Use a single socket for InterfaceChoice.Default

When using multiple sockets with multi-cast, the outgoing
socket's responses could be read back on the incoming socket,
which leads to duplicate processing and could fill up the
incoming buffer before it could be processed.

This behavior manifested with error similar to
`OSError: [Errno 105] No buffer space available`

By using a single socket with InterfaceChoice.Default
we avoid this case.

zeroconf/__init__.py

@@ -2255,8 +2255,7 @@ def new_socket(
 def add_multicast_member(
     listen_socket: socket.socket,
     interface: Union[str, Tuple[Tuple[str, int, int], int]],
-    apple_p2p: bool = False,
-) -> Optional[socket.socket]:
+) -> None:
     # This is based on assumptions in normalize_interface_choice
     is_v6 = isinstance(interface, tuple)
     err_einval = {errno.EINVAL}
@@ -2293,13 +2292,20 @@ def add_multicast_member(
         else:
             raise
 
+
+def new_respond_socket(
+    interface: Union[str, Tuple[Tuple[str, int, int], int]],
+    apple_p2p: bool = False,
+) -> Optional[socket.socket]:
+    is_v6 = isinstance(interface, tuple)
     respond_socket = new_socket(
         ip_version=(IPVersion.V6Only if is_v6 else IPVersion.V4Only),
         apple_p2p=apple_p2p,
         bind_addr=cast(Tuple[Tuple[str, int, int], int], interface)[0] if is_v6 else (cast(str, interface),),
     )
     log.debug('Configuring socket %s with multicast interface %s', respond_socket, interface)
     if is_v6:
+        iface_bin = struct.pack('@I', cast(int, interface[1]))
         respond_socket.setsockopt(_IPPROTO_IPV6, socket.IPV6_MULTICAST_IF, iface_bin)
     else:
         respond_socket.setsockopt(
@@ -2321,11 +2327,19 @@ def create_sockets(
 
     normalized_interfaces = normalize_interface_choice(interfaces, ip_version)
 
+    # If we are using InterfaceChoice.Default we can use
+    # a single socket to listen and respond.
+    if not unicast and interfaces is InterfaceChoice.Default:
+        for i in normalized_interfaces:
+            add_multicast_member(cast(socket.socket, listen_socket), i)
+        return listen_socket, [listen_socket]
+
     respond_sockets = []
 
     for i in normalized_interfaces:
         if not unicast:
-            respond_socket = add_multicast_member(cast(socket.socket, listen_socket), i, apple_p2p=apple_p2p)
+            add_multicast_member(cast(socket.socket, listen_socket), i)
+            respond_socket = new_respond_socket(i, apple_p2p=apple_p2p)
         else:
             respond_socket = new_socket(
                 port=0,
@@ -2494,6 +2508,7 @@ def __init__(
             interfaces, unicast, ip_version, apple_p2p=apple_p2p
         )
         log.debug('Listen socket %s, respond sockets %s', self._listen_socket, self._respond_sockets)
+        self.multi_socket = unicast or interfaces is not InterfaceChoice.Default
 
         self.listeners = []  # type: List[RecordUpdateListener]
         self.browsers = {}  # type: Dict[ServiceListener, ServiceBrowser]
@@ -2513,8 +2528,9 @@ def __init__(
         self.listener = Listener(self)
         if not unicast:
             self.engine.add_reader(self.listener, cast(socket.socket, self._listen_socket))
-        for s in self._respond_sockets:
-            self.engine.add_reader(self.listener, s)
+        if self.multi_socket:
+            for s in self._respond_sockets:
+                self.engine.add_reader(self.listener, s)
         self.reaper = Reaper(self)
 
         self.debug = None  # type: Optional[DNSOutgoing]
@@ -2978,8 +2994,9 @@ def close(self) -> None:
             if not self.unicast:
                 self.engine.del_reader(cast(socket.socket, self._listen_socket))
                 cast(socket.socket, self._listen_socket).close()
-            for s in self._respond_sockets:
-                self.engine.del_reader(s)
+            if self.multi_socket:
+                for s in self._respond_sockets:
+                    self.engine.del_reader(s)
             self.engine.join()
 
             # shutdown the rest