From 436cdf1fc898097a8cbe6226ad32a4055a6ca4d9 Mon Sep 17 00:00:00 2001
From: Michael Lutz <michi@icosahedron.de>
Date: Mon, 8 Mar 2021 21:16:41 +0100
Subject: [PATCH] Fix #8825: [OpenGL] Don't clear cursor cache from the game
 loop thread.

---
 src/video/opengl.cpp | 26 ++++++++++++++++++--------
 src/video/opengl.h   |  1 +
 2 files changed, 19 insertions(+), 8 deletions(-)

diff --git a/src/video/opengl.cpp b/src/video/opengl.cpp
index 64ea02cf2d..26f6f4b4dc 100644
--- a/src/video/opengl.cpp
+++ b/src/video/opengl.cpp
@@ -1051,6 +1051,19 @@ void OpenGLBackend::DrawMouseCursor()
 
 void OpenGLBackend::PopulateCursorCache()
 {
+	if (this->clear_cursor_cache) {
+		/* We have a pending cursor cache clear to do first. */
+		this->clear_cursor_cache = false;
+		this->last_sprite_pal = (PaletteID)-1;
+
+		Sprite *sp;
+		while ((sp = this->cursor_cache.Pop()) != nullptr) {
+			OpenGLSprite *sprite = (OpenGLSprite *)sp->data;
+			sprite->~OpenGLSprite();
+			free(sp);
+		}
+	}
+
 	for (uint i = 0; i < _cursor.sprite_count; ++i) {
 		SpriteID sprite = _cursor.sprite_seq[i].sprite;
 
@@ -1070,14 +1083,11 @@ void OpenGLBackend::PopulateCursorCache()
  */
 void OpenGLBackend::ClearCursorCache()
 {
-	this->last_sprite_pal = (PaletteID)-1;
-
-	Sprite *sp;
-	while ((sp = this->cursor_cache.Pop()) != nullptr) {
-		OpenGLSprite *sprite = (OpenGLSprite *)sp->data;
-		sprite->~OpenGLSprite();
-		free(sp);
-	}
+	/* If the game loop is threaded, this function might be called
+	 * from the game thread. As we can call OpenGL functions only
+	 * on the main thread, just set a flag that is handled the next
+	 * time we prepare the cursor cache for drawing. */
+	this->clear_cursor_cache = true;
 }
 
 /**
diff --git a/src/video/opengl.h b/src/video/opengl.h
index 3919179e40..c17a8536d9 100644
--- a/src/video/opengl.h
+++ b/src/video/opengl.h
@@ -63,6 +63,7 @@ private:
 
 	LRUCache<SpriteID, Sprite> cursor_cache;   ///< Cache of encoded cursor sprites.
 	PaletteID last_sprite_pal = (PaletteID)-1; ///< Last uploaded remap palette.
+	bool clear_cursor_cache = false;           ///< A clear of the cursor cache is pending.
 
 	OpenGLBackend();
 	~OpenGLBackend();