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When two `rwx sandbox exec` commands run concurrently and both trigger auto-create, the storage lock was released before calling StartSandbox, allowing the second process to also find no session and create a duplicate. The second write would overwrite the first session key, orphaning the first sandbox (running remotely but invisible locally). Fix by passing the already-held lock from ExecSandbox/ResetSandbox into StartSandbox, so the "check if session exists → create → persist" sequence is atomic. StartSandbox releases the lock after persisting the initial session.
tonywok
approved these changes
Mar 11, 2026
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Slack thread
I attempted to get an integration test wrapped around this race condition, but I could not reliably get it to fail without the code change. The unit tests have been red/green verified though, which is the best we can do with this particular race.
Summary
rwx sandbox execcommands run concurrently (e.g. Claude Code running rubocop and rspec in parallel), both find no existing session and each creates a new sandbox. The secondSetSessioncall overwrites the first because they share the same storage key (cwd:branch:configFile), orphaning the first sandbox.ExecSandbox/ResetSandboxintoStartSandbox, making the "check → create → persist" sequence atomic. The second process blocks until the first has persisted, then finds the existing session and reuses it.ResetSandboxwhere the lock was released beforeStartSandbox.Test plan
TestService_ExecSandbox_ConcurrentAutoCreate— two goroutines callExecSandboxconcurrently; asserts only oneInitiateRuncall occurs and both return the same RunIDTestService_StartSandbox_StorageLock— verifiesStartSandboxaccepts a pre-held lock, persists the session, and releases the lock (including on error)expected: 1, actual: 2without the fix