append_run_event() without an explicit seq called _next_seq(path), which
re-read and re-parsed the entire journal file on every append — O(n) per
append, O(n^2) over a run. Direct callers therefore paid a full file
reparse per event; only RunJournalWriter avoided it via its own in-memory
counter.
Introduce a module-level _SEQ_CACHE keyed by run-journal file path. The
per-path _lock_for(path) serializes same-path reserve->append so seqs
stay monotonic and file order matches; both RunJournalWriter and the free
append_run_event now draw from this one cache (_reserve_next_seq), so
seqs stay gapless even when both write the same path — the writer's
private counter is removed to keep a single source of truth. An explicit
caller-supplied seq advances the cache (_note_assigned_seq) so it can
never be re-issued. delete_run_journal() now also evicts cache entries
for the removed session so a run re-created at the same path restarts
numbering at 1.
Guard every structural _SEQ_CACHE access (reserve/note/evict) with a
dedicated _SEQ_CACHE_LOCK. The eviction sweep in delete_run_journal
iterates the whole dict; the per-path locks the append path holds do NOT
serialize against it, so a concurrent append on another session inserted
a fresh key mid-iteration and raised "dictionary changed size during
iteration" (the caller swallows it, but the eviction then aborts and
leaks stale seqs, so a re-created run resumed a stale number instead of
restarting at 1). A stress test races deletes against cross-session
appends and asserts no such error.
Rollback: revert this commit; _next_seq(path) re-read behaviour returns.
No on-disk format change, so existing journals stay compatible.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
