Audit JSONL Write Failures — 469 occurrences (~8% data loss)

[mickume]

AuditJsonlSink.emit_audit_event() in audit.py:226-233 opens the file with open(path, "a") for every single event, with no file locking. With
parallel = 5, concurrent appends cause OSError. The JSONL file ended up with 5,466 lines, meaning ~469 events were silently dropped.

Root cause: No file lock or buffered writer. Each emit_audit_event call opens/closes the file independently.

[mickume]

Starting fix session on branch fix/545-audit-jsonl-write-failures-469-occurrences-8-data-loss... (run: 20260426_172016_7d7c03)

[mickume]

Triage Report

Summary: AuditJsonlSink.emit_audit_event() in agent_fox/knowledge/audit.py (lines 226-233) opens the JSONL file with open(path, 'a') on every call with no file locking. When parallel=5 sessions concurrently call emit_audit_event(), the open/write/close cycle is not atomic — competing writers collide at the OS level, triggering OSError which is silently swallowed, causing ~8% event data loss. The fix must acquire an exclusive advisory file lock (e.g., fcntl.flock) around each append operation.

Affected files:

• agent_fox/knowledge/audit.py
• tests/unit/knowledge/test_audit_sink.py

Acceptance Criteria

AC-1: Concurrent writes from multiple threads to the same AuditJsonlSink produce no data loss.

• Preconditions: A single AuditJsonlSink instance targeting the same JSONL file path is shared across N threads (N >= 5).
• Expected: After all threads complete, the JSONL file contains exactly N * events_per_thread lines, each a valid JSON object.
• Assertion: Write a test using threading.Thread that spawns 5 threads, each emitting 20 AuditEvents via the same sink. Assert that the output file has exactly 100 lines, each parseable as JSON with the correct event_type.

AC-2: emit_audit_event uses an exclusive file lock to serialize concurrent appends.

• Preconditions: The audit.py source file is inspected after the fix is applied.
• Expected: The emit_audit_event method acquires an exclusive lock (e.g., fcntl.flock with LOCK_EX, or equivalent) before writing and releases it after, guaranteeing atomicity of the open-write-close cycle.
• Assertion: grep -n 'flock|LOCK_EX|FileLock|threading.Lock' agent_fox/knowledge/audit.py returns at least one match inside or near the emit_audit_event method body.

AC-3: Single-threaded sequential writes still produce correct output after the fix.

• Preconditions: An AuditJsonlSink is constructed in a fresh temp directory.
• Expected: Emitting two events sequentially produces a JSONL file with exactly 2 valid JSON lines, matching event IDs and types (regression guard for existing TS-40-14 test).
• Assertion: The existing test_writes_lines and test_json_format tests in tests/unit/knowledge/test_audit_sink.py continue to pass without modification.

AC-4: Write failure (OSError) is still caught and logged as a warning, not raised.

• Preconditions: AuditJsonlSink is initialized with a non-existent unwritable path.
• Expected: emit_audit_event logs a WARNING containing 'Failed to write audit event' and does not raise, regardless of locking implementation.
• Assertion: The existing test_write_failure test in tests/unit/knowledge/test_audit_sink.py passes without modification.

AC-5: No new third-party dependencies are introduced for the locking mechanism.

• Preconditions: pyproject.toml is inspected before and after the fix.
• Expected: The fix uses only stdlib modules (e.g., fcntl on POSIX, or msvcrt on Windows, or threading.Lock for in-process coordination) — no new entries appear in the [project.dependencies] section of pyproject.toml.
• Assertion: git diff pyproject.toml shows no added lines under [project.dependencies].

(run: 20260426_172016_7d7c03)

[mickume]

Fix Review Report

Overall verdict: PASS

Summary: All 5 acceptance criteria satisfied. The fix adds a threading.Lock to AuditJsonlSink.init and acquires it around the open-write-close cycle in emit_audit_event, serializing concurrent appends. A new test_concurrent_writes_no_data_loss test exercises 5 threads × 20 events and asserts exactly 100 valid JSON lines. Existing tests (test_writes_lines, test_json_format, test_write_failure) continue to pass. No new dependencies. make check passes: 4442 tests green, ruff clean.

Per-criterion verdicts

• ✅ AC-1: PASS
  • Evidence: test_concurrent_writes_no_data_loss spawns 5 threads × 20 events = 100 expected lines. Test passes: the JSONL file contains exactly 100 lines, each parseable as JSON with event_type 'run.start'. Confirmed via pytest -v output.
• ✅ AC-2: PASS
  • Evidence: grep for 'threading.Lock' in agent_fox/knowledge/audit.py returns two matches: line 222 (self._lock = threading.Lock()) in init and line 231 (docstring reference). The lock is acquired at line 236 via 'with self._lock:' wrapping the open-write-close cycle inside emit_audit_event (lines 228-240). Serialization of concurrent appends is guaranteed.
• ✅ AC-3: PASS
  • Evidence: test_writes_lines (TS-40-14) emits 2 events sequentially and asserts exactly 2 lines in the JSONL file. test_json_format (TS-40-14) verifies all fields are present and correct. Both tests pass without modification (confirmed via pytest -v).
• ✅ AC-4: PASS
  • Evidence: test_write_failure (TS-40-16) creates a sink with path '/nonexistent/path/audit', emits an event, and asserts that a WARNING containing 'Failed to write audit event' is logged without raising. Test passes without modification (confirmed via pytest -v).
• ✅ AC-5: PASS
  • Evidence: git diff HEAD~1 -- pyproject.toml produces empty output — no changes to pyproject.toml. The fix uses only stdlib module 'threading' (import added at audit.py line 14). No new third-party dependencies introduced.

(run: 20260426_172016_7d7c03)

[mickume]

Fix complete on branch fix/545-audit-jsonl-write-failures-469-occurrences-8-data-loss. Changes have been merged into develop. Create a PR from that branch to land them on main. (run: 20260426_172016_7d7c03)