MSE: Implement TrackDefaultList object

Source/modules/mediasource/TrackDefaultList.cpp

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "config.h"
+
+#include "modules/mediasource/TrackDefaultList.h"
+
+#include "bindings/core/v8/ExceptionState.h"
+#include "core/dom/ExceptionCode.h"
+
+namespace blink {
+
+TrackDefaultList* TrackDefaultList::create(const HeapVector<Member<TrackDefault>>& trackDefaults, ExceptionState& exceptionState)
+{
+    // Per 11 Dec 2014 Editor's Draft
+    // https://w3c.github.io/media-source/#trackdefaultlist
+    // When this method is invoked, the user agent must run the following steps:
+    // 1. If |trackDefaults| contains two or more TrackDefault objects with the
+    //    same type and the same byteStreamTrackID, then throw an
+    //    InvalidAccessError and abort these steps.
+    //    Note: This also applies when byteStreamTrackID contains an empty
+    //    string and ensures that there is only one "byteStreamTrackID
+    //    independent" default for each TrackDefaultType value.
+    TypeAndIDToTrackDefaultMap typeAndIDToTrackDefaultMap;

[sof, 2014/12/12 09:37:00]

How about inverting this and create the empty object first, proceeding to fill
up its hash map and vector? As it is now, you create a hash map locally and pass
it, causing a new copy to be created. Which seems wasteful.

(The vector reference is copied also, so the comment made earlier from another
reviewer about potential use-after-free, is not an issue afaict.) 

[wolenetz, 2014/12/12 19:18:31]

Haha :). Yeah, I noticed there was an extra copy once I moved the logic out of
ctor and was considering passing a pointer to a TypeAndIDToTrackDefaultMap, but
thought that would introduce maintainer confusion down the road. The UAF comment
was on the SourceBuffer.trackDefaults CR @
https://codereview.chromium.org/754463008/diff/1/Source/modules/mediasource/S....
I haven't looked deeper into that one yet but I believe I was similarly confused
about how a UAF could occur since this ::create copies the supplied const&
HeapVector.

Sure, I'll update the object to have some private initialize method in the next
step (which would throw exception). 

[philipj_slow, 2014/12/12 20:25:21]

typeAndIDToTrackDefaultMap ought to be possible to contain to this function
alone, per the comment in the header.

[wolenetz, 2014/12/12 21:48:05]

Done.

+    for (const auto& trackDefault : trackDefaults) {
+        TypeAndID key = TypeAndID(trackDefault->type(), trackDefault->byteStreamTrackID());
+        if (!typeAndIDToTrackDefaultMap.add(key, trackDefault).isNewEntry) {
+            exceptionState.throwDOMException(InvalidAccessError, "Duplicate TrackDefault type (" + key.first + ") and byteStreamTrackID (" + key.second + ")");
+            return nullptr;
+        }
+    }
+
+    // 2. Store a shallow copy of |trackDefaults| in this new object so the values can
+    //    be returned by the accessor methods.
+    // This step is done in constructor initializer.
+    return new TrackDefaultList(trackDefaults, typeAndIDToTrackDefaultMap);
+}
+
+TrackDefault* TrackDefaultList::item(unsigned index) const
+{
+    // Per 11 Dec 2014 Editor's Draft
+    // https://w3c.github.io/media-source/#trackdefaultlist
+    // When this method is invoked, the user agent must run the following steps:
+    // 1. If |index| is greater than or equal to the length attribute then
+    //    return undefined and abort these steps.
+    if (index >= m_trackDefaults.size())
+        return 0;
+
+    // 2. Return the |index|'th TrackDefault object in the list.
+    return m_trackDefaults[index].get();
+}
+
+TrackDefaultList::TrackDefaultList(const HeapVector<Member<TrackDefault>>& trackDefaults, const TypeAndIDToTrackDefaultMap& typeAndIDToTrackDefaultMap)
+    : m_trackDefaults(trackDefaults)
+    , m_typeAndIDToTrackDefaultMap(typeAndIDToTrackDefaultMap)
+{
+}
+
+void TrackDefaultList::trace(Visitor* visitor)
+{
+    visitor->trace(m_trackDefaults);
+    visitor->trace(m_typeAndIDToTrackDefaultMap);
+}
+
+} // namespace blink