Prior to this change _cached_session_lags_disk() called
Session.load_metadata_only(sid) on every LRU cache hit, parsing the
full sidecar JSON (~15-20ms even for a 1.3MB sidecar on Celeron+ eMMC).
For draft auto-saves hitting get_session() on every keystroke debounce
(~400ms cadence while typing), that 15-20ms multiplied out to ~75% of
the request's wall time on the Chromebook.
Reorder the check: read only the JSON metadata prefix
(_persisted_message_count, already defined for the LRU-eviction path)
to compare message counts. For INACTIVE sessions
(no active_stream_id, no pending_user_message) with matching counts,
the cache is provably at parity with disk — anchor-scene records cannot
have advanced without the message count advancing too, and the full
Session.load_metadata_only() is skipped. For ACTIVE sessions, or when
counts differ, fall through to the original anchor-scene comparison
path so correctness is preserved.
Measured cost reduction on the request hot path: 15-20ms -> <1ms on
the typical cache-hit case (inactive session, count matches). The
slow path is unchanged for cases that genuinely need the full reload.
No public API change. No behavior change observable on user-facing
flows. The LRU eviction safety invariant (#4765) is preserved —
_session_is_evictable() still calls _persisted_message_count under
the same conditions, and Tier 1a adds NO new eviction gate.
Konstantin M
ec31891c60