tmcast.py: Handle SIGINT/SIGTERM

See nexedi/nxdtest@b0cf277d for details
and for why signal handling is implemented like this.

namespace_demo/tmcast.py

@@ -12,8 +12,9 @@ from socket import socket, AF_INET, AF_INET6, SOCK_DGRAM, IPPROTO_UDP,
 import socket as net
 from struct import pack
 
-from golang import b, u
-from golang import sync, context
+from golang import b, u, go, func, defer, chan
+from golang import os as gos, sync, context, syscall
+from golang.os import signal
 
 
 group4 = '224.1.1.1'
@@ -94,6 +95,7 @@ def rxloop(ctx, sk):
         print("rx: %s" % u(pkt))
 
 
+@func
 def main():
     action = sys.argv[1]
 
@@ -107,7 +109,24 @@ def main():
     if G is None:
         raise RuntimeError("4/6 must be specified")
 
-    wg = sync.WorkGroup(context.background())
+    # prepare ctx that is canceled on SIGINT/SIGTERM
+    ctx, cancel = context.with_cancel(context.background())
+    sigq = chan(1, dtype=gos.Signal)
+    signal.Notify(sigq, syscall.SIGINT, syscall.SIGTERM)
+    def _():
+        signal.Stop(sigq)
+        sigq.close()
+    defer(_)
+    def _():
+        sig, ok = sigq.recv_()
+        if not ok:
+            return
+        print("# %s" % sig)
+        cancel()
+    go(_)
+    defer(cancel)
+
+    wg = sync.WorkGroup(ctx)
     if "tx" in action:
         sktx = mjoin_tx(*G)
         wg.go(txloop, sktx, b(sys.argv[2]))