json-rpc: Don't let users send messages that are handled internally

We cannot let users use `sendcustommsg` to inject messages that are handled
internally since it could result in our internal state tracking being borked.

lightningd/peer_control.c

@@ -2385,6 +2385,7 @@ static struct command_result *json_sendcustommsg(struct command *cmd,
 	struct peer *peer;
 	struct subd *owner;
 	u8 *msg;
+	int type;
 
 	if (!param(cmd, buffer, params,
 		   p_req("node_id", param_node_id, &dest),
@@ -2392,6 +2393,17 @@ static struct command_result *json_sendcustommsg(struct command *cmd,
 		   NULL))
 		return command_param_failed();
 
+	type = fromwire_peektype(msg);
+	if (wire_type_is_defined(type)) {
+		return command_fail(
+		    cmd, JSONRPC2_INVALID_REQUEST,
+		    "Cannot send messages of type %d (%s). It is not possible "
+		    "to send messages that have a type managed internally "
+		    "since that might cause issues with the internal state "
+		    "tracking.",
+		    type, wire_type_name(type));
+	}
+
 	peer = peer_by_id(cmd->ld, dest);
 	if (!peer) {
 		return command_fail(cmd, JSONRPC2_INVALID_REQUEST,

tools/gen/header_template

@@ -26,6 +26,15 @@ enum ${enum_set['name']} {
 ## The 'name' functions for the enums
 % for enum_set in enum_sets:
 const char *${enum_set['name']}_name(int e);
+
+/**
+ * Determine whether a given message type is defined as a message.
+ *
+ * Returns true if the message type is part of the message definitions we have
+ * generated parsers for, false if it is a custom message that cannot be
+ * handled internally.
+ */
+bool ${enum_set['name']}_is_defined(u16 type);
 % endfor
 
 ## Structs for subtypes + tlv messages

tools/gen/impl_template

@@ -31,6 +31,18 @@ const char *${enum_set['name']}_name(int e)
 	snprintf(invalidbuf, sizeof(invalidbuf), "INVALID %i", e);
 	return invalidbuf;
 }
+
+bool ${enum_set['name']}_is_defined(u16 type)
+{
+	switch ((enum ${enum_set['name']})type) {
+    % for msg in enum_set['set']:
+	case ${msg.enum_name()}:;
+    % endfor
+	      return true;
+	}
+	return false;
+}
+
 % endfor
 ## START PARTIALS
 ## Subtype and TLV-msg towire_