command/message: make messages/subscriptions local to the current partition

doc/protocol.rst

@@ -1354,6 +1354,9 @@ additional services.
 New messages are indicated by the ``message``
 idle event.
 
+If your MPD instance has multiple partitions, note that
+client-to-client messages are local to the current partition.
+
 :command:`subscribe {NAME}`
     Subscribe to a channel.  The channel is created if it
     does not exist already.  The name may consist of

src/command/MessageCommands.cxx

@@ -78,7 +78,12 @@ handle_channels(Client &client, gcc_unused Request args, Response &r)
 	assert(args.empty());
 
 	std::set<std::string> channels;
+
+	const auto &partition = client.GetPartition();
 	for (const auto &c : *client.GetInstance().client_list) {
+		if (&c.GetPartition() != &partition)
+			continue;
+
 		const auto &subscriptions = c.GetSubscriptions();
 		channels.insert(subscriptions.begin(),
 				subscriptions.end());
@@ -119,8 +124,11 @@ handle_send_message(Client &client, Request args, Response &r)
 
 	bool sent = false;
 	const ClientMessage msg(channel_name, message_text);
+
+	const auto &partition = client.GetPartition();
 	for (auto &c : *client.GetInstance().client_list)
-		if (c.PushMessage(msg))
+		if (&c.GetPartition() == &partition &&
+		    c.PushMessage(msg))
 			sent = true;
 
 	if (sent)