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Let a second concurrent first save replace the first one's state file [patch] - #108
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Two saves that both found no state file raced on File.Move, and the loser threw "already exists". The swap now overwrites on targets that support it, and on netstandard falls back to File.Replace when the move loses the race. Fixes #106 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_017vfJW21vjGVn4pw5oZhxSF
The SDK's build rewrote it, and the change is not part of this fix. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_017vfJW21vjGVn4pw5oZhxSF
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Fixes #106
What was wrong
JsonFilePersistenceProvider.SaveStateAsyncswapped its temp file in withFile.Move(temp, path)whenever the state file didn't exist yet. Two saves that both saw no file raced. The one that moved second threwIOException: ... already exists, its temp file was deleted, and the caller's save failed. That race only exists before the first save, so it hits an app's very first run.Change
The swap moved into
SwapIntoPlace:File.Replace, as before.NETCOREAPP3_0_OR_GREATER):File.Move(temp, path, overwrite: true), which is still an atomic rename.File.Move. If that throwsIOExceptionand the file now exists, it falls back toFile.Replace.Either way the last writer wins, the same as when the file already exists. The atomicity from #105 doesn't change.
I left out the optional per-path
SemaphoreSlimordering. It changes behaviour beyond this bug and can be its own change.Tests
ConcurrentFirstSaves_ToNewFile_BothSucceed. It runs 200 iterations of two concurrentSaveStateAsynccalls against a path that doesn't exist yet. It checks that neither throws, the file loads back to one of the two states, and no temp file is left behind.IOException(JsonFilePersistenceProvider.cs:83).Generated by Claude Code