The tests/test_radio_dish test implements is_multicast_available() to

verify that multicast is working on the system before running tests
that relies on this, or skipping those test otherwise.

In some circumstances this check itself hangs due to the recvfrom()
call being blocking waiting for data. The test is eventually killed
with a SIGALARM signal.

./config/test-driver: line 112: 1227308 Alarm clock             "$@" >> "$log_file" 2>&1
FAIL: tests/test_radio_dish

FAIL: tests/test_radio_dish
===========================

FAIL tests/test_radio_dish (exit status: 142)

$ tests/test_radio_dish
tests/test_radio_dish.cpp:511:test_leave_unjoined_fails:PASS
tests/test_radio_dish.cpp:512:test_join_too_long_fails:PASS
tests/test_radio_dish.cpp:513:test_long_group:PASS
tests/test_radio_dish.cpp:514:test_join_twice_fails:PASS
tests/test_radio_dish.cpp:515:test_radio_bind_fails_ipv4:PASS
tests/test_radio_dish.cpp:516:test_radio_bind_fails_ipv6:PASS
tests/test_radio_dish.cpp:517:test_dish_connect_fails_ipv4:PASS
tests/test_radio_dish.cpp:518:test_dish_connect_fails_ipv6:PASS
tests/test_radio_dish.cpp:519:test_radio_dish_tcp_poll_ipv4:PASS
tests/test_radio_dish.cpp:520:test_radio_dish_tcp_poll_ipv6:PASS
tests/test_radio_dish.cpp:521:test_radio_dish_udp_ipv4:PASS
tests/test_radio_dish.cpp:522:test_radio_dish_udp_ipv6:PASS
Alarm clock

With this commit, making the recvfrom() call non-blocking. The test
does noy hang, and the non-available multicast is detected and the
tests needing it are skipped:

PASS: tests/test_radio_dish

$ tests/test_radio_dish
tests/test_radio_dish.cpp:510:test_leave_unjoined_fails:PASS
tests/test_radio_dish.cpp:511:test_join_too_long_fails:PASS
tests/test_radio_dish.cpp:512:test_long_group:PASS
tests/test_radio_dish.cpp:513:test_join_twice_fails:PASS
tests/test_radio_dish.cpp:514:test_radio_bind_fails_ipv4:PASS
tests/test_radio_dish.cpp:515:test_radio_bind_fails_ipv6:PASS
tests/test_radio_dish.cpp:516:test_dish_connect_fails_ipv4:PASS
tests/test_radio_dish.cpp:517:test_dish_connect_fails_ipv6:PASS
tests/test_radio_dish.cpp:518:test_radio_dish_tcp_poll_ipv4:PASS
tests/test_radio_dish.cpp:519:test_radio_dish_tcp_poll_ipv6:PASS
tests/test_radio_dish.cpp:520:test_radio_dish_udp_ipv4:PASS
tests/test_radio_dish.cpp:521:test_radio_dish_udp_ipv6:PASS
tests/test_radio_dish.cpp:431:test_radio_dish_mcast_ipv4:IGNORE: No multicast available
tests/test_radio_dish.cpp:431:test_radio_dish_no_loop_ipv4:IGNORE: No multicast available
tests/test_radio_dish.cpp:431:test_radio_dish_mcast_ipv6:IGNORE: No multicast available
tests/test_radio_dish.cpp:431:test_radio_dish_no_loop_ipv6:IGNORE: No multicast available

-----------------------
16 Tests 0 Failures 4 Ignored
OK

tests/test_radio_dish.cpp

@@ -321,7 +321,7 @@ static bool is_multicast_available (int ipv6_)
         struct sockaddr_in *mcast_ipv4 = &mcast.ipv4;
 
         any_ipv4->sin_family = AF_INET;
-        any_ipv4->sin_port = htons (5555);
+        any_ipv4->sin_port = htons (port);
 
         rc = test_inet_pton (AF_INET, "0.0.0.0", &any_ipv4->sin_addr);
         if (rc == 0) {
@@ -395,15 +395,14 @@ static bool is_multicast_available (int ipv6_)
 
     msleep (SETTLE_TIME);
 
-    rc = sendto (send_sock, msg, static_cast<socklen_t> (strlen (msg)), 0,
-                 &mcast.generic, sl);
+    rc = sendto (send_sock, msg, strlen (msg), 0, &mcast.generic, sl);
     if (rc < 0) {
         goto out;
     }
 
     msleep (SETTLE_TIME);
 
-    rc = recvfrom (bind_sock, buf, sizeof (buf) - 1, 0, NULL, 0);
+    rc = recvfrom (bind_sock, buf, sizeof (buf) - 1, MSG_DONTWAIT, NULL, 0);
     if (rc < 0) {
         goto out;
     }