SessionStore._entries was read and mutated without synchronisation,
causing race conditions when multiple platforms (Telegram + Discord)
received messages concurrently on the same gateway process. Two threads
could simultaneously pass the session_key check and create duplicate
sessions for the same user, splitting conversation history.
- Added threading.Lock to protect all _entries / _loaded mutations
- Split _ensure_loaded() into public wrapper + internal _ensure_loaded_locked()
- SQLite I/O is performed outside the lock to avoid blocking during
slow disk operations
- _save() stays inside the lock since it reads _entries for serialization
Cherry-picked from PR #3012 by Kewe63. Removed unrelated changes
(delivery.py case-sensitivity, hermes_state.py schema tracking) and
stripped the UTC timezone switch to keep the change focused on threading.
Co-authored-by: Kewe63 <Kewe63@users.noreply.github.com>
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