feat(logging): per-session log files for isolated debugging - #112

Review Summary

Verdict: APPROVE with nits — The implementation is correct, well-structured, and follows best practices. A few minor issues noted below.

Step 3 — PR Description Review

Minor issues:

1. Missing placeholder in path: The description reads:

   logs/session-.log

   This is missing the <session_id> placeholder. Should be:

   logs/session-<session_id>.log

2. Missing topic parameter: The description says setup_session_logging(session_id) but the actual function signature is setup_session_logging(session_id, topic). The topic parameter (used for the initial delimiter line) is omitted from the description.

Step 4 — Code Review

observability/session_logging.py ✅

• Path resolution (_get_log_dir): Uses Path(__file__).resolve().parent.parent.parent.parent / "logs" — identical to server.py line 34. Consistent. ✅
• SessionFilter: Clean logging.Filter subclass. Injects session_id into LogRecord correctly. ✅
• setup_session_logging: Properly creates directory, configures FileHandler with DEBUG level, sets formatter, adds SessionFilter, registers on root logger. Writes initial delimiter line using handler.handle() (which runs filters — correct approach). ✅
• teardown_session_logging: Removes handler from root logger and closes it. Idempotent-safe. ✅
• Error handling: mkdir and FileHandler raise OSError (docstring accurate). ✅

sessions.py — _run_session() wiring ✅

Good:

• Setup in try/except with a clear warning message and exc_info=True ✅
• Teardown in finally block ✅
• Lazy import pattern consistent with the rest of the module ✅
• _session_log_handler: logging.Handler | None type annotation ✅

One structural concern (minor):

There is a gap between the setup block (lines 297–310) and the main try: block (line 325). Lines 317–324 (cancel event creation + lazy imports) execute outside the inner try/finally. If those intermediate lines raise, the per-session handler would leak (added to root logger but never removed).

# Lines 297-310: setup (ok, caught)
_session_log_handler = ...  # might succeed

# Lines 317-324: intermediate code (NOT covered)
cancel_event = asyncio.Event()     # very unlikely to fail
from deepresearch.orchestrator ... # very unlikely to fail

# Line 325: inner try/finally starts
try:
    ...
finally:
    # teardown only runs if we reached line 325
    if _session_log_handler:
        teardown_session_logging(_session_log_handler)

In practice, asyncio.Event() and from imports virtually never raise. But if they did (e.g., import error from a dependency change), the handler would leak. To be maximally robust, consider wrapping the entire post-setup body in a single try/finally, or adding an additional guard. Not a blocker.

ADR-0003 & Design Doc Updates ✅

Accurate and consistent with code:

• Version bumped to 1.4 (ADR) and 1.6 (design doc) ✅
• Per-session enhancement noted in ADR §File-Based Logging section ✅
• Design doc §6.5 matches implementation: format, level, lifecycle, safety ✅
• New positive/negative consequences listed ✅

One minor doc mismatch:

Both documents state that the per-session log captures "all loggers under the deepresearch.* namespace." However, the handler is added to the root logger (logging.getLogger()), meaning all loggers (including third-party) will write to the per-session file. This is consistent with how the global deepresearch.log handler works (also on root logger), so it's behaviorally correct — but the "scope" description in §6.5 is technically narrower than reality. Consider updating §6.5 to say "the root logger (which includes all deepresearch.* loggers)."

ADR-0049 / The Ladder Compliance

Lean already. Ship.

• Stdlib only: No new dependencies introduced. logging.Filter, logging.FileHandler, pathlib.Path — all stdlib. ✅
• No unnecessary abstractions: SessionFilter is a minimal, idiomatic logging.Filter subclass. _get_log_dir() exists only to share path resolution with server.py. No interfaces, factories, or speculative generality. ✅
• No ponytail comments needed: The implementation is simple enough that intentional-shortcut annotations are unnecessary. ✅

"Not lazy about" check ✅

• Trust-boundary validation: session_id is auto-generated (uuid.uuid4()[:8]), not user-supplied. No injection risk. ✅
• Data-loss error handling: Graceful degradation — setup failure warns and continues. ✅
• Security: Log paths derived from internal IDs, not user input. ✅

Tests

No test file for the new module was found. The existing .testers_done marker suggests testers have signed off, but session_logging.py has 114 lines of new production code without dedicated tests. Consider adding basic tests for setup_session_logging (happy path + error path) and teardown_session_logging (including idempotency).

Summary of Issues

None of these are blockers. The code is correct, well-documented, and ready to merge.