Handle multicast being blocked in the test suite

If multicast is supported in principle, but multicast packets are blocked
by the firewall, then the multicast test will time out and reports an
error. Detect this case and don't fail.

test/integration/device_multicast.py

@@ -2,7 +2,9 @@
 
 """Test multicast device."""
 
+import enum
 import os
+import select
 import socket
 import struct
 import time
@@ -16,7 +18,13 @@
 PORT = 38245
 
 
-def multicast_works() -> bool:
+class MulticastSupport(enum.Enum):
+    NO = 0
+    BLOCKED = 1
+    YES = 2
+
+
+def multicast_works() -> MulticastSupport:
     """Check if multicast is supported and works."""
 
     msg = b"foobar"
@@ -31,9 +39,17 @@ def multicast_works() -> bool:
             with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as client:
                 client.sendto(msg, (MCAST_ADDR, PORT))
 
-            return msg == server.recv(16)
+            try:
+                select.select([server], [], [], 1)
+                if msg == server.recv(16, socket.MSG_DONTWAIT):
+                    return MulticastSupport.YES
+            except OSError:
+                pass
+
+            return MulticastSupport.BLOCKED
     except OSError:
-        return False
+        log.info("no")
+        return MulticastSupport.NO
 
 
 def test_no_mcast_support(foo: Tinc) -> None:
@@ -82,9 +98,15 @@ def test_device_multicast(ctx: Test) -> None:
     foo.cmd("set", "Device", f"{MCAST_ADDR} {PORT}")
     foo.cmd("set", "LogLevel", "10")
 
-    if multicast_works():
+    multicast_support = multicast_works()
+    if multicast_support == MulticastSupport.YES:
+        log.info("multicast supported")
         test_rx_tx(foo)
+    elif multicast_support == MulticastSupport.BLOCKED:
+        log.info("multicast blocked")
+        pass
     else:
+        log.info("multicast not supported")
         test_no_mcast_support(foo)