xwrite: poll on non-blocking FDs
write(2) can hit the same EAGAIN/EWOULDBLOCK errors as read(2), so busy-looping on a non-blocking FD is a waste of resources. Currently, I do not know of a way for this happen: * the NonBlocking directive in systemd does not apply to stdin, stdout, or stderr. * xinetd provides no way to set the non-blocking flag at all But theoretically, it's possible a careless C10K HTTP server could use pipe2(..., O_NONBLOCK) to setup a pipe for git-http-backend with only the intent to use non-blocking reads; but accidentally leave non-blocking set on the write end passed as stdout to git-upload-pack. Followup-to: 1079c4be ("xread: poll on non blocking fds") Signed-off-by: NEric Wong <e@80x24.org> Reviewed-by: NJeff King <peff@peff.net> Signed-off-by: NJunio C Hamano <gitster@pobox.com>
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