Fix #8874: show a warning when a NewGRF scan is requested multiple times from the console (#9022)

src/console_cmds.cpp

@@ -1351,7 +1351,9 @@ DEF_CONSOLE_CMD(ConRescanNewGRF)
 		return true;
 	}
 
-	RequestNewGRFScan();
+	if (!RequestNewGRFScan()) {
+		IConsoleWarning("NewGRF scanning is already running. Please wait until completed to run again.");
+	}
 
 	return true;
 }

src/openttd.cpp

@@ -1450,11 +1450,15 @@ static void DoAutosave()
  * done in the game-thread, and not in the draw-thread (which most often
  * triggers this request).
  * @param callback Optional callback to call when NewGRF scan is completed.
+ * @return True when the NewGRF scan was actually requested, false when the scan was already running.
  */
-void RequestNewGRFScan(NewGRFScanCallback *callback)
+bool RequestNewGRFScan(NewGRFScanCallback *callback)
 {
+	if (_request_newgrf_scan) return false;
+
 	_request_newgrf_scan = true;
 	_request_newgrf_scan_callback = callback;
+	return true;
 }
 
 void GameLoop()

src/openttd.h

@@ -81,6 +81,6 @@ void HandleExitGameRequest();
 
 void SwitchToMode(SwitchMode new_mode);
 
-void RequestNewGRFScan(struct NewGRFScanCallback *callback = nullptr);
+bool RequestNewGRFScan(struct NewGRFScanCallback *callback = nullptr);
 
 #endif /* OPENTTD_H */