bpo-30644: Mention how to disable signal fd wakeup

[pitrou]

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[mention-bot]

@pitrou, thanks for your PR! By analyzing the history of the files in this pull request, we identified @birkenfeld, @loewis and @taleinat to be potential reviewers.

[pitrou]

@njsmith

[njsmith]

Yeah, good idea and lgtm

If you want to get really fancy, I guess you could also mention that on unixes it's an arbitrary FD, but on Windows it should be a socket handle (not an FD at all).

[pitrou]

If I'm reading signalmodule.c correctly (especially the functions signal_set_wakeup_fd and trip_signal), we support both regular file descriptors and sockets on Windows (but not native Windows handles).

[njsmith]

we support both regular file descriptors and sockets on Windows (but not native Windows handles).

Ah, looks like you're right. In practice though I don't think there are any uses for passing in a regular file descriptor, because only sockets can be set in non-blocking mode, and python sockets are always represented as socket handles and not wrapped into an fd.