Check for contiguous buffer state.

When we are reading large image data (i.e.: people using webp to stream
video instead of the native webm format; I'm looking at you, Giphy!)
we can run into the situation where the available data is not in a
contiguous buffer, and we need to either buffer additional data or
re-buffer from the start. If we don't do this, we can run into issues
because of buffer over-reading (causing corrupted data if allocated or
more likely crashes if not allocated).
Re-buffering is expensive, but this should be rare and limited to
dealing with unintended use for animated image formats.

This resolves #940.

1 files changed, 7 insertions, 0 deletions

diff --git a/image/SourceBuffer.h b/image/SourceBuffer.h
index 64727e65e..6f2c74d33 100644
--- a/image/SourceBuffer.h
+++ b/image/SourceBuffer.h
@@ -174,6 +174,13 @@ public:
     return mState == READY ? mData.mIterating.mNextReadLength : 0;
   }
 
+  /// If we're ready to read, returns whether or not everything available thus
+  /// far has been in the same contiguous buffer.
+  bool IsContiguous() const {
+    MOZ_ASSERT(mState == READY, "Calling IsContiguous() in the wrong state");
+    return mState == READY ? mData.mIterating.mChunk == 0 : false;
+  }
+  
   /// @return a count of the chunks we've advanced through.
   uint32_t ChunkCount() const { return mChunkCount; }