Metal compositor: custom cursor sprite cast to the wrong swapchain type on the VK-via-Metal fallback (#1941) #1948

Summary

On the Vulkan-app-on-Metal-compositor fallback (oxr_session_populate_vk_with_metal_native, used on macOS when vk_native is unavailable), a custom runtime-drawn cursor sprite (XrCursorDepthDrawInfoDXR::image != XR_NULL_HANDLE, XR_DXR_cursor_depth v5, #1941) is handed to the Metal compositor as the client Vulkan swapchain, and the Metal compositor casts it to its native swapchain type. This was found by reading the code while reviewing #1946 and has not been reproduced. The default look (image = XR_NULL_HANDLE) is unaffected.

Evidence

• src/xrt/state_trackers/oxr/oxr_session_gfx_vk_native.c:166: on this route sess->compositor = &xcvk->base (a comp_vk_client wrapper), while sess->xcn is the comp_metal compositor (:187 is_vk_via_metal = true, and is_metal_native_compositor = true). The app's swapchains are therefore struct client_vk_swapchain (src/xrt/compositor/client/comp_vk_client.h:46), which wrap the native one in ->xscn.
• src/xrt/state_trackers/oxr/oxr_session_frame_end.c:2052 (cursor_draw_submit): req.image = sc->swapchain; hands over that client swapchain unchanged.
• src/xrt/compositor/metal/comp_metal_compositor.m:4160 (metal_cursor_draw): struct comp_metal_swapchain *msc = metal_swapchain(req->image);. metal_swapchain() (:579) is a plain cast, so msc->image_count / msc->images[] are read from a client_vk_swapchain.

The expected result is a garbage image count or texture: the cursor is not drawn, or an invalid MTLTexture is sampled and the app may crash.

Likely fix

In cursor_draw_submit (or in the Metal hand-off), resolve the native swapchain for wrapped sessions: on is_vk_via_metal take ((struct client_vk_swapchain *)sc->swapchain)->xscn. The macOS GL-on-Metal interop route may need the same check for its client swapchains. Alternatively, report willDraw = XR_FALSE for a custom sprite on wrapped sessions until the hand-off resolves it.

Not done

There is no repro yet: on macOS the reference Vulkan cube takes vk_native, not this fallback.