node_has_undrained_write_ack scanned ack_write_queue on every
reservation, including the per-request-per-chunk prepare path whose
node ids are freshly minted and can never be queued. Track the max
node id ever appended to the queue: any queued id is <= max by
construction (no monotonicity assumption needed for correctness), so
fresh ids exit on one integer compare and the scan remains only for
re-reservations of old ids (the rare write_backup fallback).
All three ack_write_queue append sites now go through a single
_append_write_ack funnel that maintains the max — the zero-owned-rank
and non-CP write acks previously would have bypassed it, allowing
false negatives on ranks owning no pages of a node.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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