These 7 test sites assert rotation behavior (fork, child sessions, lock
contention, logging session-context follows id rotation, boundary hooks fire
on rotation). Pin each builder to in_place=False explicitly so they keep
exercising the retained rotation fallback regardless of the global default
(flipped to True in #38763). Rotation stays a working opt-out fallback and
deserves continued coverage — these are NOT deleted.
Pinned sites:
- test_compression_concurrent_fork._build_agent_with_db
- test_compression_logging_session_context._build_agent_with_db
- test_compression_rotation_state._build_agent_with_db
- test_compression_boundary_hook._make_agent (2 helpers: CompressionBoundaryHook + SessionCompressEvent)
- test_compression_concurrent_sessions._build_agent_with_db
test_concurrent_compressions_same_session_serialize relied on a
time.sleep(0.25) inside the stubbed compressor to make the two threads
overlap inside the per-session lock window. Under CI CPU starvation that
sleep is insufficient: one thread can acquire -> compress -> rotate ->
RELEASE the lock before the other reaches try_acquire, so both acquire on
the shared session_id and both compress (the recurring 'Expected exactly
one agent to compress, got 2' failure on shard test (1)).
Replace the timing dependency with a threading.Barrier(2) wrapped around
the shared db's try_acquire_compression_lock: both threads rendezvous
immediately before the real (atomic) acquire, guaranteeing genuine
simultaneous contention regardless of scheduling. The real lock logic is
unchanged and still picks exactly one winner — this only fixes the test's
overlap guarantee. Restored after join so the post-join lock-leak
assertion hits the unwrapped method.
Verified: 20/20 plain + 15/15 under all-core CPU stress (load avg ~4.6),
where the old version flaked.
