At the 2,000-record threshold the UI thread serialised and fsynced the whole catalog (1-2 s
at 85k photos after streaming). Now `persist()` starts `Journal::snapshot_in_background`:
- the worker writes `catalog.snap` for op N from a clone of the catalog (photos are Arc-shared,
so the clone is cheap) through `Store::background_writer` (a second FsStore handle; MemStore
opt-in; none on wasm/web, which stays synchronous);
- appends go on as before (fsynced before every command returns) and are also kept in memory
as the "tail" (records > N);
- `Journal::poll` (from `persist` and the per-frame `persist_if_dirty`) sees the worker done and
atomically replaces the log with exactly the tail.
Crash points: before the snapshot rename -> old snapshot + whole log; between the rename and the
log rewrite -> new snapshot + whole log (records <= N skipped as stale, existing rule); after ->
new snapshot + tail. A failed snapshot keeps the log whole and retries after another
max_records records (no retry storm). At most one snapshot in flight; a synchronous snapshot
(close, Optimize Library), reopening a library and dropping the Journal first wait for it, so an
older snapshot can never replace a newer one. On-disk format unchanged.
Tests (crates/catalog/src/tests_background.rs, a gated store holds the worker mid-flight and
"crashes" by copying the files): edits during a background snapshot survive crashes at each
point and the trimmed log holds exactly seq N+1.. in order; failure loses nothing and backs
off; sync snapshot waits (no regression); drop waits; real FsStore with a leftover partial
temp file; a proptest over interleavings of edits / start / land / finish / sync snapshot /
crash. Mutations checked: trimming the log to empty, or not waiting in `snapshot`, fail them.
Engine: `threshold_compaction_runs_in_the_background` (crash copy mid-run, reopen without close).
`library.info` -> persistence gains snapshotRunning, failedSnapshots, maxBlockingMs and
lastSnapshot.{background, blockingMs}.
persist_bench, same session (load avg ~160), UI-thread time per threshold compaction:
85k photos: synchronous 0.85-1.7 s (median 1.25 / 1.34 s) -> background: blocking median
45 / 74 ms (clone 1.3-154 ms + log rewrite 9-23 ms); worker 1.2-2.4 s; 230-380
single-op appends during it at median 4-6 ms (same as without)
10k photos: synchronous 72-233 ms -> blocking median 12.5 / 14 ms
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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