Root cause of the silent-crash-after-hours cluster (#4765/#2233/#4633): the
global in-memory SESSIONS LRU evicted with a blind popitem(last=False), which
could drop an actively streaming or not-yet-persisted session (data loss) and
was capped only via an env var. On long-running installs the effective result
was unbounded RAM growth until segfault.
- Add _session_is_evictable(): a session is evictable ONLY when it is not
streaming (no active_stream_id), has no in-flight turn (no pending_user_message
/ pending_started_at), and its full state is proven on disk (sidecar
message_count >= in-memory count; metadata-only stubs and zero-message shells
are trivially safe).
- Add _evict_sessions_over_cap(): replaces all 8 blind popitem loops across
models.py, routes.py, streaming.py. Walks the LRU oldest-first and removes only
provably-safe entries; never acquires LOCK/stream locks itself (caller holds
LOCK) so no lock-ordering deadlock. May briefly exceed the cap rather than ever
evict an active/unsaved session.
- Make the cap configurable via config.yaml webui.sessions_cache_max
(get_sessions_cache_max()); precedence config.yaml -> HERMES_WEBUI_SESSIONS_MAX
(legacy) -> DEFAULT_SESSIONS_CACHE_MAX=300. No new HERMES_* env var. Invalid or
<1 values fall back so a typo can never disable the bound.
- Evicted sessions lazily reload from their JSON sidecar via the existing
get_session() accessor; no call sites changed. _index.json sidebar behavior
unchanged (the sidebar reads the index, not SESSIONS).
- Add tests/test_issue4765_sessions_lru_eviction.py (8 tests): eviction past
cap, active/streaming never evicted, unsaved/stale-tail never evicted, lazy
reload with identical content, and no-data-loss under heavy churn.
- README: document the config.yaml key + safety semantics.
Fixes#4765.
nesquena-hermes
6caaced8eb