Review folds on the #71775 salvage (dossier findings 1+2):
- self._lock becomes an RLock and the mutation primitives
(_replace_entry, _persist) are now self-locking, so the deferred
single-use-token refresh path — which deliberately runs its
cross-process flock + OAuth network I/O OUTSIDE the pool lock —
still serializes its pool mutations against concurrent
select()/rotation. In-lock callers re-acquire reentrantly.
- Dropped _refresh_pending_entries' redundant second _replace_entry:
_refresh_entry already merges the refreshed entry internally.
Adds tests/agent/test_credential_pool_deferred_refresh.py pinning both
invariants: select() must NOT hold the lock during the refresh window
(the PR's whole point), and the post-refresh mutations MUST contend on
the lock (blocking-thread probe).
kshitijk4poor