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fix: bound command_capture output by size, not just time - #1318
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command_capture accumulated a child's stdout and stderr into unbounded Vecs, so it was bounded only by the timeout. A command that produces output quickly and steadily stays well inside its deadline while buffering arbitrarily much memory. Retain only the most recent bytes of each stream, tail-first, behind a default per-stream cap. Tail-preserving is deliberate: the collectors that motivated this filter for the newest lines, so head truncation would discard exactly the signal they keep. Record the dropped-byte count on Capture so the bundle can note the truncation instead of silently shipping a partial capture. Add command_capture_capped for the rare caller whose output is legitimately larger than the default.
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Included review availability: 3 reviews are currently available. Your included PR review attempts over the past 7 days set your current allowance at 4 reviews per hour. 📝 WalkthroughWalkthroughCommand output capture now limits each stream to an 8 MiB tail, reports discarded bytes, supports explicit capacities, and preserves drop counts for successful and timeout results. The capped API is publicly re-exported and covered by tail-retention tests. ChangesBounded command capture
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The change bounds diagnostic command output memory use while preserving the newest output and reporting truncation; no actionable merge-blocking risk remains after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant Caller
participant command_capture
participant ChildProcess
participant Tail
participant Capture
Caller->>command_capture: invoke command
command_capture->>ChildProcess: start and read output
ChildProcess->>Tail: provide stdout and stderr bytes
Tail-->>Capture: return retained tails and dropped-byte counts
Capture-->>Caller: return successful or timeout capture
Suggested reviewers: Poem
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Description checkExplanation The description explains the change and rationale, and it provides detailed verification results. It does not use the template's exact Summary and Testing headings and omits the Checklist, but the required information is mostly present.
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🧪 Generate unit tests (beta)
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Inline comments:
In `@crates/diagnostics/src/capture.rs`:
- Around line 92-97: Update Tail::extend to use ring-buffer semantics after
reaching cap, avoiding repeated Vec front-draining and near-full-tail copies;
preserve retained-tail ordering and dropped-byte accounting, and materialize the
ordered bytes as Vec<u8> when constructing Capture. Add a regression test for
command_capture_capped using multiple reads of large output to verify bounded
buffering and correct final tail contents.
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📒 Files selected for processing (2)
crates/diagnostics/src/capture.rscrates/diagnostics/src/lib.rs
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Commit pushed:
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Routing-Key: inbox-route/I_kwDOSUhdos8AAAABNxFrSg
command_capturein the diagnostics crate buffered a child's stdout andstderr into unbounded
Vecs — capture was bounded by the timeout only, so afast, steady producer could stay inside its deadline while buffering
arbitrarily much memory. Each stream now keeps only its most recent bytes
behind a default 8 MiB per-stream cap, tail-first (the command-shaped
collectors filter for the newest lines, so a head cap would drop exactly the
signal they keep). The discarded-byte count rides back on
Captureso abundle can record the truncation;
command_capture_cappedraises the cap fora caller whose output is legitimately larger.
Verification
cargo fmt --all --check — clean
cargo clippy --workspace --locked -- -D warnings — no warnings
cargo build --workspace --locked — ok
cargo test --workspace --locked — ok (diagnostics: 9 passed, incl. size_cap_keeps_the_tail_and_reports_the_drop)
Summary by CodeRabbit
New Features
Bug Fixes