Bug 1888634 - Disallow captureStream with canvas transferred to DOM w…

…orker. r=gfx-reviewers,lsalzman

This behaviour has yet to be defined by the spec and/or WPT, see:

w3c/mediacapture-fromelement#65
w3c/mediacapture-extensions#26
web-platform-tests/wpt#21102

Differential Revision: https://phabricator.services.mozilla.com/D207582

dom/canvas/OffscreenCanvas.cpp

@@ -358,6 +358,21 @@ UniquePtr<OffscreenCanvasCloneData> OffscreenCanvas::ToCloneData(
     return nullptr;
   }
 
+  // Check if we are using HTMLCanvasElement::captureStream. This is not
+  // defined by the spec yet, so it is better to fail now than implement
+  // something not compliant:
+  // https://github.com/w3c/mediacapture-fromelement/issues/65
+  // https://github.com/w3c/mediacapture-extensions/pull/26
+  // https://github.com/web-platform-tests/wpt/issues/21102
+  if (mDisplay && NS_WARN_IF(mDisplay->UsingElementCaptureStream())) {
+    ErrorResult rv;
+    rv.ThrowNotSupportedError(
+        "Cannot transfer OffscreenCanvas bound to element using "
+        "captureStream.");
+    MOZ_ALWAYS_TRUE(rv.MaybeSetPendingException(aCx));
+    return nullptr;
+  }
+
   auto cloneData = MakeUnique<OffscreenCanvasCloneData>(
       mDisplay, mWidth, mHeight, mCompositorBackendType, mTextureType,
       mNeutered, mIsWriteOnly, mExpandedReader);

dom/canvas/OffscreenCanvasDisplayHelper.cpp

@@ -61,6 +61,22 @@ void OffscreenCanvasDisplayHelper::DestroyCanvas() {
   mWorkerRef = nullptr;
 }
 
+bool OffscreenCanvasDisplayHelper::CanElementCaptureStream() const {
+  MutexAutoLock lock(mMutex);
+  return !!mWorkerRef;
+}
+
+bool OffscreenCanvasDisplayHelper::UsingElementCaptureStream() const {
+  MutexAutoLock lock(mMutex);
+
+  if (NS_WARN_IF(!NS_IsMainThread())) {
+    MOZ_ASSERT_UNREACHABLE("Should not call off main-thread!");
+    return !!mCanvasElement;
+  }
+
+  return mCanvasElement && mCanvasElement->UsingCaptureStream();
+}
+
 CanvasContextType OffscreenCanvasDisplayHelper::GetContextType() const {
   MutexAutoLock lock(mMutex);
   return mType;

dom/canvas/OffscreenCanvasDisplayHelper.h

@@ -57,6 +57,9 @@ class OffscreenCanvasDisplayHelper final {
   void DestroyCanvas();
   void DestroyElement();
 
+  bool CanElementCaptureStream() const;
+  bool UsingElementCaptureStream() const;
+
   already_AddRefed<mozilla::gfx::SourceSurface> GetSurfaceSnapshot();
   already_AddRefed<mozilla::layers::Image> GetAsImage();
   UniquePtr<uint8_t[]> GetImageBuffer(int32_t* aOutFormat,

dom/html/HTMLCanvasElement.cpp

@@ -857,6 +857,20 @@ already_AddRefed<CanvasCaptureMediaStream> HTMLCanvasElement::CaptureStream(
     return nullptr;
   }
 
+  // Check if we transferred the OffscreenCanvas to a DOM worker. This is not
+  // defined by the spec yet, so it is better to fail now than implement
+  // something not compliant:
+  // https://github.com/w3c/mediacapture-fromelement/issues/65
+  // https://github.com/w3c/mediacapture-extensions/pull/26
+  // https://github.com/web-platform-tests/wpt/issues/21102
+  if (mOffscreenDisplay &&
+      NS_WARN_IF(!mOffscreenDisplay->CanElementCaptureStream())) {
+    aRv.ThrowNotSupportedError(
+        "Capture stream not supported when OffscreenCanvas transferred to "
+        "worker");
+    return nullptr;
+  }
+
   auto stream = MakeRefPtr<CanvasCaptureMediaStream>(window, this);
 
   nsCOMPtr<nsIPrincipal> principal = NodePrincipal();

dom/html/HTMLCanvasElement.h

@@ -356,6 +356,8 @@ class HTMLCanvasElement final : public nsGenericHTMLElement,
 
   layers::ImageContainer* GetImageContainer() const { return mImageContainer; }
 
+  bool UsingCaptureStream() const { return !!mRequestedFrameRefreshObserver; }
+
  protected:
   bool mResetLayer;
   bool mMaybeModified;  // we fetched the context, so we may have written to the