rest: pin cacheControlWriter.FlushError errors.Is discriminator (#172)#182
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Mutation-surfaced gap (wave-1 review of #164/PR #169, sibling of the #163 rollback pin): broadening the rollback discriminator from errors.Is(err, http.ErrNotSupported) to err != nil passed the entire suite. The dangerous direction is a stamp rollback after a flush whose commit really happened — a genuine I/O error returns only after net/http has snapshotted the headers onto the wire, so the stamp and the latch must KEEP. Mirrors TestSentry_PanicAfterGenuineFlushErrorKeepsRepanicSemantics: reuse flushErrorWriter (sentry_test.go, same package — reciprocal to sentry_test.go's use of noFlushWriter) as the delegate, drive a plain and a wrapped non-refusal flush error through the chain, assert the stamp keeps on the live map and a trailing 404 cannot reopen the latched commit decision. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Comment-only. The file-header doctrine said stamp-presence reads rr.Result().Header; the #172 battery necessarily violates it — flushErrorWriter intercepts the flush before any WriteHeader reaches the recorder, so no snapshot exists and rr.Result() memoization would blind the post-404 cannot-reopen check. Document the exception at both the doctrine and the assertion. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Closes #172.
Test-only pin closing a surviving mutant in
cacheControlWriter.FlushError: the stamp-rollback discriminatorerrors.Is(err, http.ErrNotSupported)had no test witnessing the genuine-error direction — broadening it toerr != nilpassed the whole suite. Two new subtests assert a genuine (plain and wrapped) flush error KEEPS the Cache-Control stamp and the commit latch holds against a later WriteHeader. Mirrors the #164 sentry pin, reuses itsflushErrorWriterfake.Second commit annotates the live-map observation exception (a delegate-intercepted flush reaches no recorder WriteHeader, so the live map is the only valid pre-404 read —
rr.Result()would memoize and blind the latch check).Reviewed (code/test/comment agents) + mutation-verified: the
err != nilbroadening dies at both assertion sites. Gate green (build/vet/test/-race).Follow-up #178 filed for two adjacent unwitnessed mutants in the same rollback block.
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