also reconsider the partials counting for dry_run. Actually wait, in dry_run mode:

```rust
for entry in entries.flatten() {
    if !dry_run {
        if let Err(error) = fs::remove_file(entry.path()) {
            tracing::debug!(path = %entry.path().display(), %error, "could not remove an abandoned partial blob");
            continue;
        }
    }
    removed += 1;
}
```

When `dry_run` is true, the file is not removed, but `removed` is incremented. This counts the partials that WOULD be removed. Correct.

But `forget`:
```rust
fn forget(index: &BTreeMap<String, Vec<PathBuf>>, gone: &[String]) {
    for hex in gone {
        for path in index.get(hex).into_iter().flatten() {
            if let Err(error) = fs::remove_file(path) {
                tracing::debug!(path = %path.display(), %error, "could not remove a stale index entry");
            }
        }
    }
}
```

No `dry_run` check. Files removed unconditionally. BUG.

Confirmed.

Let me also re-check that I'm right about the Mutex bypass:

In routes/admin.rs:
```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    ...
}
```

`app.sweeper.sweep(dry_run)` calls `Sweeper::sweep` which is synchronous and does not acquire the lock. Yes, confirmed.

The comment in sweep.rs says:
> /// Held for the whole of a sweep, so this module's one-at-a-time promise is
> /// something the type keeps rather than something every caller remembers.

And `run` does acquire it. But `sweep` (the sync method) doesn't, and admin uses `sweep` directly.

OK, let me now think about whether there are any other issues:

1. Could there be a race between the admin route calling `sweeper.sweep` and the background task calling `sweeper.run`? Yes, both can run concurrently. Documented invariant violated.

2. Could two admins call `sweeper.sweep` concurrently? Yes, no Mutex protection. They would race.

3. The age_of function returns Duration::default() on error. Duration::default() is zero. So a blob with an unreadable mtime gets age=0, which is "very recent". This is the OPPOSITE of what you want - you want unreadable blobs to be treated as oldest so they get evicted first (in case they can't be read for some other reason).

Actually, the comment says:
> The failure flattens rather than propagating: a sweep that gave up because one
> blob carried an odd timestamp would be a sweep that never ran.

So they want to continue, not abort. Setting age=0 (treat as "just arrived") means such a blob will be among the LAST to be evicted if the store is over the ceiling. This means a blob with bad metadata could prevent other valid blobs from being evicted. Probably not a bug worth flagging.

But wait, actually for the orphan-detection path:
```rust
let remove = !index.contains_key(hex)
    || (remaining >= self.max_bytes && candidate.age >= self.min_age);
```

For orphans (no index entry), age doesn't matter. So this isn't an issue for orphans.

For age-based eviction, a blob with age=0 won't be evicted until age >= min_age. So this blob sticks around. Not a bug.

For listing in `held`, the blob's size is counted. Fine.

OK no major issue there.

Let me also check the candidates sort. `sort_by_key(|candidate| candidate.age)`. Age is Duration. Duration is `Copy` and `Ord`. So sort by Duration works.

For two blobs with the same age, the order is whatever Vec's sort produces (stable, so insertion order). Fine.

Alright let me think about whether the spawn_sweeps comment claim about MissedTickBehavior is correct.

`MissedTickBehavior::Delay`:
> the ticks that would have been missed are not fired. Instead, the timer is reset to fire one tick after the previous tick completed.

So if a sweep takes 20 minutes and interval is 15 minutes, the next tick fires 15 minutes after the sweep ends. The comment says exactly this. Correct.

OK I think the four issues I identified are the main ones. Let me also briefly think about:

- What if `app.sweeper.run().await` is called when the runtime is shutting down? The task gets aborted. The lock is held by an aborted task. With `tokio::sync::Mutex`, an aborted task releases the lock. OK.

- What about error handling in the sweep loop? `app.metrics.record_sweep(...)` - if this errors, what happens? Looking at metrics, `record_sweep` does atomic fetches, never errors. OK.

- What about the `spawn_sweeps` task panicking? It would terminate, no retry. The proxy would never sweep again. But `sweeper.run()` returns Reclaimed, not Result. No panic source there.

- Is there an issue with `SweepQuery` field not being public? The handler uses `query.dry_run` from within the same module. That's fine in Rust since same module.

Let me also think about the metrics counter names. The Prometheus convention is `_total` suffix. Both new counters have it. Fine.

But wait, the metrics are labeled `sweep_blobs_removed` and `sweep_bytes_reclaimed`. But `record_sweep` uses parameter names `blobs` and `bytes`. The struct fields are `sweep_blobs_removed` and `sweep_bytes_reclaimed`. Internally consistent except for the order in `values()`.

OK so my findings:

1. **HIGH** - Metrics counter name/value order mismatch (services/proxy/src/metrics.rs:88-95) - the values() array is in [cache_hits, cache_misses, upstream_failures, policy_refusals, sweep_bytes_reclaimed, sweep_blobs_removed] order while COUNTERS is in [cache_hits, cache_misses, upstream_failures, policy_refusals, sweep_blobs_removed, sweep_bytes_reclaimed] order. Prometheus gets blobs/bytes counts swapped.

2. **HIGH** - `forget` removes index entries in dry_run mode (services/proxy/src/sweep.rs:140) - The docs say `?dry_run=true` reports what a sweep would reclaim without removing anything. But `forget(&index, &gone)` runs unconditionally and `gone` includes all candidates regardless of dry_run. A dry run deletes index entries.

3. **MEDIUM** - Admin sweep handler bypasses the mutex (services/proxy/src/routes/admin.rs:76) - Module docs promise "One sweep at a time" and the Mutex is supposed to enforce that. `Sweeper::run` (used by background) acquires it; `Sweeper::sweep` (used by admin handler) does not. Concurrent sweeps are now possible, violating the invariant.

4. **LOW** - `tokio::time::interval` first tick fires immediately (services/proxy/src/main.rs:114) - The comment promises "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill." But the default `interval` fires its first tick at time 0, so a fresh proxy sweeps immediately. Either the comment or the code should change to defer the first tick (e.g., via `interval_at` or a pre-loop `.tick().await`).

Let me also double-check Bug 3 (admin bypasses mutex). Looking again:

```rust
pub async fn run(&self) -> Reclaimed {
    let _running = self.running.lock().await;
    self.sweep(false)
}

pub fn sweep(&self, dry_run: bool) -> Reclaimed {
    // No lock
    ...
}
```

And in admin.rs:
```rust
let reclaimed = app.sweeper.sweep(dry_run);
```

The admin route calls `sweep`, not `run`. The Mutex is bypassed. Confirmed.

Let me also check if there is a potential issue with `record_sweep` being called from the admin route too. Looking at admin.rs:

```rust
fn reclaim(app: &App, headers: &HeaderMap, dry_run: bool) -> Result<Response, Error> {
    authorise(app, headers)?;

    let reclaimed = app.sweeper.sweep(dry_run);
    if !dry_run {
        app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
    }
    ...
}
```

So admin also calls `record_sweep`. And spawn_sweeps also calls it:
```rust
let reclaimed = app.sweeper.run().await;
app.metrics.record_sweep(reclaimed.removed, reclaimed.bytes);
```

Both call `record_sweep(removed, bytes)`. Args: first blobs, second bytes. Method:
```rust
pub fn record_sweep(&self, blobs: u64, bytes: u64) {
    self.sweep_blobs_removed.fetch_add(blobs, Ordering::Relaxed);
    self.sweep_bytes_reclaimed
        .fetch_add(bytes, Ordering::Relaxed);
}
```

So caller's first arg `removed` -> `sweep_blobs_removed`, second arg `bytes` -> `sweep_bytes_reclaimed`. Internally consistent in writes.

But the render pairs COUNTERS[4]="sweep_blobs_removed_total" with values[4]=sweep_bytes_reclaimed, which is the bytes count under the blobs name. This is the bug.

OK I'm confident in my findings. Let me finalize the report.

Actually let me look once more at the Interval issue. Looking at tokio docs:

> The first tick of the interval completes immediately.

So `interval(Duration::from_mins(15))` first tick fires at t=0. The comment in spawn_sweeps says:
> /// The first tick is one interval away, which is what keeps a proxy that has only
> /// just started from sweeping a cache it has not begun to fill.

But the actual behavior is that the first tick fires immediately, NOT after one interval. So either:
- The comment is wrong (the behavior is what the code does)
- The code is wrong (the behavior should be what the comment promises)

If you want the first tick at t=interval, use `tokio::time::interval_at(tokio::time::Instant::now() + period, period)` instead of `interval(period)`.

Either way it's an inconsistency. The user-visible effect: a fresh proxy sweeps its (empty) cache immediately on startup. Harmless.

Severity: low. Maintainability/correctness of docs.

Alright, my final report.

Actually, looking once more at the metrics bug, the doc comment at line 30-32:
```
/// Name and help for each counter, in the order [`Metrics::values`] reads them.
/// Keeping the two side by side is what stops a fifth counter from being rendered
/// under a fourth one's name.
```

But now there are six. And the order ISN'T the order `values()` reads them. The new ones are swapped. So the very thing the comment promises (correctness via ordering) is now broken.

Let me also re-examine if there are any other issues I might have missed.

Looking at the `IndexEntry` import in sweep.rs:
```rust
use crate::store::IndexEntry;
```

And in store.rs:
```rust
#[derive(Debug, Deserialize, Serialize)]
pub struct IndexEntry {
    pub version_id: String,
    pub digest: String,
}
```

The struct is `pub`, so importable. Fine.

Looking at `Digest::parse` and `Digest::hex`:
```rust
.and_then(|record| Digest::parse(&record.digest).ok())
.map(|digest| digest.hex().to_owned())
```

The `Digest::hex()` method exists (used in store.rs `blob_path_in`). Let me check the digest module to be safe.
└ 7972 tok · 137.0 tok/s · 32K ctx · 3199ms ttft · 58.2s wall
🔴 `bug` services/proxy/src/metrics.rs:88-95: The `values()` array orders the two new counters as `[..., sweep_bytes_reclaimed, sweep_blobs_removed]`, but the `COUNTERS` table at line 33-58 lists them as `[..., sweep_blobs_removed_total, sweep_bytes_reclaimed_total]`. `render()` pairs them positionally, so Prometheus sees blobs_removed reporting the bytes total and bytes_reclaimed reporting the blob count — exactly the swap the "Keeping the two side by side" comment is supposed to prevent.

🔴 `bug` services/proxy/src/sweep.rs:140: `forget(&index, &gone)` is invoked unconditionally at the end of `sweep`, and `gone` accumulates hex digests regardless of `dry_run` (line 137). The route is documented as `?dry_run=true` reports what a sweep would reclaim without removing anything, but a dry run still `unlink`s the index files via `forget`, leaving blobs on disk with no index entries and forcing a refetch on the next cache lookup.

🟠 `bug` services/proxy/src/routes/admin.rs:76: `reclaim` calls `app.sweeper.sweep(dry_run)` (the synchronous entry point at sweep.rs:90), bypassing the `tokio::sync::Mutex` that `Sweeper::run` (sweep.rs:82-85) acquires to honour the module's "One sweep at a time" promise. The background task and any number of concurrent admin requests can now run sweeps in parallel, each iterating candidates over a total the other is mutating.

🟡 `bug` services/proxy/src/main.rs:113: `tokio::time::interval(app.cfg.sweep_interval)` fires its first tick at t=0, but the surrounding doc comment on line 106-108 claims "The first tick is one interval away, which is what keeps a proxy that has only just started from sweeping a cache it has not begun to fill." A fresh proxy sweeps an empty store immediately on startup; either the comment or the choice of constructor (`interval_at`) is wrong.