The find-or-create dance — SELECT, then a SAVEPOINT INSERT that recovers (not a
full rollback) on IntegrityError when a concurrent worker inserted first — was
hand-rolled identically in 4 async sites: ArtistService.find_or_create,
TagService.find_or_create, ExtensionService._find_or_create_artist and
._find_or_create_source. Divergent copies of exactly this pattern are how the
duplicate-row/race bugs in reference_scalar_one_or_none_duplicates crept in, so
it now lives once in services/db_helpers.get_or_create (returns (row, created);
factory adds+flushes+returns the row; caller owns the outer commit).
Over-DRY guard: SourceService's IntegrityError sites RAISE DuplicateSourceError
(reject-on-conflict, a different concept) — left alone. Importer._get_or_create
is the lone SYNC consumer (already shared by 2 callers) — stays separate, can't
cross the sync/async boundary. §8b: no hand-rolled async find-or-create remains.
Test: get_or_create creates then returns existing without re-invoking the factory.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
