streams/xp_socket: eliminate poll() when MSG_DONTWAIT is available

If there is a zero timeout and MSG_DONTWAIT is available (or the
socket is non-blocking), the poll() call is not necessary, and we can
just call recv() right away.

Before this change:

 poll([{fd=4, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, 0) = 0 (Timeout)
 poll([{fd=4, events=POLLIN|POLLERR|POLLHUP}], 1, 60000) = 1 ([{fd=4, revents=POLLIN}])
 recvfrom(4, "HTTP/1.1 301 Moved Permanently\r\n"..., 8192, MSG_DONTWAIT, NULL, NULL) = 348
 poll([{fd=4, events=POLLIN|POLLPRI|POLLERR|POLLHUP}], 1, 0) = 1 ([{fd=4, revents=POLLIN}])
 recvfrom(4, "", 1, MSG_PEEK, NULL, NULL) = 0

After this change:

 recvfrom(4, 0x7ffe0cc719a0, 1, MSG_PEEK|MSG_DONTWAIT, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
 poll([{fd=4, events=POLLIN|POLLERR|POLLHUP}], 1, 60000) = 1 ([{fd=4, revents=POLLIN}])
 recvfrom(4, "HTTP/1.1 301 Moved Permanently\r\n"..., 8192, MSG_DONTWAIT, NULL, NULL) = 348
 recvfrom(4, "", 1, MSG_PEEK|MSG_DONTWAIT, NULL, NULL) = 0

The first poll() is replaced by recvfrom(), and the third poll() is
omitted completely.

Author: MaxKellermann

main/streams/xp_socket.c

@@ -337,15 +337,16 @@ static int php_sockop_set_option(php_stream *stream, int option, int value, void
 
 				if (sock->socket == -1) {
 					alive = 0;
-				} else if (php_pollfd_for(sock->socket, PHP_POLLREADABLE|POLLPRI, &tv) > 0) {
+				} else if ((value == 0 && (MSG_DONTWAIT || !sock->is_blocked)) || php_pollfd_for(sock->socket, PHP_POLLREADABLE|POLLPRI, &tv) > 0) {
+					/* the poll() call was skipped if the socket is non-blocking (or MSG_DONTWAIT is available) and if the timeout is zero */
 #ifdef PHP_WIN32
 					int ret;
 #else
 					ssize_t ret;
 #endif
 					int err;
 
-					ret = recv(sock->socket, &buf, sizeof(buf), MSG_PEEK);
+					ret = recv(sock->socket, &buf, sizeof(buf), MSG_PEEK|MSG_DONTWAIT);
 					err = php_socket_errno();
 					if (0 == ret || /* the counterpart did properly shutdown*/
 						(0 > ret && err != EWOULDBLOCK && err != EAGAIN && err != EMSGSIZE)) { /* there was an unrecoverable error */