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test(tier-guard): module-list check counts the xdist probe module while it is on disk #2341
Description
Describe the bug
TestTierTests.test_every_module_resolves_to_exactly_one_tier (tests/test_tier_guard.py) compares _tiers.suite_modules() with a bare TESTS_ROOT.rglob('test_*.py'). suite_modules() excludes the xdist probe module (_tiers.PROBE_MODULE_NAME, tests/protocols/test_zzz_xdist_guard_probe_unit.py). The rglob does not, so the test fails whenever XdistSubprocessTests has the probe on disk at the same moment.
Evidence
On main at bfe184bd6, the Python 3.15 on Windows (experimental) leg of run 40413168147 failed with First differing element 530: 'tests/protocols/transport/test_mptcp_roundtrip_unit.py' / 'tests/protocols/test_zzz_xdist_guard_probe_unit.py'. The experimental: main push runs CI report posted it as a warning.
Expected behavior
The on-disk scan skips what suite_modules() skips: the probe module and __pycache__, through _tiers.is_probe_module. A test creates the probe file and checks that the guard still passes.
Activity
test(tier-guard): skip the xdist probe module in the on-disk module s…
JarryShaw commentedon Oct 26, 2026
Fixed on
mainin 15c2e57 (a test-only push). The module-list check scans the disk throughon_disk_test_modules(), which skips the probe exactly assuite_modules()does. A new test writes the probe and checks that the two lists agree; it fails without the fix.tests/test_tier_guard.pyandtests/test_tier_guard_xdist.pygive 146 passed.test: keep the tier-guard scan clear of the xdist probe module