metal: SDL_RenderReadPixels on macOS synchronizes the render target's texture data if it's managed, before reading from it.
diff --git a/src/render/metal/SDL_render_metal.m b/src/render/metal/SDL_render_metal.m
index d5e009d..06e4ef6 100644
--- a/src/render/metal/SDL_render_metal.m
+++ b/src/render/metal/SDL_render_metal.m
@@ -1196,14 +1196,28 @@ METAL_RenderReadPixels(SDL_Renderer * renderer, const SDL_Rect * rect,
METAL_RenderData *data = (__bridge METAL_RenderData *) renderer->driverdata;
METAL_ActivateRenderCommandEncoder(renderer, MTLLoadActionLoad, NULL);
- // Commit any current command buffer, and waitUntilCompleted, so any output is ready to be read.
[data.mtlcmdencoder endEncoding];
+ id<MTLTexture> mtltexture = data.mtlpassdesc.colorAttachments[0].texture;
+
+#ifdef __MACOSX__
+ /* on macOS with managed-storage textures, we need to tell the driver to
+ * update the CPU-side copy of the texture data.
+ * NOTE: Currently all of our textures are managed on macOS. We'll need some
+ * extra copying for any private textures. */
+ if (mtltexture.storageMode == MTLStorageModeManaged) {
+ id<MTLBlitCommandEncoder> blit = [data.mtlcmdbuffer blitCommandEncoder];
+ [blit synchronizeResource:mtltexture];
+ [blit endEncoding];
+ }
+#endif
+
+ /* Commit the current command buffer and wait until it's completed, to make
+ * sure the GPU has finished rendering to it by the time we read it. */
[data.mtlcmdbuffer commit];
[data.mtlcmdbuffer waitUntilCompleted];
data.mtlcmdencoder = nil;
data.mtlcmdbuffer = nil;
- id<MTLTexture> mtltexture = data.mtlpassdesc.colorAttachments[0].texture;
MTLRegion mtlregion = MTLRegionMake2D(rect->x, rect->y, rect->w, rect->h);
// we only do BGRA8 or RGBA8 at the moment, so 4 will do.