Handle disappearing proc stat reads in Fleet cleanup regression - #946

Scope

Fix the Fleet producer cleanup test's /proc/<pid>/stat race: when the descendant exits during the read, ProcessLookupError or FileNotFoundError now means it is no longer active. A small helper reads once and retains the zombie (Z) check.

• Base commit: 3c01bdddc66896f8e36f9f21d452710808a9557e.
• Lane: lane:foundation.
• Owned path: tests/test_local_pages_fleet_regressions.py only.
• One commit: 7720287b09e498874756fadf3ee2e61021b44073.

SOTA sources

• Python 3.13 exception documentation: ProcessLookupError, corresponding to errno.ESRCH; FileNotFoundError, corresponding to errno.ENOENT. The installed Python is 3.13.16; CPython v3.13.16, Objects/exceptions.c contains both errno-to-exception registrations.
• Linux proc(5) and proc_pid_stat(5): /proc/<pid>/stat, field (3) is process state and Z is zombie.
• Linux kernel proc documentation, process-specific subdirectories: operations on open proc descriptors for dead processes usually fail with ESRCH.

Evidence-class table

Local commands run

$ python3 -m unittest -v tests.test_local_pages_fleet_regressions.ProcessStatRegressionTests
Before fix: exit 1; FAILED (errors=2).
After fix: exit 0; Ran 2 tests; OK.

$ python3 -m unittest -v tests.test_local_pages_fleet_regressions tests.test_local_pages_fleet_data
exit 0; Ran 53 tests in 0.659s; OK; no skips.

$ python3 scripts/validate.py
FULL, exit 0; 70 components, 11283 hashed files, 4 profiles, 239 receipts.

$ git diff --check
exit 0.

The fail-before run mechanically extracted main's exact status.exists() and status.read_text().split()[2] != 'Z' expression without changing behavior, then mocked read_text raising each disappearance error. Both subtests failed on that expression before exception handling was added. The actual cleanup assertion now uses the same helper exercised by those regressions.

The commit and native-shell push use repository hooks; no --no-verify is used. Tests ran normally, without a RAM gate, resource cap or nice wrapper. Hosted CI and the CC's Claude read remain separate acceptance gates.

Decision record

This bounded test fix follows the CC's Fleet /proc race slice. Handling disappearance at the read removes the existence/read race while retaining the existing zombie check. The helper catches only ProcessLookupError and FileNotFoundError; other read failures still surface. No separate architecture decision is introduced.

Host evidence

Not applicable: no host receipt or platform status changes.

Checklist

• No GitHub Actions or workflow permissions changed; existing pins remain.
• No secrets were printed, logged or committed, and no required secret was added.
• No paid hosting, subscription or billing surface was introduced.
• Peer-owned files and worktrees were preserved.
• Exactly one commit; the draft remains gated on the CC's Claude head read.