fix: check the frame QueryInterface result in dxgi capture

Both AcquireNextFrame paths cast the IDXGIResource to ID3D11Texture2D
without looking at the HRESULT. ohgodwhat() then dereferences the null
pointer in GetDesc(), and get_texture() hands a null texture to the vram
encoder. Return the error instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011sw75MSAz7PTqrSALdStXe

libs/scrap/src/dxgi/mod.rs

@@ -48,6 +48,7 @@ pub struct Capturer {
     duplication: ComPtr<IDXGIOutputDuplication>,
     fastlane: bool,
     surface: ComPtr<IDXGISurface>,
+    readable: ComPtr<ID3D11Texture2D>,
     texture: ComPtr<ID3D11Texture2D>,
     width: usize,
     height: usize,
@@ -163,6 +164,7 @@ impl Capturer {
             duplication: ComPtr(duplication),
             fastlane: desc.DesktopImageInSystemMemory == TRUE,
             surface: ComPtr(ptr::null_mut()),
+            readable: ComPtr(ptr::null_mut()),
             texture: ComPtr(ptr::null_mut()),
             width: display.width() as usize,
             height: display.height() as usize,
@@ -346,19 +348,19 @@ impl Capturer {
         if self.fastlane {
             wrap_hresult((*self.duplication.0).MapDesktopSurface(&mut rect))?;
         } else {
-            self.surface = ComPtr(self.ohgodwhat(frame.0)?);
+            self.ohgodwhat(frame.0)?;
             wrap_hresult((*self.surface.0).Map(&mut rect, DXGI_MAP_READ))?;
         }
         Ok((rect.pBits, rect.Pitch))
     }
 
     // copy from GPU memory to system memory
-    unsafe fn ohgodwhat(&mut self, frame: *mut IDXGIResource) -> io::Result<*mut IDXGISurface> {
+    unsafe fn ohgodwhat(&mut self, frame: *mut IDXGIResource) -> io::Result<()> {
         let mut texture: *mut ID3D11Texture2D = ptr::null_mut();
-        (*frame).QueryInterface(
+        wrap_hresult((*frame).QueryInterface(
             &IID_ID3D11Texture2D,
             &mut texture as *mut *mut _ as *mut *mut _,
-        );
+        ))?;
         let texture = ComPtr(texture);
 
         #[allow(invalid_value)]
@@ -370,24 +372,37 @@ impl Capturer {
         texture_desc.CPUAccessFlags = D3D11_CPU_ACCESS_READ;
         texture_desc.MiscFlags = 0;
 
-        let mut readable = ptr::null_mut();
-        wrap_hresult((*self.device.0).CreateTexture2D(
-            &mut texture_desc,
-            ptr::null(),
-            &mut readable,
-        ))?;
-        (*readable).SetEvictionPriority(DXGI_RESOURCE_PRIORITY_MAXIMUM);
-        let readable = ComPtr(readable);
+        // Avoid per-frame staging texture allocation and the kernel allocation churn it causes.
+        let mut current: D3D11_TEXTURE2D_DESC = mem::zeroed();
+        if !self.surface.is_null() {
+            (*self.readable.0).GetDesc(&mut current);
+        }
+        if current.Width != texture_desc.Width
+            || current.Height != texture_desc.Height
+            || current.Format != texture_desc.Format
+        {
+            let mut readable = ptr::null_mut();
+            wrap_hresult((*self.device.0).CreateTexture2D(
+                &mut texture_desc,
+                ptr::null(),
+                &mut readable,
+            ))?;
+            (*readable).SetEvictionPriority(DXGI_RESOURCE_PRIORITY_MAXIMUM);
+            let readable = ComPtr(readable);
 
-        let mut surface = ptr::null_mut();
-        (*readable.0).QueryInterface(
-            &IID_IDXGISurface,
-            &mut surface as *mut *mut _ as *mut *mut _,
-        );
+            let mut surface = ptr::null_mut();
+            wrap_hresult((*readable.0).QueryInterface(
+                &IID_IDXGISurface,
+                &mut surface as *mut *mut _ as *mut *mut _,
+            ))?;
 
-        (*self.context.0).CopyResource(readable.0 as *mut _, texture.0 as *mut _);
+            self.readable = readable;
+            self.surface = ComPtr(surface);
+        }
 
-        Ok(surface)
+        (*self.context.0).CopyResource(self.readable.0 as *mut _, texture.0 as *mut _);
+
+        Ok(())
     }
 
     pub fn frame<'a>(&'a mut self, timeout: UINT) -> io::Result<Frame<'a>> {
@@ -485,10 +500,10 @@ impl Capturer {
             }
 
             let mut texture: *mut ID3D11Texture2D = ptr::null_mut();
-            (*frame.0).QueryInterface(
+            wrap_hresult((*frame.0).QueryInterface(
                 &IID_ID3D11Texture2D,
                 &mut texture as *mut *mut _ as *mut *mut _,
-            );
+            ))?;
             let texture = ComPtr(texture);
             self.texture = texture;