unix: don't crash listener if remote end disconnects.

In some scenarios, Ansible's worker seems to exit early, resulting in
EPIPE during .recv() or .send(). Log an error and gracefully disconnect
in that case.

mitogen/unix.py

@@ -78,21 +78,38 @@ class Listener(mitogen.core.BasicStream):
         self.receive_side = mitogen.core.Side(self, self._sock.fileno())
         router.broker.start_receive(self)
 
-    def on_receive(self, broker):
-        sock, _ = self._sock.accept()
+    def _accept_client(self, sock):
         sock.setblocking(True)
+        try:
             pid, = struct.unpack('>L', sock.recv(4))
+        except socket.error:
+            LOG.error('%r: failed to read remote identity: %s',
+                      self, sys.exc_info()[1])
+            return
 
         context_id = self._router.id_allocator.allocate()
         context = mitogen.parent.Context(self._router, context_id)
         stream = mitogen.core.Stream(self._router, context_id)
-        stream.accept(sock.fileno(), sock.fileno())
         stream.name = u'unix_client.%d' % (pid,)
         stream.auth_id = mitogen.context_id
         stream.is_privileged = True
-        self._router.register(context, stream)
+
+        try:
             sock.send(struct.pack('>LLL', context_id, mitogen.context_id,
                                   os.getpid()))
+        except socket.error:
+            LOG.error('%r: failed to assign identity to PID %d: %s',
+                      self, pid, sys.exc_info()[1])
+            return
+
+        stream.accept(sock.fileno(), sock.fileno())
+        self._router.register(context, stream)
+
+    def on_receive(self, broker):
+        sock, _ = self._sock.accept()
+        try:
+            self._accept_client(sock)
+        finally:
             sock.close()