resumeSession's warm-cache fast-path trusted the
storedSessionId -> runtimeId -> ClientSessionState mapping without
checking the cached state still BELONGS to the session being resumed.
A pooled profile backend that gets idle-reaped and respawned
(pruneSecondaryGateways) re-mints runtime ids, so a recycled id can
resolve to a live-but-DIFFERENT session's cache entry. The only
existing guard was a session.usage 404 -- that catches a fully-dead
runtime id, but a recycled id still 200s, so the fast-path happily
painted the wrong transcript under the current route (open chat A,
chat B loads).
Fold the belongs-to check into a single takeWarmCache() helper used at
BOTH cache reads -- the early transcript-keep decision and the fast-path
itself -- so a cross-wired entry can't even briefly flash a stale
transcript before the full resume repaints. On a mismatch the helper
purges both stale map entries and reports a miss, falling through to a
full resume that rebinds a correct runtime id. The full-resume path
already guards its final paint with isCurrentResume(), so only the
cached fast-path was missing the belongs-to check.
Pre-existing bug from the initial desktop app (#20059); not introduced
by the session-switch perf work (#49807), which left these lines
untouched.
Tests: two cases in use-session-actions.test.tsx driven through a
harness that owns the two cache maps -- a cross-wired mapping is
rejected + purged (the bug), and a correctly-wired cache still serves
from memory with no needless refetch (no perf regression).
Supersedes #50464 by @professorpalmer, reimplemented to also guard the
early transcript-keep read (whole-class fix, not just the fast-path).
Co-authored-by: professorpalmer <professorpalmer@users.noreply.github.com>
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