the idle sweep from #71463 left three growth paths open. _peers_cache had no
bound at all, _session_observation (from #98941) grew one entry per session
id and kept orphans when an init failed after add_peers, and a burst of
distinct sessions inside one ttl window was not bounded. the sweep also
evicted sessions whose messages had not reached honcho yet, which in
"session" write mode drops the only copy, and _flush_session re-inserted an
evicted session into the cache with no observation flags, so recall for it
routed from the config snapshot instead of its server config.
_cache and _sessions_cache now cap at 128 entries and _peers_cache at 512,
evicting least recently used first (dict order, refreshed on every hit).
a session with unsynced messages is never evicted by the sweep or the cap.
_configure_session_peers returns the synced flags and get_or_create stores
them under _cache_lock next to the cache entry, so the observation dict can
hold no id the cache does not; eviction drops both. recall reads go through
_cached_session, which stamps updated_at, so a read-only session survives
the idle sweep. follows #71461, #71463, #98936.
Erosika
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