lightningd: check rpc names collisions with builtin commands

lightningd/plugin.c

@@ -1393,11 +1393,18 @@ static const char *plugin_rpcmethod_add(struct plugin *plugin,
 	if (!jsonrpc_command_add(plugin->plugins->ld->jsonrpc, cmd, usage)) {
 		struct plugin *p =
 		    find_plugin_for_command(plugin->plugins->ld, cmd->name);
-		return tal_fmt(
-		    plugin,
-		    "Could not register method \"%s\", a method with "
-		    "that name is already registered by plugin %s",
-		    cmd->name, p->cmd);
+		if (p)
+			return tal_fmt(
+			    plugin,
+			    "Could not register method \"%s\", a method with "
+			    "that name is already registered by plugin %s",
+			    cmd->name, p->cmd);
+		else
+			return tal_fmt(plugin,
+				       "Could not register method \"%s\", a "
+				       "builtin method with "
+				       "that name is already registered",
+				       cmd->name);
 	}
 	tal_arr_expand(&plugin->methods, cmd->name);
 	return NULL;

tests/test_plugin.py

@@ -6134,3 +6134,21 @@ def test_bwatch_blockdepth_watch_no_fire_before_start_block(node_factory, bitcoi
 
     # Clean up
     l1.rpc.delblockdepthwatch(owner=owner, start_block=future_start)
+
+
+def test_command_collision(node_factory):
+    def xpay_duplicate(plugin):
+        @plugin.method("xpay")
+        def on_xpay(plugin):
+            return {}
+    l1 = node_factory.get_node(inline_plugin=xpay_duplicate)
+    l1.daemon.logsearch_start = 0
+    l1.daemon.wait_for_log("a method with that name is already registered by plugin")
+
+    def builtin_duplicate(plugin):
+        @plugin.method("getinfo")
+        def on_getinfo(plugin):
+            return {}
+    l2 = node_factory.get_node(inline_plugin=builtin_duplicate)
+    l2.daemon.logsearch_start = 0
+    l2.daemon.wait_for_log("a builtin method with that name is already registered")