Bug 1775495 - Avoid trigger extra async image decodes when sync decod…

…es are requested. r=jrmuizel

When we request a sync decode, if there is an outstanding pending
async decode, but we don't have all the network data, we would end up
triggering an extra async decode. This patch ensures that we only
trigger sync decodes if they will actually execute as sync.

Differential Revision: https://phabricator.services.mozilla.com/D150008

image/RasterImage.cpp

@@ -339,12 +339,25 @@ LookupResult RasterImage::LookupFrame(const OrientedIntSize& aSize,
     return LookupResult(MatchType::NOT_FOUND);
   }
 
+  // We want to trigger a decode if and only if:
+  // 1) There is no pending decode
+  // 2) There is no acceptable size decoded
+  // 3) The pending decode has not produced a frame yet, a sync decode is
+  // requested, and we have all the source data. Without the source data, we
+  // will just trigger another async decode anyways.
+  //
+  // TODO(aosmond): We should better handle case 3. We should actually return
+  // TEMPORARY_ERROR or NOT_READY if we don't have all the source data and a
+  // sync decode is requested. If there is a pending decode and we have all the
+  // source data, we should always be able to block on the frame's monitor --
+  // perhaps this could be accomplished by preallocating the first frame buffer
+  // when we create the decoder.
   const bool syncDecode = aFlags & FLAG_SYNC_DECODE;
   const bool avoidRedecode = aFlags & FLAG_AVOID_REDECODE_FOR_SIZE;
   if (result.Type() == MatchType::NOT_FOUND ||
       (result.Type() == MatchType::SUBSTITUTE_BECAUSE_NOT_FOUND &&
        !avoidRedecode) ||
-      (syncDecode && !avoidRedecode && !result)) {
+      (syncDecode && !avoidRedecode && !result && LoadAllSourceData())) {
     // We don't have a copy of this frame, and there's no decoder working on
     // one. (Or we're sync decoding and the existing decoder hasn't even started
     // yet.) Trigger decoding so it'll be available next time.