Bug 1473173 - Report MediaResource as seekable if we know we will never seek into underlying transport. r=bryce

The MediaCache reads content by block of BLOCK_SIZE bytes (currently 32kB). As such, if a content being fetched is less than BLOCK_SIZE, it will always be fully read from offset = 0. We can then seek within this buffered content regardless of the underlying HTTP connection supporting request-range or not.

test_bug686942.html is ultimately testing a racy behaviour: it attempts to seek into a resource that isn't always seekable (http.js doesn't report Range -Request support for small files). The seekable range becomes then the buffered range which at the time of loadedmetadata may not be set yet.

Differential Revision: https://phabricator.services.mozilla.com/D3028

--HG--
extra : moz-landing-system : lando

dom/media/ChannelMediaResource.cpp

@@ -295,6 +295,9 @@ ChannelMediaResource::OnStartRequest(nsIRequest* aRequest,
 
   mCacheStream.NotifyDataStarted(mLoadID, startOffset, seekable, length);
   mIsTransportSeekable = seekable;
+  if (mFirstReadLength < 0) {
+    mFirstReadLength = length;
+  }
 
   mSuspendAgent.Delegate(mChannel);
 
@@ -317,7 +320,14 @@ bool
 ChannelMediaResource::IsTransportSeekable()
 {
   MOZ_ASSERT(NS_IsMainThread());
-  return mIsTransportSeekable;
+  // We Report the transport as seekable if we know we will never seek into
+  // the underlying transport. As the MediaCache reads content by block of
+  // BLOCK_SIZE bytes, so the content length is less it will always be fully
+  // read from offset = 0 and we can then always successfully seek within this
+  // buffered content.
+  return mIsTransportSeekable ||
+         (mFirstReadLength > 0 &&
+          mFirstReadLength < MediaCacheStream::BLOCK_SIZE);
 }
 
 nsresult

dom/media/ChannelMediaResource.h

@@ -255,6 +255,8 @@ protected:
   bool mClosed = false;
   // The last reported seekability state for the underlying channel
   bool mIsTransportSeekable = false;
+  // Length of the content first reported.
+  int64_t mFirstReadLength = -1;
   RefPtr<Listener> mListener;
   // A mono-increasing integer to uniquely identify the channel we are loading.
   uint32_t mLoadID = 0;