dos: Load on main thread

I tried the interrupt handler approach but couldn't get it to work,
so for now we're back to loading and updating the progress bar
synchronously on DOS.

Source/interfac.cpp

@@ -36,6 +36,10 @@
 #include "controls/touch/renderers.h"
 #endif
 
+#ifdef __DJGPP__
+#define LOAD_ON_MAIN_THREAD
+#endif
+
 namespace devilution {
 
 namespace {
@@ -235,6 +239,64 @@ void DrawCutsceneForeground()
 	RenderPresent();
 }
 
+struct {
+	uint32_t loadStartedAt;
+	EventHandler prevHandler;
+	bool skipRendering;
+	bool done;
+	uint32_t drawnProgress;
+	std::array<SDL_Color, 256> palette;
+} ProgressEventHandlerState;
+
+void InitRendering()
+{
+	// Blit the background once and then free it.
+	DrawCutsceneBackground();
+	if (RenderDirectlyToOutputSurface && PalSurface != nullptr) {
+		// Render into all the backbuffers if there are multiple.
+		const void *initialPixels = PalSurface->pixels;
+		if (DiabloUiSurface() == PalSurface)
+			BltFast(nullptr, nullptr);
+		RenderPresent();
+		while (PalSurface->pixels != initialPixels) {
+			DrawCutsceneBackground();
+			if (DiabloUiSurface() == PalSurface)
+				BltFast(nullptr, nullptr);
+			RenderPresent();
+		}
+	}
+	FreeCutsceneBackground();
+
+	// The loading thread sets `logical_palette`, so we make sure to use
+	// our own palette for the fade-in.
+	PaletteFadeIn(8, ProgressEventHandlerState.palette);
+}
+
+void CheckShouldSkipRendering()
+{
+	if (!ProgressEventHandlerState.skipRendering) return;
+	const bool shouldSkip = ProgressEventHandlerState.loadStartedAt + *GetOptions().Gameplay.skipLoadingScreenThresholdMs > SDL_GetTicks();
+	if (shouldSkip) return;
+	ProgressEventHandlerState.skipRendering = false;
+	if (!HeadlessMode) InitRendering();
+}
+
+bool HandleProgressBarUpdate()
+{
+	CheckShouldSkipRendering();
+	SDL_Event event;
+	// We use the real `PollEvent` here instead of `FetchMessage`
+	// to process real events rather than the recorded ones in demo mode.
+	while (PollEvent(&event) != 0) {
+		CheckShouldSkipRendering();
+		if (event.type != SDL_QUIT) {
+			HandleMessage(event, SDL_GetModState());
+		}
+		if (ProgressEventHandlerState.done) return false;
+	}
+	return true;
+}
+
 void DoLoad(interface_mode uMsg)
 {
 	IncProgress();
@@ -401,6 +463,9 @@ void DoLoad(interface_mode uMsg)
 			LogError("Failed to send WM_ERROR {}", SDL_GetError());
 			SDL_ClearError();
 		}
+#ifdef LOAD_ON_MAIN_THREAD
+		HandleProgressBarUpdate();
+#endif
 		return;
 	}
 
@@ -410,48 +475,9 @@ void DoLoad(interface_mode uMsg)
 		LogError("Failed to send WM_DONE {}", SDL_GetError());
 		SDL_ClearError();
 	}
-}
-
-struct {
-	uint32_t loadStartedAt;
-	EventHandler prevHandler;
-	bool skipRendering;
-	bool done;
-	uint32_t drawnProgress;
-	std::array<SDL_Color, 256> palette;
-} ProgressEventHandlerState;
-
-void InitRendering()
-{
-	// Blit the background once and then free it.
-	DrawCutsceneBackground();
-	if (RenderDirectlyToOutputSurface && PalSurface != nullptr) {
-		// Render into all the backbuffers if there are multiple.
-		const void *initialPixels = PalSurface->pixels;
-		if (DiabloUiSurface() == PalSurface)
-			BltFast(nullptr, nullptr);
-		RenderPresent();
-		while (PalSurface->pixels != initialPixels) {
-			DrawCutsceneBackground();
-			if (DiabloUiSurface() == PalSurface)
-				BltFast(nullptr, nullptr);
-			RenderPresent();
-		}
-	}
-	FreeCutsceneBackground();
-
-	// The loading thread sets `logical_palette`, so we make sure to use
-	// our own palette for the fade-in.
-	PaletteFadeIn(8, ProgressEventHandlerState.palette);
-}
-
-void CheckShouldSkipRendering()
-{
-	if (!ProgressEventHandlerState.skipRendering) return;
-	const bool shouldSkip = ProgressEventHandlerState.loadStartedAt + *GetOptions().Gameplay.skipLoadingScreenThresholdMs > SDL_GetTicks();
-	if (shouldSkip) return;
-	ProgressEventHandlerState.skipRendering = false;
-	if (!HeadlessMode) InitRendering();
+#ifdef LOAD_ON_MAIN_THREAD
+	HandleProgressBarUpdate();
+#endif
 }
 
 void ProgressEventHandler(const SDL_Event &event, uint16_t modState)
@@ -525,6 +551,9 @@ void ProgressEventHandler(const SDL_Event &event, uint16_t modState)
 		app_fatal("Unknown progress mode");
 		break;
 	}
+#ifdef LOAD_ON_MAIN_THREAD
+	HandleProgressBarUpdate();
+#endif
 }
 
 } // namespace
@@ -578,6 +607,9 @@ void IncProgress(uint32_t steps)
 			LogError("Failed to send WM_PROGRESS {}", SDL_GetError());
 			SDL_ClearError();
 		}
+#ifdef LOAD_ON_MAIN_THREAD
+		HandleProgressBarUpdate();
+#endif
 	}
 }
 
@@ -629,6 +661,11 @@ void ShowProgress(interface_mode uMsg)
 	}
 
 	// Begin loading
+#ifdef LOAD_ON_MAIN_THREAD
+	const uint32_t start = SDL_GetTicks();
+	DoLoad(uMsg);
+	LogVerbose("Loading finished in {}ms", SDL_GetTicks() - start);
+#else
 	static interface_mode loadTarget;
 	loadTarget = uMsg;
 	SdlThread loadThread = SdlThread([]() {
@@ -636,28 +673,9 @@ void ShowProgress(interface_mode uMsg)
 		DoLoad(loadTarget);
 		LogVerbose("Load thread finished in {}ms", SDL_GetTicks() - start);
 	});
-
-	const auto processEvent = [&](const SDL_Event &event) {
-		CheckShouldSkipRendering();
-		if (event.type != SDL_QUIT) {
-			HandleMessage(event, SDL_GetModState());
-		}
-		if (ProgressEventHandlerState.done) {
-			loadThread.join();
-			return false;
-		}
-		return true;
-	};
-
-	while (true) {
-		CheckShouldSkipRendering();
-		SDL_Event event;
-		// We use the real `PollEvent` here instead of `FetchMessage`
-		// to process real events rather than the recorded ones in demo mode.
-		while (PollEvent(&event)) {
-			if (!processEvent(event)) return;
-		}
-	}
+	while (HandleProgressBarUpdate()) { }
+	loadThread.join();
+#endif
 }
 
 } // namespace devilution