Mutation gate: RemoveCheckAsync block-removal mutant is reported surviving in about half of the sweeps #312

What happens

Stryker reports the block-removal mutant of EfBurstRepository.RemoveCheckAsync's body (Anichron.Core/Data/Repository/BurstRepository.cs, { → {}) as Survived in roughly half of the sweeps. In the other runs it reports it Killed by BurstRepositoryTests.RemoveCheckAsync_CheckedPhoto_DeletesItsRowAtTheNextSaveAsync. With the 100% gate, every sweep that loses this kill fails Mutation and therefore CI gate, whatever the PR changed.

The kill is real. With the body removed by hand, RemoveCheckAsync_CheckedPhoto_DeletesItsRowAtTheNextSaveAsync fails, because the row is still there. A "Survived" means the covering test passed, so the mutant was not active while it ran.

Evidence (2026-10-24)

Covering tests in every run: RemoveCheckAsync_UncheckedPhoto_ChangesNothingAsync and RemoveCheckAsync_CheckedPhoto_DeletesItsRowAtTheNextSaveAsync.

What this rules out:

• Concurrency is not the trigger. The mutant survived the single-threaded sweep, so this does not look like the concurrency under-reporting in ADR 0001, or not only that.
• feat(worker): Similar photos join bursts through OpenCV feature matching (#298) #308 did not introduce it. Master flips the same way. Master's earlier green sweeps (bdb9e0d, e19c04f, 8547e1d) were lucky draws.
• The test class's shape is not the cause. On feat(worker): Similar photos join bursts through OpenCV feature matching (#298) #308, BurstRepositoryTests gained two tests. Moving those tests into their own class, which left BurstRepositoryTests identical to master's, still showed the mutant surviving in 2 of 3 runs.

It was first measured on 8547e1d (#297), where RemoveCheckAsync was added, so the flip may be as old as the method.

Not yet known

• Why this mutant's activation is lost. Candidates:
  • The mutant is in an async method whose only statement awaits FindAsync. Stryker's MTP runner may read the active mutant on a path an async continuation does not see.
  • The test seeds through one AnichronDbContext and asserts through another, built by AnichronDbContexts.InMemory(name) with the cached service provider. CLAUDE.md names EF's cached model as a source of mutants that flip between runs.
• Whether other mutants flip the same way less often. The divergence job compares counts, and this one moves them by 1.

Workaround in use

#308 reran Mutation until the gate passed, with the maintainer's agreement, because the flip predates it.