lightningd: allow sending of even messages.

It's your funeral!

Signed-off-by: Rusty Russell <[email protected]>

doc/lightning-sendcustommsg.7.md

@@ -16,11 +16,9 @@ top, not for direct use by end-users.
 
 The message must be a hex encoded well-formed message, including the 2-byte
 type prefix, but excluding the length prefix which will be added by the RPC
-method. The messages must not use even-numbered types, since these may require
-synchronous handling on the receiving side, and can cause the connection to be
-dropped. The message types may also not use one of the internally handled
+method. The message types may not be one of the internally handled
 types, since that may cause issues with the internal state tracking of
-Core Lightning.
+Core Lightning. We do (as of *v23.11*) allow sending of even types, but note that peers (as per the spec) will disconnect on receiving unknown even types.
 
 The node specified by `node_id` must be a peer, i.e., it must have a direct
 connection with the node receiving the RPC call, and the connection must be
@@ -33,7 +31,7 @@ not have a connection, or are synchronous daemons that do not handle
 spontaneous messages.
 
 On the reveiving end a plugin may implement the `custommsg` plugin hook and
-get notified about incoming messages.
+get notified about incoming messages, and allow additional unknown even types in their getmanifest response.
 
 RETURN VALUE
 ------------

lightningd/connect_control.c

@@ -782,16 +782,6 @@ static struct command_result *json_sendcustommsg(struct command *cmd,
 		    type, peer_wire_name(type));
 	}
 
-	if (type % 2 == 0) {
-		return command_fail(
-		    cmd, JSONRPC2_INVALID_REQUEST,
-		    "Cannot send even-typed %d custom message. Currently "
-		    "custom messages are limited to odd-numbered message "
-		    "types, as even-numbered types might result in "
-		    "disconnections.",
-		    type);
-	}
-
 	peer = peer_by_id(cmd->ld, dest);
 	if (!peer) {
 		return command_fail(cmd, JSONRPC2_INVALID_REQUEST,