lua: fix completion retrieval for cdata objects

Cdata objects cannot have an `__autocomplete` metamethod (it's our custom
metamethod). To work this around, we should also lookup the
`__autocomplete` method using an object's `__index` metamethod.

Part of #9107

NO_CHANGELOG=<not a reported bug>
NO_DOC=<bugfix>

src/box/lua/console.c

@@ -1108,6 +1108,17 @@ lua_rl_getcompletion(lua_State *L)
 	/* use __autocomplete metamethod if it's present */
 	lua_pushstring(L, "__autocomplete");
 	lua_rawget(L, -2);
+	if (lua_isnil(L, -1)) {
+		lua_pop(L, 1);
+		/*
+		 * __autocomplete may not always be part of the metatable (e.g.,
+		 * in case of cdata), so we also want to trigger the __index
+		 * metamethod instead of simply doing a raw lookup in the
+		 * metatable.
+		 */
+		lua_pushstring(L, "__autocomplete");
+		lua_gettable(L, -3);
+	}
 	if (lua_isfunction(L, -1)) {
 		lua_replace(L, -2);
 		lua_insert(L, -2);

test/app-luatest/gh_6305_autocomplete_metamethod_test.lua

@@ -45,6 +45,14 @@ g.test__autocomplete = function()
                                                    })
    -- the right way - should add 'auto' comletion
    setmetatable(tab, {__autocomplete = function() return {auto = true} end})
+   t.assert_items_equals(tabcomplete('table11.'), {'table11.',
+                                                   'table11.first',
+                                                   'table11.second',
+                                                   'table11.auto',
+                                                   })
+   -- __autocomplete should also be looked up in __index
+   local autocompletion = function() return {auto = true} end
+   setmetatable(tab, {__index = {__autocomplete = autocompletion}})
    t.assert_items_equals(tabcomplete('table11.'), {'table11.',
                                                    'table11.first',
                                                    'table11.second',