Re-derivation of #23254 (@devsart95) on today's flush loop. The turn
flush in _flush_messages_to_session_db wrote one BEGIN IMMEDIATE
transaction per message row; a typical agent turn (user + assistant +
tool results) paid 3-8 transactions -- and, off WAL (the default on
macOS while the WAL-reset guard is active), 3-8 fsyncs -- per turn.
Adds SessionDB.append_messages_batch: same row shape as append_message
(shared _prepare_message_row serializer + _MESSAGE_INSERT_SQL column
list, so the two writers cannot drift), same compression-lock and
compression-closed guards, one aggregated session-counter UPDATE, one
transaction for the whole batch. Row serialization stays outside the
write lock.
The flush loop now collects the turn's new rows and writes them in one
call. All-or-nothing pairs exactly with the persisted-marker stamping:
on failure no rows landed and no markers were stamped, so the next
flush re-writes the whole tail (same recovery contract as before,
minus the partial-prefix case that could double-count).
Measured (same harness, 5-message turn, journal_mode=DELETE,
synchronous=FULL): 2.32ms -> 0.83ms median per turn flush (64% faster,
5 fsyncs -> 1). On WAL the win is smaller but the atomicity fix holds.
devsart95
06ae5b6faa