`Curl_poll` may not ignore EINTR if 0 file descriptors are passed

[pitrou]

I encountered this potential issue when trying to investigate a user report.

In the general case, Curl_poll will translate EINTR into a no-error return so that the caller may retry:

curl/lib/select.c

Lines 312 to 318 in a9f8fe2

	r = poll(ufds, nfds, pending_ms);
	if(r <= 0) {
	if((r == -1) && (SOCKERRNO == EINTR))
	/* make EINTR from select or poll not a "lethal" error */
	r = 0;
	return r;
	}

However, if no valid fds are passed, Curl_poll delegates to Curl_wait_ms which doesn't try to eliminate EINTR errors:

curl/lib/select.c

Lines 94 to 104 in a9f8fe2

	r = poll(NULL, 0, (int)timeout_ms);
	#else
	{
	struct timeval pending_tv;
	r = select(0, NULL, NULL, NULL, curlx_mstotv(&pending_tv, timeout_ms));
	}
	#endif /* HAVE_POLL_FINE */
	#endif /* USE_WINSOCK */
	if(r)
	r = -1;
	return r;

Curl_poll is used from Curl_multi_wait where it's not obvious whether a non-zero number of file descriptors is always found.

This might be a red herring, but I suppose it wouldn't hurt fixing Curl_wait_ms (and/or Curl_poll) in any case.

[bagder]

@pitrou are you up to making a PR for this?

[pitrou]

I can try to do that.