Fail loudly when two migrations share a version number#186
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constkolesnyak wants to merge 1 commit into
Open
Fail loudly when two migrations share a version number#186constkolesnyak wants to merge 1 commit into
constkolesnyak wants to merge 1 commit into
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run_migrations skips anything at or below the recorded schema version, so if two files claim the same number one silently wins and the other never runs — on every database, forever, with no error. It surfaces much later as a missing column or index, and which of the two is missing depends on filesystem iteration order, so it reproduces on some machines and not others. discover_migrations now raises on a collision, naming both files. Failing at import is the lesser evil: the collision is a packaging mistake caught in seconds once it is named, whereas a silently skipped migration costs hours and can corrupt data in the meantime. Adds tests/test_migrations_runner.py (there were none), including a regression check that the real on-disk migration set has no duplicates — so a collision introduced later fails in CI rather than at a user startup. 113 tests pass.
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run_migrationsskips anything at or below the recorded schema version, so if two files claim the same number, one silently wins and the other never runs — on every database, forever, with no error.It surfaces much later as a missing column or index, and which of the two is missing depends on filesystem iteration order, so it reproduces on some machines and not others.
We hit this three times downstream while syncing this repo. The worst instance crash-looped the service 279 times before anyone noticed what the traceback actually meant.
Fix
discover_migrations()now raises on a collision, naming both files.Failing at import is the lesser evil here: a duplicate version is a packaging mistake that takes seconds to fix once it is named, whereas a silently skipped migration costs hours and can leave data half-migrated in the meantime.
Tests
tests/test_migrations_runner.pyis new — there were no runner tests. Beyond the guard itself it includes a regression check that the real on-disk migration set has no duplicates, so a collision introduced later fails in CI rather than at a user's startup.113 tests pass.