[mi_datagram] turn the RX sockets to non-blocking

As we have multiple procs reading from the same datagram sockets, we may end up with a mixing between the procs woken up by OS and the procs doing the reading. So some procs (even if were woken up) may have nothing to read.
To be resilient, better do non-blocking reading and igonre the EAGAIN or EWOULDBLOCK.

(cherry picked from commit 12e705e)

Author: bogdan-iancu

modules/mi_datagram/datagram_fnc.c

@@ -113,6 +113,17 @@ int  mi_init_datagram_server(sockaddr_dtgram *addr, unsigned int socket_domain,
 		return -1;
 	}
 
+	/* Turn non-blocking mode on for rx*/
+	flags = fcntl(socks->rx_sock, F_GETFL);
+	if (flags == -1) {
+		LM_ERR("fcntl failed on RX socket: %s\n", strerror(errno));
+		goto err_both;
+	}
+	if (fcntl(socks->rx_sock, F_SETFL, flags | O_NONBLOCK) == -1) {
+		LM_ERR("fcntl: set non-blocking failed for RX socket: %s\n",strerror(errno));
+		goto err_both;
+	}
+
 	switch(socket_domain) {
 		case AF_LOCAL:
 			LM_DBG("we have a unix socket: %s\n", addr->unix_addr.sun_path);
@@ -451,15 +462,14 @@ int mi_datagram_callback(int rx_sock, void *_tx_sock, int was_timeout)
 				(struct sockaddr*)&reply_addr, &reply_addr_len);
 
 	if (ret < 0) {
-		LM_ERR("recvfrom %d: (%d) %s\n", ret, errno, strerror(errno));
 		if ((errno == EINTR) ||
 			(errno == EAGAIN) ||
 			(errno == EWOULDBLOCK) ||
 			(errno == ECONNREFUSED)) {
 			LM_DBG("got %d (%s), going on\n", errno, strerror(errno));
 			return 0;
 		}
-		LM_DBG("error in recvfrom\n");
+		LM_ERR("recvfrom %d: (%d) %s\n", ret, errno, strerror(errno));
 		return -1;
 	}