The review-identified race: saves are async (owner snapshots content,
worker pool writes), nothing serialized per file, and every write of a
file used the SAME deterministic temp ('.<name>.tmp'). Two overlapping
writes (autosave x autosave, retry x autosave, or the synchronous
FlushAll on GoToBrowser/Shutdown x a worker write) interleaved on the
shared temp and could rename a byte-mixture into place; even without
interleaving, last-rename-wins could promote a STALE snapshot.
Owner-side protocol (logic.go, requestSave + result handler):
- at most one write in flight per file (writeInFlight maps filename ->
the file version whose content the in-flight write carries);
- a save requested while one is in flight is deferred (savePending) and
re-issued by the write's result handler with a FRESH snapshot, so
'last rename wins' coincides with 'newest snapshot wins';
- on success the SNAPSHOT version (not the current one) is recorded as
written, so an edit that arrived during the write leaves the file
dirty and triggers the re-issue;
- FlushAll (GoToBrowser, Shutdown) defers via the same protocol instead
of writing concurrently on the shared temp;
- shutdown drain: on done the owner waits (bounded 5 s) for in-flight
writes and armed retries to settle before exiting, so the post-exit
synchronous FlushAll and workerPool.Stop cannot race a straggling
worker write;
- retry timer now sends a non-blocking token (no timer-goroutine stall
on a full channel); emitFrame no longer blocks on a slow/gone main
(frames are snapshots; the next emission wins) - also required so the
drain can never deadlock on frame delivery.
Mechanism (real/filesystem.go):
- WriteFileAtomic uses a unique per-call temp ('.<name>.tmp.<pid>.<seq>'),
making same-file staging-file interleaving structurally impossible even
if the serialization regressed (defense in depth);
- each successful write best-effort removes stale temps of the same file
(crash leftovers, plus the legacy deterministic name for upgraded
installs); a failed write removes its own temp.
Tests:
- write_serialization_test.go (e2e): a counting FS wrapper proves the
peak concurrent same-file saves is 1 across two deliberately
overlapping autosaves (2 s saves; the second edit lands inside the
first save's window and its token is deferred, then re-issued with the
newer content), and that a Flush during an in-flight save adds no
concurrent writer and the newest snapshot still wins. Mutation-verified:
disabling the deferral fails it with peak = 2. (The pool's WriteFileTask
calls FS.WriteFile, not WriteFileAtomic - the real FS is atomic only
because WriteFile delegates to WriteFileAtomic; the wrapper mirrors that
delegation or the overlap window does not exist.)
- filesystem_test.go: stale-temp test updated to the new pattern, also
covering the legacy name and asserting a different file's temp is
untouched.
- real_file_fuzz_test.go: stray-temp check matches both patterns.
Docs: architecture.md 6.5 rewritten (protocol invariants), spec.md
autosave line, development_plan.md v11 + Phase 12.
On-device smoke: open, type, autosave lands exact content on disk, no
temp files left, clean relaunch. Full suite green under -race.
Residuals (documented): no fsync before rename (power-loss window only);
external-change detection absent; a drain-deadline exit with a straggling
write can only lose freshness (unique temps keep every rename a complete
snapshot).
Greg Pomerantz
6c6a0c1a27