AsyncUdpSocket::poll_recv fairness #2981
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## Description The existing AsyncUdpSocket::poll_recv polled the UDP IPv4 socket first, only if that has no datagrams would it poll UDP IPv6 and only also that had no datagrams to deliver was the relay socket polled. This highly favoured IPv4 relay traffic, potentially starving the relay traffic. This change makes it prefer a different source on each poll. Meaning if multiple sockets have data to deliver a fairer delivery happens. Closes #2981. ## Breaking Changes <!-- Optional, if there are any breaking changes document them, including how to migrate older code. --> ## Notes & open questions - Using Ordering::Relaxed is probably fine. I believe normally this polling would only happen from a single task (the Quinn endpoint driver) and even if it was polled concurrently from different threads it would not be that bad and still an improvement on the current order. - I'm not particularly fond of the implementation, but the macros is the best I could come up with. Maybe there's something cleverer which can be done. ## Change checklist - [x] Self-review. - [x] Documentation updates following the [style guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text), if relevant. - [x] Tests if relevant. - [x] All breaking changes documented. --------- Co-authored-by: Philipp Krüger <[email protected]>
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AsyncUdpSocket::poll_recvmultiplexes receiving datagrams from 3 different sources:Currently they are polled in this specific order: first IPv4, only if that returns
Pendingpoll IPv6 and only if also that returnsPendingdoes it poll the relay channel. If one of the polls returnsReadythe remaining ones are not polled.This unfairly penalises the relay channel.
A possible solution is for the MagicSock to have buffers for datagrams that are ready. Whenever there is something in the buffers return it as
Poll::Readywithout polling any of the sources. If the buffers are empty always poll all the sources and store anyReadyresults that are not immediately returned in a buffer.This is one of the issues found from #2951.
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