Address PR #62871 review: with the default retain_async=True, aretain_batch
returns when the write is accepted, not when it's durable/recall-visible, so
draining the local writer queue (task_done) is not a read-after-write signal.
The next-turn prefetch could still recall before the just-completed turn was
observable on the server.
- Track the async operation_id/operation_ids returned by aretain_batch
- _wait_for_retains_drained now applies two ordered, budget-bounded barriers:
(1) local writer queue drains, then (2) tracked server-side async ops report
completion via operations.get_operation_status (an explicit read-after-write
condition). NotFound (completed+evicted) counts as done; transient errors
keep waiting until the deadline
- Completed ops are removed from the pending set so later prefetches don't
re-poll them; the whole wait stays off the reply path
- Add TestPrefetchServerRetainVisibility: op-id tracking (single/multiple),
no-op tracking when retain_async=False, prefetch waiting for server
completion before recall, timeout fallback on a wedged op, and NotFound /
transient-error status handling
1be353bf9c