Bug 1383682 - Part 3. Prevent imgRequestProxy from leaking the current state when validating. r=tnikkel

There are two other means from which a caller can get the current state
which originally ignored validation -- GetImageStatus and
StartDecodingWithResult. These methods are used by layout in some
circumstances to decide whether or not the image is ready to display. As
observed in some web platform tests, in particular
css/css-backgrounds-3/background-size-031.html, we may actually validate
and purge the cache for images under test. The state given by the
aforementioned methods was misleading, because validation changed it.
Now they take into account validation, and do not imply any particular
state while validation is in progress.

image/imgRequestProxy.cpp

@@ -560,8 +560,11 @@ imgRequestProxy::CancelAndForgetObserver(nsresult aStatus)
 NS_IMETHODIMP
 imgRequestProxy::StartDecoding(uint32_t aFlags)
 {
-  // Flag this, so we know to transfer the request if our owner changes
-  mDecodeRequested = true;
+  // Flag this, so we know to request after validation if pending.
+  if (IsValidating()) {
+    mDecodeRequested = true;
+    return NS_OK;
+  }
 
   RefPtr<Image> image = GetImage();
   if (image) {
@@ -578,8 +581,11 @@ imgRequestProxy::StartDecoding(uint32_t aFlags)
 bool
 imgRequestProxy::StartDecodingWithResult(uint32_t aFlags)
 {
-  // Flag this, so we know to transfer the request if our owner changes
-  mDecodeRequested = true;
+  // Flag this, so we know to request after validation if pending.
+  if (IsValidating()) {
+    mDecodeRequested = true;
+    return false;
+  }
 
   RefPtr<Image> image = GetImage();
   if (image) {
@@ -736,8 +742,16 @@ imgRequestProxy::GetImage(imgIContainer** aImage)
 NS_IMETHODIMP
 imgRequestProxy::GetImageStatus(uint32_t* aStatus)
 {
-  RefPtr<ProgressTracker> progressTracker = GetProgressTracker();
-  *aStatus = progressTracker->GetImageStatus();
+  if (IsValidating()) {
+    // We are currently validating the image, and so our status could revert if
+    // we discard the cache. We should also be deferring notifications, such
+    // that the caller will be notified when validation completes. Rather than
+    // risk misleading the caller, return nothing.
+    *aStatus = imgIRequest::STATUS_NONE;
+  } else {
+    RefPtr<ProgressTracker> progressTracker = GetProgressTracker();
+    *aStatus = progressTracker->GetImageStatus();
+  }
 
   return NS_OK;
 }