net: stop monitoring select exceptions outside Windows #36716

pull Sjors wants to merge 2 commits into bitcoin:master from Sjors:2026/10/windows-select-exceptions changing 3 files +23 −0

Sjors commented at 4:13 AM on October 31, 2026: member

Alternative approach to #36688, citing the relevant parts of its description:

There is currently nothing preventing us to receive TCP urgent data, which would never be returned by recv(). Linux's poll() is indifferent to this, but the select() socket monitoring may keep reporting it as pending on some platforms.

This would not prevent progress nor lead to a crash anymore after #36665, but it's still undesirable.

Fix this by ignoring the exceptionSet. Except on Windows, where we instead disconnect if its used.

We could instead have set SO_OOBINLINE on our sockets, so recv() returns any urgent data sent to us (and likely leads us to disconnect the peer). But this goes against the spirit of RFC 6093, as documented in the second commit.

net: stop monitoring select exceptions outside Windows

Windows needs select's exception set to detect failed non-blocking
connects. On other platforms, a TCP urgent pointer can leave the
exception set pending even when no data is readable. Only monitor it
on Windows, and disconnect established P2P peers if it is set.

Co-authored-by: Antoine Poinsot <mail@antoinep.com>

1fde57f1de

doc: explain why P2P does not set SO_OOBINLINE 8754e5c1c4
DrahtBot added the label P2P on Oct 31, 2026
DrahtBot commented at 4:13 AM on October 31, 2026: contributor


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External sites
- Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36716
- Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36716.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36716#author-ralph


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pinheadmz commented at 11:38 AM on October 31, 2026: member

Mine as well apply the same logic to HTTP, probably right here:

https://github.com/bitcoin/bitcoin/blob/5669fda5ed85ac739c1678c03bd7c25193d099c6/src/httpserver.cpp#L916-L918

I wrote functional test for the first PR, could be adapted to ensuring HTTP disconnects on urgent data:

https://gist.github.com/pinheadmz/f5bb77865f870865e44c54d5751b569a

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#36665 sock: align exception monitoring between poll() and select() backends #36688 net: receive TCP urgent data inline

net: stop monitoring select exceptions outside Windows #36716

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