test(cache): fix Windows flake in the torn-file concurrency test - #3921
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Assessment: 🟢 APPROVE
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What this PR does
Fixes a Windows-only CI flake in
TestFileCache_concurrentStoreNeverYieldsTornFile(seen e.g. in this run):The test races a reader loop (
os.ReadFile) againstStore's temp-file+rename publishes to prove readers never observe a torn write. On POSIX that's a clean race: rename is atomic and the destination stays openable. On Windows,MoveFileEx(MOVEFILE_REPLACE_EXISTING)can make the destination transiently unopenable, so the reader occasionally getsERROR_SHARING_VIOLATION— a timing artifact of the writer itself, not a torn write.The reader loop already tolerated
os.ErrNotExist(file not published yet); it now also tolerates the Windows sharing violation, via a build-tagged test helper following the existingisLockUnavailablepattern (pkg/tools/builtin/plan/lock_windows.go). The test's actual assertion — successfully read bytes always parse as valid JSON — is unchanged.