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 tarantool#9107

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

src/box/lua/console.c

@@ -1094,6 +1094,30 @@ lua_rl_dmadd(dmlist *ml, const char *p, size_t pn, const char *s, int suf)
 	return 0;
 }
 
+/*
+ * Try to retrieve object's `__autocomplete` method.
+ *
+ * @param L Lua state
+ * @param obj_idx object's index on Lua stack
+ * @param mt_idx object metatable's index on Lua stack
+ */
+static void
+lua_rl_get_autocomplete_metamethod(lua_State *L, int obj_idx, int mt_idx)
+{
+	lua_pushstring(L, "__autocomplete");
+	lua_rawget(L, mt_idx);
+	if (!lua_isnil(L, -1))
+		return;
+	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, obj_idx);
+}
+
 /*
  * Get table from __autocomplete function if it's present
  * Use __index field of metatable of object as a fallback
@@ -1105,9 +1129,9 @@ lua_rl_getcompletion(lua_State *L)
 		lua_pop(L, 1);
 		return 0;
 	}
-	/* use __autocomplete metamethod if it's present */
-	lua_pushstring(L, "__autocomplete");
-	lua_rawget(L, -2);
+	/* use __autocomplete method if it's present */
+	int top = lua_gettop(L);
+	lua_rl_get_autocomplete_metamethod(L, top - 1, top);
 	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',