fix(observability): a failing trace disk no longer stalls the server

[t3dotgg]

When the trace file cannot be written (full disk, EACCES, EIO), the trace sink put every failed record back in its buffer, and each new span retried the whole backlog. After about 5 minutes of a failed disk this took about 40% of the event loop and 35 to 65 MB of heap. Then buffer.unshift(...) threw RangeError, which lost the backlog and killed the timed flush fiber.

Fix

• A failed write drops the rest of that batch and counts it. The buffer stays at one batch (256 records) or less. Writes start again on their own when the disk recovers.
• The sink logs one warning when writes start failing, and one info line with the dropped count when they recover.
• The flush runs without the inherited makeTraceSink span. That span has ended but lives as long as the sink, so the tracer logger would add every log line to it and never free them.
• Docs: T3CODE_TRACE_BATCH_WINDOW_MS defaults to 1000 ms, not 200 ms.

Tradeoff

A short failure (for example Windows EBUSY on the rotate rename) now loses that batch (256 records or fewer) instead of retrying it. The recovery line counts the loss. This only affects the local trace file, not user data.

Verification

• vp test run packages/shared/src/observability.test.ts (24 passed). The new test puts a directory at the trace path so every append fails, pushes 1,024 records, and runs timed flushes with TestClock. It checks writes of 256, 256, 256, 256, then 1 record (no growing backlog), one warning, a recovery line with droppedCount: 1029, silent healthy flushes, a new warning for a second failure, and 0 events on the makeTraceSink span.
• The test fails with the old unshift retry, with a warning on every flush, and without the parent span removal.
• vp lint, vp fmt, and typecheck for shared, server, and desktop.
• Merges cleanly with #13760 (same file, different lines) in either order.

Made by Claude Opus 5.5 (1M context) in Claude Code, running in T3 Code.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Trace logging now warns when writes begin failing and reports the number of records dropped once writes recover. The warning is limited to once per failure episode; if writes fail again after recovery, a new warning is reported. Failures and recoveries detected within the same flush window are treated as one episode.
• Documentation
  • Updated the documented default trace flush window to 1,000 ms.

[macroscopeapp]

Approvability

Verdict: Approved at dd32fb6

Macroscope's review found this PR approvable — This focused observability bug fix bounds failed trace writes, prevents server stalls, and adds targeted coverage for failure and recovery behavior. The documentation-only default correction matches the runtime value already present at the base commit, so no product default is changed.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+12 B (+0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+3 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.5 KiB	+9 B (+0.1%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	56.3 KiB	+44 B (+0.1%)	66.4 KiB	✅
Codex	Live turn messages	9	10	+1 (+11.1%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+18 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+4 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+14 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 295d7cb · PR result: dd32fb6 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 114.0 KiB
• Claude decoded thread snapshot: 114.7 KiB

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📝 Walkthrough

Walkthrough

The trace sink tracks write-failure episodes, counts dropped records, and logs recovery after a successful write. Tests cover timed flushes, failures, and recovery. The documented default trace batch window changes to 1000 ms.

Changes

Observability

Layer / File(s)	Summary
Trace flush failure handling packages/shared/src/observability.ts, packages/shared/src/observability.test.ts, docs/operations/observability.md	The sink warns once during a failure episode and logs the accumulated dropped count after recovery. The test checks failed batches, recovery, and a subsequent failure. The flush effect removes Tracer.ParentSpan from its context. The documentation sets the default flush window to 1000 ms.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: bahlo

Merge Risk: ⚪ Minimal · up to dd32f

The trace sink reports failures by flush window as intended. No actionable issue remains that should delay merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 2…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly and concisely describes the main fix: preventing failed trace-disk writes from stalling the server.
Description check	✅ Passed	The description clearly explains the failure mode, the fix, tradeoffs, documentation change, and verification steps. It does not include the template's Checklist heading, but the required items are mo…

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Inline comments:
In `@packages/shared/src/observability.ts`:
- Line 456: Update `flushUnsafe()` to retain each write-outcome transition so a
recovery followed by another failure is not collapsed into one episode, and have
`flush` report the recorded transitions, preserving the corresponding drop-count
reporting.

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