fix(quinn-udp): drain full batch on partial sendmsg_x (Apple)#2
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`sendmsg_x` behaves like `sendmmsg`: it delivers as many messages as the interface queue accepts and returns that count, which can be fewer than requested when the queue fills mid-batch. The Apple fast-datapath send discarded the count and always returned `Ok(())`, silently dropping the un-sent tail of a partially-sent batch. Under load this shows up as upstream UDP loss (and collapsed TCP throughput) with no NIC-level drops. Loop until the whole batch is drained, surfacing the error only when no messages were sent so the caller can still back-pressure on `ENOBUFS`. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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sendmsg_xbehaves likesendmmsg: it delivers as many messages as the interface queue accepts and returns that count, which can be fewer than requested when the queue fills mid-batch. The Apple fast-datapath send discarded the count and always returnedOk(()), silently dropping the un-sent tail of a partially-sent batch. Under load this shows up as upstream UDP loss (and collapsed TCP throughput) with no NIC-level drops.Loop until the whole batch is drained, surfacing the error only when no messages were sent so the caller can still back-pressure on
ENOBUFS.