Formspec: Show/update the player inv using core.show_formspec

'core.show_formspec' now shows and updates the inventory formspec as if
it was opened using the hotkey on client-side.

doc/lua_api.md

@@ -6872,9 +6872,10 @@ Formspec
 * `core.show_formspec(playername, formname, formspec)`
     * `playername`: name of player to show formspec
     * `formname`: name passed to `on_player_receive_fields` callbacks.
-      It should follow the `"modname:<whatever>"` naming convention.
-    * `formname` must not be empty, unless you want to reshow
-      the inventory formspec without updating it for future opens.
+        * It should follow the `"modname:<whatever>"` naming convention.
+        * If empty: Reshows the inventory formspec. Servers and
+          clients >= 5.13.0 will update the inventory formspec
+          (`ObjectRef:set_inventory_formspec`) for future opens.
     * `formspec`: formspec to display
 * `core.close_formspec(playername, formname)`
     * `playername`: name of player to close formspec

src/client/game.cpp

@@ -2714,11 +2714,21 @@ void Game::handleClientEvent_DeathscreenLegacy(ClientEvent *event, CameraOrienta
 
 void Game::handleClientEvent_ShowFormSpec(ClientEvent *event, CameraOrientation *cam)
 {
-	m_game_formspec.showFormSpec(*event->show_formspec.formspec,
-		*event->show_formspec.formname);
+	auto &fs = event->show_formspec;
 
-	delete event->show_formspec.formspec;
-	delete event->show_formspec.formname;
+	if (fs.formname->empty() && !fs.formspec->empty()) {
+		// Overwrite the inventory formspec
+		LocalPlayer *player = client->getEnv().getLocalPlayer();
+		player->inventory_formspec = *fs.formspec;
+
+		m_game_formspec.showPlayerInventory();
+	} else {
+		m_game_formspec.showFormSpec(*fs.formspec,
+			*fs.formname);
+	}
+
+	delete fs.formspec;
+	delete fs.formname;
 }
 
 void Game::handleClientEvent_ShowCSMFormSpec(ClientEvent *event, CameraOrientation *cam)

src/server.cpp

@@ -3397,6 +3397,11 @@ bool Server::showFormspec(const char *playername, const std::string &formspec,
 	if (!player)
 		return false;
 
+	if (formname.empty() && !formspec.empty()) {
+		// Overwrite the inventory formspec
+		player->inventory_formspec = formspec;
+	}
+
 	SendShowFormspecMessage(player->getPeerId(), formspec, formname);
 	return true;
 }