On Windows, fs.promises.writeFile truncates the target file to 0 bytes
before writing. Since loadSessionStore reads the file synchronously
without holding the write lock, a concurrent read can observe the empty
file, fail to parse it, and fall through to an empty store — causing the
agent to lose its session context.
Changes:
- saveSessionStoreUnlocked (Windows path): write to a temp file first,
then rename it onto the target. If rename fails due to file locking,
retry 3 times with backoff, then fall back to copyFile (which
overwrites in-place without truncating to 0 bytes).
- loadSessionStore: on Windows, retry up to 3 times with 50ms
synchronous backoff (via Atomics.wait) when the file is empty or
unparseable, giving the writer time to finish. SharedArrayBuffer is
allocated once and reused across retry attempts.
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