Codechange: Generalise the delayed blitter change to a generic video driver command queue.

(cherry picked from commit 91b8ce073f)

src/video/video_driver.hpp

@@ -22,6 +22,7 @@
 #include <mutex>
 #include <thread>
 #include <vector>
+#include <functional>
 #if defined(__MINGW32__)
 #include "../3rdparty/mingw-std-threads/mingw.mutex.h"
 #include "../3rdparty/mingw-std-threads/mingw.condition_variable.h"
@@ -41,7 +42,7 @@ class VideoDriver : public Driver {
 	const uint DEFAULT_WINDOW_HEIGHT = 480u; ///< Default window height.
 
 public:
-	VideoDriver() : fast_forward_key_pressed(false), fast_forward_via_key(false), is_game_threaded(true), change_blitter(nullptr) {}
+	VideoDriver() : fast_forward_key_pressed(false), fast_forward_via_key(false), is_game_threaded(true) {}
 
 	/**
 	 * Mark a particular area dirty.
@@ -183,12 +184,16 @@ public:
 	}
 
 	/**
-	 * Queue a request to change the blitter. This is not executed immediately,
-	 * but instead on the next draw-tick.
+	 * Queue a function to be called on the main thread with game state
+	 * lock held and video buffer locked. Queued functions will be
+	 * executed on the next draw tick.
+	 * @param func Function to call.
 	 */
-	void ChangeBlitter(const char *new_blitter)
+	void QueueOnMainThread(std::function<void()> &&func)
 	{
-		this->change_blitter = new_blitter;
+		std::lock_guard<std::mutex> lock(this->cmd_queue_mutex);
+
+		this->cmd_queue.emplace_back(std::forward<std::function<void()>>(func));
 	}
 
 	void GameLoopPause();
@@ -337,11 +342,29 @@ protected:
 	static void GameThreadThunk(VideoDriver *drv);
 
 private:
+	std::mutex cmd_queue_mutex;
+	std::vector<std::function<void()>> cmd_queue;
+
+	/** Execute all queued commands. */
+	void DrainCommandQueue()
+	{
+		std::vector<std::function<void()>> cmds{};
+
+		{
+			/* Exchange queue with an empty one to limit the time we
+			 * hold the mutex. This also ensures that queued functions can
+			 * add new functions to the queue without everything blocking. */
+			std::lock_guard<std::mutex> lock(this->cmd_queue_mutex);
+			cmds.swap(this->cmd_queue);
+		}
+
+		for (auto &f : cmds) {
+			f();
+		}
+	}
+
 	void GameLoop();
 	void GameThread();
-	void RealChangeBlitter(const char *repl_blitter);
-
-	const char *change_blitter; ///< Request to change the blitter. nullptr if no pending request.
 };
 
 #endif /* VIDEO_VIDEO_DRIVER_HPP */