Credential runner signal test: write with os.write so a signal cannot re-enter the buffered stdout - #601

Scope

One test fixture in tests/test_credential_run.py (ProcessTests.test_exit_code_and_signal_propagate). The fixture's readiness line and its signal handler now write with os.write instead of print. The seven signals, their expected exit codes, the exact stdout assertion and the runner (scripts/credential_run.py) are unchanged. Lane: foundation.

Why: the hosted macOS suite of #592 failed once on this test (run 36946376402, job 110649337077: SIGQUIT subtest, 1 != 9; 9,483 tests, 1 failure). The Codex lane's source diagnosis on #592 named buffered-stream reentrancy as the lead. A handler that prints while the interrupted print still holds the buffered stdout raises RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>, so the command exits 1 instead of its code. CPython runs signal handlers inside the buffered writer's flush loop (PyErr_CheckSignals after a raw write), which is where the window opens.

SOTA sources

• Python 3.13 documentation, io module, "Reentrancy": buffered objects are not reentrant, and a signal handler that re-enters one raises RuntimeError (https://docs.python.org/3.13/library/io.html#reentrancy).
• Python 3.13 documentation, signal module, "Execution of Python signal handlers" (https://docs.python.org/3.13/library/signal.html#execution-of-python-signal-handlers).
• Python 3.13 documentation, os.write: an unbuffered write on a file descriptor (https://docs.python.org/3.13/library/os.html#os.write).

Evidence-class table

The local demonstration forces the window with a blocked write; the hosted failure reached the same window through scheduling. It shows the mechanism, not that it was the cause of that run.

Local commands run

• Mechanism, old handler against new handler in a child process blocked on a full pipe: print: exit 1 | RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>; os.write: exit 9.
• python3 -B -m unittest tests.test_credential_run.ProcessTests.test_exit_code_and_signal_propagate, 40 runs: 40 passed.
• python3 -B -m unittest tests.test_credential_run: 92 tests, OK.
• python3 -B scripts/validate.py: passed, 8,976 hashed files.

Both suites ran with the temporary directory outside /tmp: while Codex sandboxes run on this host, an empty /tmp/.git mount target appears and the runner correctly refuses a store inside a git work tree (9 of 150 runs of the unmodified test on main exited 1 with unsafe: inside_git_worktree for that reason). That is a property of this host under load, not of the test.

Decision record

None; a test-fixture repair.

Host evidence

None changed.

Checklist

• No workflow or action changes.
• No secrets are printed, logged or committed; no new required secret was introduced.
• No new paid hosting, subscription or billing surface was introduced.
• Peer-owned untracked files and worktrees were preserved.

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