Make UDP packet reception work right with non-blocking sockets and enable.

We need this to permit a UDP receiving iperf3 server to listen on its
control channel.

The fix for non-blocking sockets basically makes sure that if we do a
read on a non-blocking sockets, but there's no data, the UDP processing
code does *not* try to do the normal protocol packet processing on the
non-existent packet.

This is part of an ongoing fix for issue #212 but also should have been
a part of the fix for issue #125.

src/iperf_server_api.c

@@ -534,8 +534,14 @@ iperf_run_server(struct iperf_test *test)
 			    FD_SET(s, &test->read_set);
 			if (s > test->max_fd) test->max_fd = s;
 
-			// If the protocol isn't UDP, set nonblocking sockets
-			if (test->protocol->id != Pudp) {
+			/* 
+			 * If the protocol isn't UDP, or even if it is but
+			 * we're the receiver, set nonblocking sockets.
+			 * We need this to allow a server receiver to
+			 * maintain interactivity with the control channel.
+			 */
+			if (test->protocol->id != Pudp ||
+			    !test->sender) {
 			    setnonblocking(s, 1);
 			}
 

src/iperf_udp.c

@@ -63,7 +63,12 @@ iperf_udp_recv(struct iperf_stream *sp)
 
     r = Nread(sp->socket, sp->buffer, size, Pudp);
 
-    if (r < 0)
+    /*
+     * If we got an error in the read, or if we didn't read anything
+     * because the underlying read(2) got a EAGAIN, then skip packet
+     * processing.
+     */
+    if (r <= 0)
         return r;
 
     sp->result->bytes_received += r;