tmcast: Don't block forever if there is no packets to receive

Or else cancellation does not work.
Amends 334a148f (tmcast.py: Handle SIGINT/SIGTERM).

namespace_demo/tmcast.py

@@ -8,7 +8,7 @@ import sys, time
 from socket import socket, AF_INET, AF_INET6, SOCK_DGRAM, IPPROTO_UDP,          \
     SOL_SOCKET, SO_REUSEADDR, IPPROTO_IP, IPPROTO_IPV6, IP_ADD_MEMBERSHIP,          \
     IPV6_JOIN_GROUP, IP_MULTICAST_TTL, IPV6_MULTICAST_HOPS, IP_MULTICAST_LOOP,          \
-    IPV6_MULTICAST_LOOP, inet_pton
+    IPV6_MULTICAST_LOOP, inet_pton, timeout as TimeoutError
 import socket as net
 from struct import pack
 
@@ -88,10 +88,14 @@ def txloop(ctx, sk, pkt):
         time.sleep(1)
 
 def rxloop(ctx, sk):
+    sk.settimeout(1)
     while 1:
         if ctx.err():
             raise ctx.err()
+        try:
             pkt = sk.recv(4096)
+        except TimeoutError:
+            continue
         print("rx: %s" % u(pkt))