Do not connect UDP sockets when broadcast is allowed

This fixes issue python#480.
The _SelectorDatagramTransport.sendto method has to be modified
so _sock.sendto is used in all cases (since there is no proper
way to tell if the socket is connected or not). Cound that be an
issue for connected sockets?

asyncio/base_events.py

@@ -916,7 +916,8 @@ def create_datagram_endpoint(self, protocol_factory,
                     if local_addr:
                         sock.bind(local_address)
                     if remote_addr:
-                        yield from self.sock_connect(sock, remote_address)
+                        if not allow_broadcast:
+                            yield from self.sock_connect(sock, remote_address)
                         r_addr = remote_address
                 except OSError as exc:
                     if sock is not None:

asyncio/selector_events.py

@@ -1083,9 +1083,11 @@ def sendto(self, data, addr=None):
         if not data:
             return
 
-        if self._address and addr not in (None, self._address):
-            raise ValueError('Invalid address: must be None or %s' %
-                             (self._address,))
+        if self._address:
+            if addr not in (None, self._address):
+                raise ValueError(
+                    'Invalid address: must be None or %s' % (self._address,))
+            addr = self._address
 
         if self._conn_lost and self._address:
             if self._conn_lost >= constants.LOG_THRESHOLD_FOR_CONNLOST_WRITES:
@@ -1096,10 +1098,7 @@ def sendto(self, data, addr=None):
         if not self._buffer:
             # Attempt to send it right away first.
             try:
-                if self._address:
-                    self._sock.send(data)
-                else:
-                    self._sock.sendto(data, addr)
+                self._sock.sendto(data, addr)
                 return
             except (BlockingIOError, InterruptedError):
                 self._loop._add_writer(self._sock_fd, self._sendto_ready)
@@ -1119,10 +1118,7 @@ def _sendto_ready(self):
         while self._buffer:
             data, addr = self._buffer.popleft()
             try:
-                if self._address:
-                    self._sock.send(data)
-                else:
-                    self._sock.sendto(data, addr)
+                self._sock.sendto(data, addr)
             except (BlockingIOError, InterruptedError):
                 self._buffer.appendleft((data, addr))  # Try again later.
                 break