fix(bench): exit before finalizers in the Python benchmark runner (#434) - #472
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The 2026-07-15 python-nightly failed with exit 139. Every scenario in `--scenario workers` completed and emitted its JSON first; the process then died with SIGSEGV during interpreter finalization. This is the crash class #228 already characterised: pyo3-async-runtimes holds its tokio runtime in process-global state, that runtime's outstanding tasks hold references to Python objects, and finalization tears those objects down while the runtime may still touch them (PyO3/pyo3#1415). The mitigation #228 settled on — exit before finalizers run — lives in `tests/_subprocess_exit.py` and is used by both chaos subprocess helpers. `scripts/benchmark_runtime.py` is the only other `asyncio.run()` entry point in the package and never got it, which is why the same crash resurfaced in the nightly benchmark rather than the chaos helpers. `os._exit` does not flush stdio. Both existing helpers pass `flush=True` on every print so they are unaffected; this script passes it on none of its ten prints, and the nightly tees stdout into the artifact `check-bench-regression.py` reads. Verified by experiment: without the explicit flushes the piped run captures zero lines while still exiting 0, so the naive mitigation would have silently emptied the benchmark artifact. The shared helper gets the same flush so the next caller does not have to know this. The nightly benchmark step also sets PYTHONFAULTHANDLER=1 — this crash was diagnosable only from the shell's "Segmentation fault" line, with no Python frame. I was not able to reproduce the SIGSEGV locally (6 runs of the failing command pinned to 2 CPUs, plus targeted attempts at the shutdown-timeout and sequential-client paths), so this is a structural mitigation of a known upstream hazard rather than a verified-by-reproduction fix.
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📝 WalkthroughWalkthroughThe changes flush standard streams before forced Python process exits. The nightly workflow also enables ChangesProcess exit diagnostics
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🔵 Low · up to The change prevents benchmark crashes during interpreter finalization and preserves buffered output, but a closed output pipe could still bypass the forced exit if flushing raises. This is a bounded issue that should have explicit owner awareness or a small hardening follow-up. Poem
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In `@awa-python/scripts/benchmark_runtime.py`:
- Around line 1081-1093: Update the benchmark shutdown path around
sys.stdout.flush(), sys.stderr.flush(), and os._exit(0) to use the existing
_flush_std_streams() behavior from tests/_subprocess_exit.py, ensuring stdio
flush failures are caught so the forced exit always occurs. Verify the Rust and
Python suites against live PostgreSQL.
Apply the same fix in `@awa-python/scripts/benchmark_runtime.py` around lines 1090
- 1092.
In `@awa-python/tests/_subprocess_exit.py`:
- Around line 10-14: Update the _flush_std_streams docstring to state that
callers may use buffered output and that the helper flushes pending
standard-stream output before os._exit; remove the inaccurate claim that callers
flush every print.
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Fix all unresolved CodeRabbit comments on this PR:
- Push a commit to this branch (recommended)
- Create a new PR with the fixes
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Review feedback on #472, and the first item is not cosmetic. The inline flushes were unguarded. When stdout is a closed pipe, flush() raises BrokenPipeError, the exception propagates out of main(), os._exit(0) is never reached, and the interpreter runs normal finalization — the exact crash path this change exists to avoid. Measured with the output piped into a reader that exits immediately: unguarded gives BrokenPipeError and exit 120 (Python's "failed to flush stdout on exit"), guarded gives exit 0. So a broken pipe silently disarmed the fix. Both flushes now tolerate OSError and ValueError, mirroring _flush_std_streams in tests/_subprocess_exit.py. Duplicated rather than imported because that module is outside this script's import path. Also corrects that helper's docstring: it claimed both callers flush per print, which stopped being true when benchmark_runtime.py started relying on a final flush for buffered output. It now states the actual contract. Verified the artifact is still intact through tee (complete output, exit 0) and test_subprocess_exit.py still passes.
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Addresses the python-nightly half of #434. See also #471, which covers the flake half.
The failure is not a flake
The 2026-07-15 python-nightly failed with exit 139 — SIGSEGV, core dumped — in
benchmark_runtime.py --scenario workers. Every scenario completed and emitted its result first:All seven scenarios produced correct output. The process died afterwards, during interpreter finalization. No throughput floor was missed and no assertion fired — this needed a different fix from the margin work in #471.
Root cause
This is the crash class #228 already characterised: pyo3-async-runtimes holds its tokio runtime in process-global state, that runtime's outstanding tasks hold references to Python objects, and finalization tears those objects down while the runtime may still touch them (PyO3/pyo3#1415). #228's own hypothesis describes this exact sequence, and its recommended mitigation — exit before the finalizer phase — is what
tests/_subprocess_exit.pyimplements.The gap:
scripts/benchmark_runtime.pyis the only otherasyncio.run()entry point in the package, and it never got the mitigation.Both chaos helpers are protected. The benchmark runner is not, which is why this resurfaced in the nightly benchmark rather than in the chaos helpers that #228 was originally about.
Corroborating:
nightly-chaos.ymlalready carries a commented-out--scenario failuresstep, disabled "due to pool exhaustion when creating multipleAsyncClientinstances in sequence", noting the Python client lifecycle fix should be tracked separately.--scenario workerscreates seven clients in sequence — the same pattern, in the step that's still enabled.The flush is the interesting part
os._exitdoes not flush stdio. Both existing helpers passflush=Trueon every print, so they were never exposed to this.benchmark_runtime.pypasses it on none of its ten prints, and the nightly pipes stdout throughteeinto the artifactcheck-bench-regression.pyreads.I checked rather than assumed, running the real command through a pipe:
os._exit(0)aloneos._exit(0)So the naive mitigation would have silently emptied the benchmark artifact while still reporting success — trading a loud crash for a quiet loss of every nightly benchmark result. The flushes are load-bearing. The shared helper gets them too, so the next caller doesn't have to know this.
Also
The nightly benchmark step now sets
PYTHONFAULTHANDLER=1. That step had no fault handler, so this crash was diagnosable only from the shell's "Segmentation fault" line, with no Python frame. Next occurrence will say more.What I did and didn't verify
Verified:
teewith the fix.test_subprocess_exit.pyandtest_cross_language.pypass — the shared helper's callers are unaffected.Not verified: I could not reproduce the SIGSEGV locally. Six runs of the exact failing command pinned to 2 CPUs all exited 0, as did targeted attempts at the shutdown-timeout path (handlers outliving
shutdown(timeout_ms=50)) and the sequential-client path (seven clients created and torn down in order).So this is a structural mitigation of a documented upstream hazard, applied to the one entry point missing it — not a fix verified against a reproduction. It makes the finalization race unreachable for this process rather than curing it.
The underlying PyO3 lifecycle issue is still live, and the disabled
--scenario failuresstep is still disabled. If you want that tracked properly I'd suggest a dedicated issue for the Python client lifecycle — the comment innightly-chaos.ymlasks for one and I couldn't find that it exists. Happy to open it.Summary by CodeRabbit