poll/select reports a listening socket as writable

[selfish]

Listening on a socket, then passing it to poll() with POLLOUT reports it as writable, despite actually doing so being an error (and unlike other platforms). See attached source file for an example.

[selfish]

The problem seems to be that ​socket_request_notification sends a notification when socket_send_avail() < B_OK, but ​TCPEndpoint::SendAvailable (which eventually gets called for socket_send_avail for TCP sockets) returns EPIPE (which is < B_OK) for cases in which the socket is not writable. The impression I get is that it meant to check for > B_OK since the same TCP method returns the (positive) free buffer size for a writable socket, for example. Flipping the check does indeed solve the issue, if this seems reasonable I'd like to submit a patch myself.

[waddlesplash]

I think the B_SELECT_ERROR check should probably account for read and write errors too? Or maybe we need to do something different for LISTEN sockets? Either way I don't think flipping the check is correct, but perhaps PulkoMandy or korli know more here.

[korli]

Had a look at FreeBSD, and they simply don't notify anything for POLLOUT on listening sockets. ​https://review.haiku-os.org/c/haiku/+/5247

[waddlesplash]

Change merged in hrev56062.