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fix(logging)!: write structured entries synchronously so a forked child keeps them (LAB-9130) - #572
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…ld keeps them StructuredLogger queued entries in a ring buffer that one cachekit-LogWriter thread per process drained. The thread started at import and nothing started it again after a fork, so every forked child (os.fork with hooks included: Gunicorn --preload, Celery prefork, multiprocessing's fork start method) buffered its entries and never wrote them, and each entry carried the parent's PID. The writer thread, the ring buffer and LogEntry are deleted. An entry is now serialised and logged on the caller's thread, after an isEnabledFor check on cachekit.logging, so a disabled level costs one level check and nothing is sampled or serialised. The pid field is read per entry. A value json cannot encode is written as its type name, never its str(), instead of the whole entry being dropped. Importing cachekit no longer starts a thread. The enable_structured_logging and Structured Logging docs described JSON output the flag does not produce; they now say what each kind of record is. BREAKING CHANGE: CachekitConfig.log_buffer_size, log_batch_size and log_flush_interval are removed, with their env vars CACHEKIT_LOG_BUFFER_SIZE, CACHEKIT_LOG_BATCH_SIZE and CACHEKIT_LOG_FLUSH_INTERVAL; they configured the deleted writer. Passing one to the constructor now raises ValidationError. A still-exported env var is ignored, so startup is unaffected.
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Summary by CodeRabbit
WalkthroughThe SDK logger now writes JSON records synchronously on the calling thread. Async logging settings and the background writer are removed. Documentation and tests describe and validate the logging behaviour and fork handling. ChangesStructured logging
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~25 minutes Change: Bug fix Merge Risk: 🟡 Moderate · up to Some logging paths in workers forked without Python’s fork hooks still bypass the safeguard. Guard those paths before merging to avoid a possible worker hang. Pre-merge checks |
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Actionable comments posted: 3
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/cachekit/logging.py:
- Around line 92-93: Update the sampling condition in the `log()` method so
`_should_sample()` is applied only to levels below ERROR; after the existing
level check, always write ERROR records, including calls using `log("ERROR",
...)`.
- Line 84: Update the field merge in the logging record construction around
_mask_pii so caller-supplied fields cannot override the generated process PID;
merge caller fields before generated metadata or reject reserved field names.
- Line 86: Update the `logger.log` JSON-encoding path so fields that fail
encoding, including mappings with unsupported keys, use a safe type-only
fallback without calling `str()` on their values; ensure the synchronous logging
call still writes the record.
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📒 Files selected for processing (9)
docs/api-reference.mdsrc/cachekit/config/nested.pysrc/cachekit/config/settings.pysrc/cachekit/logging.pytests/performance/ir_budget.pytests/unit/test_config_env_fallback.pytests/unit/test_l1_memory_bounds.pytests/unit/test_structured_logging.pytests/utils/tracemalloc_isolation.py
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- src/cachekit/config/settings.py
- tests/unit/test_l1_memory_bounds.py
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…able field StructuredLogger.log() mapped only DEBUG/INFO/WARNING/ERROR, so CRITICAL, FATAL and WARN were written at INFO and sampled; the old writer resolved them with getattr(logging, name). They now map to their stdlib levels, and nothing at ERROR or above is sampled through log(), matching error(). json.dumps now runs on the caller's thread, so a field it raises on (a non-str dict key, a cycle, a too-deep nest) reached the caller and could replace the exception it was logging. Such a field is now written as its type name, like any other value json cannot encode, and the rest of the entry is kept. Caller fields are merged before the generated ones, so a pid= keyword can no longer make an entry report another process.
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The C encoder's depth limit is the thread's stack since 3.14, so a 100_000-deep nest encodes on a runner with an unlimited stack and the too-deep case failed on the free-threaded lane. The pure-Python encoder stops at sys.getrecursionlimit() on every supported interpreter.
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…fork hooks A fork made from C (uWSGI without --py-call-uwsgi-fork-hooks) skips logging's after-fork lock reset. If a parent thread held logging's module lock or a handler lock at the fork, the child's first structured entry hung: the level check takes the module lock when its level is not cached, and the handler takes its own. Before entries were written synchronously that child only queued them, so the hang is new with this branch. Such a child is detected the way the L1 take-over and the invalidation listener already detect it (l1_cache._forked_without_hooks), and writes nothing there, as those paths log nothing there. A child forked by Python (os.fork, multiprocessing, Gunicorn, Celery) is unaffected and writes its entries with its own pid. The C-fork test now holds logging's module lock and the handler's lock in threads that end before the fork, with no level check cached; without the guard the child hangs until its alarm.
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…ucturedLogger docs/performance.md and the instruction-budget harness still described the deleted log writer thread and its one-day flush interval. Both now name only the L1 cleanup worker, and the harness no longer says that import cachekit starts a thread. The main merge brought an SWR fork test that stopped the log writers before forking; there are none to stop now. The module docstring now says that log() and error() write no entry in a child forked without at-fork hooks, rather than every StructuredLogger method: cache_operation() and connection_pool_utilization() call logging as they did before. The C-fork test uses the shared run_fresh helper. cache_operation() logs through self.logger with the module's level table instead of a local import, a dead five-level map and a second getLogger. error() keeps its own _enabled() gate: routing it through log() would make a caller field named level or message a TypeError.
df6e614
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StructuredLoggerput its entries in a ring buffer that onecachekit-LogWriterthread per process drained. That thread started atimport cachekit, and nothing started it again after a fork. So every forked child buffered its entries and never wrote them: a plainos.fork()with every at-fork hook run, Gunicorn--preload, Celery prefork, andmultiprocessingwith the fork start method. Every entry'spidwas also the parent's, cached when the logger was built. The entries lost were the CachekitIO backend's and client's debug lines and the sampledpool_utilizationentry.Change
LogEntryare deleted. An entry is serialised and logged on the caller's thread. Itspidis read per entry.log()checksisEnabledForoncachekit.loggingbefore it samples, so below that logger's level a call costs one level check, and nothing is sampled or serialised. Before, every sampled DEBUG entry was built and queued whatever the level. Nothing at ERROR or above is sampled, througherror()orlog(), andlog()writes WARN, CRITICAL and FATAL at their own levels.jsoncannot encode is written as its type name ("<ValueError>"), never itsstr(), which may carry a cache key. So is a fieldjsonraises on (a non-str dict key, a cycle, a too-deep nest): the rest of the entry is kept, and the call never raises into its caller. Before, the writer dropped the whole entry without a word.pid,thread_id,logger,leveland the rest) win over a caller field of the same name.import cachekitno longer starts a thread.--py-call-uwsgi-fork-hooks) writes no entry.logging's after-fork lock reset did not run there, so the level check or a handler lock a parent thread held at the fork would hang the caller. It is detected the way the L1 take-over and the invalidation listener already detect it, and like them it logs nothing there. A child forked by Python writes its entries.Behaviour change (breaking)
CachekitConfig.log_buffer_size,log_batch_sizeandlog_flush_intervalare removed, withCACHEKIT_LOG_BUFFER_SIZE,CACHEKIT_LOG_BATCH_SIZEandCACHEKIT_LOG_FLUSH_INTERVAL. They configured the deleted writer. Passing one to the constructor now raisesValidationError. A still-exported env var is ignored, so startup is unaffected.CACHEKIT_LOG_SAMPLING_RATEstays.Tests
tests/unit/test_structured_logging.py: anerror()entry is written before the call returns, with this process'spid; PII-named fields are masked; an unencodable value, and a fieldjsonraises on, are written as their type only; generated fields win over same-named caller fields; the level check comes before sampling, and a disabled level reaches neither; each stdlib level name is written at its level; nothing at ERROR or above is sampled; building a logger starts no thread.os.fork()child writes itserror()entry, and itspidfield is the child's.ctypes.PyDLL(None).fork, run in a fresh interpreter thatimport cachekitleaves single-threaded) whilelogging's module lock and the handler's lock are held by threads it lacks, with no level check cached: itsdebug()anderror()return, write nothing and request no thread start. Without the guard the child hangs until its alarm.tests/unit/test_config_env_fallback.py: the three removed env vars are ignored at load.test_l1_memory_bounds.py, the tracemalloc isolation helper and the instruction-count harness no longer stop or park the writer.All new tests fail on
main. Local runs: the affected suites on 3.12, 3.14.3 and 3.14t, and the fork tests 25 times on 3.14t, 15 on 3.14.3 and 15 on 3.12, with no failure.uv run pytest -m "not slow"matchesmainapart from the new tests: the only failures are the Redis provider integration tests, which need a live Redis, andtest_cachekit_type_preservation_serialized, both failing onmaintoo.Docs:
docs/performance.mdand the instruction-budget harness (no log writer or flush interval left),docs/api-reference.md(the Structured Logging section, including which forked children write, and theenable_structured_loggingentry, which described JSON output that flag does not produce) and theMonitoringConfigdocstring. Companion docs-site change: cachekit-io/docs#173.Squash with this as the commit message footer:
BREAKING CHANGE: CachekitConfig.log_buffer_size, CachekitConfig.log_batch_size and CachekitConfig.log_flush_interval are removed, along with their env vars CACHEKIT_LOG_BUFFER_SIZE, CACHEKIT_LOG_BATCH_SIZE and CACHEKIT_LOG_FLUSH_INTERVAL. They configured the background log writer thread, which is deleted: structured log entries are now written synchronously. Passing one of these fields to the constructor now raises ValidationError. A still-exported env var is ignored, so startup is unaffected.
BEGIN_COMMIT_OVERRIDE
fix(logging)!: write structured entries synchronously so a forked child keeps them (LAB-9130) (#572)
BREAKING CHANGE: CachekitConfig.log_buffer_size, CachekitConfig.log_batch_size and CachekitConfig.log_flush_interval are removed, along with their env vars CACHEKIT_LOG_BUFFER_SIZE, CACHEKIT_LOG_BATCH_SIZE and CACHEKIT_LOG_FLUSH_INTERVAL. They configured the background log writer thread, which is deleted: structured log entries are now written synchronously. Passing one of these fields to the constructor now raises ValidationError. A still-exported env var is ignored, so startup is unaffected.
END_COMMIT_OVERRIDE
Closes LAB-9130