MSE-in-Workers: Introduce MediaSourceTracer

In preparation for moving the MediaSource core interface into the
MediaSourceAttachment interface, this change factors out a necessary
class for retaining the existing same-thread Oilpan collection of idle
unreferenced attached HTMLMediaElement+MediaSource object groups.

Specifically, adds MediaSourceTracer to specialize further in later
changes in modules concrete implementations how to handle Oilpan tracing
of an HTMLMediaElement and a MediaSource in same- versus cross-thread
attachments. Includes same-thread concrete specialization in this
change. Having this as a separate entity reduces complexity and
fragility on both of those sides of the attachment. Note that a same
thread non-oilpan concrete attachment cannot use "Member" since the
attachment class is unmanaged. Hence the need for this MediaSourceTracer
to encapsulate that "Member" linkage.

BUG=878133

Change-Id: I4153ade37a1d4303af2604c2c2e18f701a79834d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2363703Reviewed-by: Philip Jägenstedt <foolip@chromium.org>
Commit-Queue: Matthew Wolenetz <wolenetz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800166}

third_party/blink/renderer/core/html/BUILD.gn

@@ -500,6 +500,8 @@ blink_core_sources("html") {
     "media/media_source_attachment.cc",
     "media/media_source_attachment.h",
     "media/media_source_registry.h",
+    "media/media_source_tracer.cc",
+    "media/media_source_tracer.h",
     "media/picture_in_picture_interstitial.cc",
     "media/picture_in_picture_interstitial.h",
     "media/remote_playback_controller.cc",

third_party/blink/renderer/core/html/media/html_media_element.cc

@@ -76,6 +76,7 @@
 #include "third_party/blink/renderer/core/html/media/media_fragment_uri_parser.h"
 #include "third_party/blink/renderer/core/html/media/media_source.h"
 #include "third_party/blink/renderer/core/html/media/media_source_attachment.h"
+#include "third_party/blink/renderer/core/html/media/media_source_tracer.h"
 #include "third_party/blink/renderer/core/html/time_ranges.h"
 #include "third_party/blink/renderer/core/html/track/audio_track.h"
 #include "third_party/blink/renderer/core/html/track/audio_track_list.h"
@@ -1196,12 +1197,14 @@ void HTMLMediaElement::LoadResource(const WebMediaPlayerSource& source,
   SetPlayerPreload();
 
   DCHECK(!media_source_);
+  DCHECK(!media_source_tracer_);
 
   bool attempt_load = true;
 
   media_source_ = MediaSourceAttachment::LookupMediaSource(url.GetString());
   if (media_source_) {
-    if (media_source_->StartAttachingToMediaElement(this)) {
+    media_source_tracer_ = media_source_->StartAttachingToMediaElement(this);
+    if (media_source_tracer_) {
       // If the associated feature is enabled, auto-revoke the MediaSource
       // object URL that was used for attachment on successful (start of)
       // attachment. This can help reduce memory bloat later if the app does not
@@ -2633,6 +2636,7 @@ void HTMLMediaElement::CloseMediaSource() {
 
   media_source_->Close();
   media_source_ = nullptr;
+  media_source_tracer_ = nullptr;
 }
 
 bool HTMLMediaElement::Loop() const {
@@ -4057,6 +4061,7 @@ void HTMLMediaElement::Trace(Visitor* visitor) const {
   visitor->Trace(current_source_node_);
   visitor->Trace(next_child_node_to_consider_);
   visitor->Trace(media_source_);
+  visitor->Trace(media_source_tracer_);
   visitor->Trace(audio_tracks_);
   visitor->Trace(video_tracks_);
   visitor->Trace(cue_timeline_);

third_party/blink/renderer/core/html/media/html_media_element.h

@@ -66,6 +66,7 @@ class EventQueue;
 class ExceptionState;
 class HTMLMediaElementControlsList;
 class MediaSource;
+class MediaSourceTracer;
 class HTMLSourceElement;
 class HTMLTrackElement;
 class MediaError;
@@ -631,6 +632,7 @@ class CORE_EXPORT HTMLMediaElement
   cc::Layer* cc_layer_;
 
   Member<MediaSource> media_source_;
+  Member<MediaSourceTracer> media_source_tracer_;
 
   // Stores "official playback position", updated periodically from "current
   // playback position". Official playback position should not change while

third_party/blink/renderer/core/html/media/media_source.h

@@ -34,6 +34,7 @@
 #include <memory>
 #include "third_party/blink/public/platform/web_time_range.h"
 #include "third_party/blink/renderer/core/core_export.h"
+#include "third_party/blink/renderer/core/html/media/media_source_tracer.h"
 #include "third_party/blink/renderer/platform/heap/handle.h"
 #include "third_party/blink/renderer/platform/wtf/forward.h"
 
@@ -61,9 +62,11 @@ class CORE_EXPORT MediaSource : public GarbageCollectedMixin {
   // attempting to attach to this object.  The WebMediaSource is not available
   // to the element initially, so between the two calls, the attachment could be
   // considered partially setup.
-  // If already attached, StartAttachingToMediaElement() returns false.
-  // Otherwise, must be in 'closed' state, and returns true to indicate
-  // attachment success.
+  // If already attached, StartAttachingToMediaElement() returns nullptr.
+  // Otherwise, must be in 'closed' state, and indicates success by returning a
+  // tracer object useful in at least same-thread attachments for enabling
+  // automatic idle unreferenced same-thread attachment object garbage
+  // collection.
   // CompleteAttachingToMediaElement() provides the MediaSource with the
   // underlying WebMediaSource, enabling parsing of media provided by the
   // application for playback, for example.
@@ -71,7 +74,8 @@ class CORE_EXPORT MediaSource : public GarbageCollectedMixin {
   // 'closed').
   // Once attached, the source uses the element to synchronously service some
   // API operations like duration change that may need to initiate seek.
-  virtual bool StartAttachingToMediaElement(HTMLMediaElement*) = 0;
+  virtual MediaSourceTracer* StartAttachingToMediaElement(
+      HTMLMediaElement*) = 0;
   virtual void CompleteAttachingToMediaElement(
       std::unique_ptr<WebMediaSource>) = 0;
 

third_party/blink/renderer/core/html/media/media_source_tracer.cc

+// Copyright 2020 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 "third_party/blink/renderer/core/html/media/media_source_tracer.h"
+
+namespace blink {
+
+void MediaSourceTracer::Trace(Visitor* visitor) const {}
+
+}  // namespace blink

third_party/blink/renderer/core/html/media/media_source_tracer.h

+// Copyright 2020 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.
+
+#ifndef THIRD_PARTY_BLINK_RENDERER_CORE_HTML_MEDIA_MEDIA_SOURCE_TRACER_H_
+#define THIRD_PARTY_BLINK_RENDERER_CORE_HTML_MEDIA_MEDIA_SOURCE_TRACER_H_
+
+#include "third_party/blink/renderer/core/core_export.h"
+#include "third_party/blink/renderer/platform/heap/handle.h"
+
+namespace blink {
+
+// Interface that encapsulates the Oilpan tracing of either same-thread or
+// cross-thread attachment of an HTMLMediaElement and MediaSource, necessary
+// since MediaSourceAttachments themselves are not managed by Oilpan. See
+// concrete implementation(s) in modules/mediasource.
+class CORE_EXPORT MediaSourceTracer
+    : public GarbageCollected<MediaSourceTracer> {
+ public:
+  virtual ~MediaSourceTracer() = default;
+  virtual void Trace(Visitor*) const;
+};
+
+}  // namespace blink
+
+#endif  // THIRD_PARTY_BLINK_RENDERER_CORE_HTML_MEDIA_MEDIA_SOURCE_TRACER_H_

third_party/blink/renderer/modules/mediasource/BUILD.gn

@@ -12,6 +12,8 @@ blink_modules_sources("mediasource") {
     "media_source_impl.h",
     "media_source_registry_impl.cc",
     "media_source_registry_impl.h",
+    "media_source_tracer_impl.cc",
+    "media_source_tracer_impl.h",
     "source_buffer.cc",
     "source_buffer.h",
     "source_buffer_list.cc",

third_party/blink/renderer/modules/mediasource/media_source_impl.cc

@@ -43,6 +43,7 @@
 #include "third_party/blink/renderer/core/html/media/html_media_element.h"
 #include "third_party/blink/renderer/core/html/track/audio_track_list.h"
 #include "third_party/blink/renderer/core/html/track/video_track_list.h"
+#include "third_party/blink/renderer/modules/mediasource/media_source_tracer_impl.h"
 #include "third_party/blink/renderer/modules/mediasource/source_buffer_track_base_supplement.h"
 #include "third_party/blink/renderer/platform/bindings/exception_messages.h"
 #include "third_party/blink/renderer/platform/bindings/exception_state.h"
@@ -831,9 +832,10 @@ void MediaSourceImpl::Close() {
   SetReadyState(ClosedKeyword());
 }
 
-bool MediaSourceImpl::StartAttachingToMediaElement(HTMLMediaElement* element) {
+MediaSourceTracer* MediaSourceImpl::StartAttachingToMediaElement(
+    HTMLMediaElement* element) {
   if (attached_element_)
-    return false;
+    return nullptr;
 
   DCHECK(IsClosed());
 
@@ -841,7 +843,7 @@ bool MediaSourceImpl::StartAttachingToMediaElement(HTMLMediaElement* element) {
       "media", "MediaSourceImpl::StartAttachingToMediaElement",
       TRACE_ID_LOCAL(this));
   attached_element_ = element;
-  return true;
+  return MakeGarbageCollected<MediaSourceTracerImpl>(element, this);
 }
 
 void MediaSourceImpl::OpenIfInEndedState() {

third_party/blink/renderer/modules/mediasource/media_source_impl.h

@@ -37,6 +37,7 @@
 #include "third_party/blink/renderer/bindings/core/v8/active_script_wrappable.h"
 #include "third_party/blink/renderer/core/execution_context/execution_context_lifecycle_observer.h"
 #include "third_party/blink/renderer/core/html/media/media_source.h"
+#include "third_party/blink/renderer/core/html/media/media_source_tracer.h"
 #include "third_party/blink/renderer/core/html/time_ranges.h"
 #include "third_party/blink/renderer/modules/event_target_modules.h"
 #include "third_party/blink/renderer/modules/mediasource/source_buffer.h"
@@ -92,7 +93,7 @@ class MediaSourceImpl final : public EventTargetWithInlineData,
   static bool isTypeSupported(const String& type);
 
   // html/media/MediaSource interface implementation
-  bool StartAttachingToMediaElement(HTMLMediaElement*) override;
+  MediaSourceTracer* StartAttachingToMediaElement(HTMLMediaElement*) override;
   void CompleteAttachingToMediaElement(
       std::unique_ptr<WebMediaSource>) override;
   void Close() override;

third_party/blink/renderer/modules/mediasource/media_source_tracer_impl.cc

+// Copyright 2020 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 "third_party/blink/renderer/modules/mediasource/media_source_tracer_impl.h"
+
+#include "third_party/blink/renderer/core/html/media/html_media_element.h"
+#include "third_party/blink/renderer/modules/mediasource/media_source_impl.h"
+
+namespace blink {
+
+MediaSourceTracerImpl::MediaSourceTracerImpl(HTMLMediaElement* media_element,
+                                             MediaSourceImpl* media_source)
+    : media_element_(media_element), media_source_(media_source) {}
+
+void MediaSourceTracerImpl::Trace(Visitor* visitor) const {
+  visitor->Trace(media_element_);
+  visitor->Trace(media_source_);
+  MediaSourceTracer::Trace(visitor);
+}
+
+}  // namespace blink

third_party/blink/renderer/modules/mediasource/media_source_tracer_impl.h

+// Copyright 2020 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.
+
+#ifndef THIRD_PARTY_BLINK_RENDERER_MODULES_MEDIASOURCE_MEDIA_SOURCE_TRACER_IMPL_H_
+#define THIRD_PARTY_BLINK_RENDERER_MODULES_MEDIASOURCE_MEDIA_SOURCE_TRACER_IMPL_H_
+
+#include "third_party/blink/renderer/core/html/media/media_source_tracer.h"
+#include "third_party/blink/renderer/platform/heap/handle.h"
+
+namespace blink {
+
+class HTMLMediaElement;
+class MediaSourceImpl;
+
+// Concrete MediaSourceTracer that enables an HTMLMediaElement and its attached
+// MediaSourceImpl on the same (main) thread to trace into each other. This
+// enables garbage collection to automatically detect and collect idle
+// attachments of these objects that have no other strong references.
+class MediaSourceTracerImpl final : public MediaSourceTracer {
+ public:
+  MediaSourceTracerImpl(HTMLMediaElement* media_element,
+                        MediaSourceImpl* media_source);
+  ~MediaSourceTracerImpl() override = default;
+
+  void Trace(Visitor* visitor) const override;
+
+ private:
+  Member<HTMLMediaElement> media_element_;
+  Member<MediaSourceImpl> media_source_;
+};
+
+}  // namespace blink
+
+#endif  // THIRD_PARTY_BLINK_RENDERER_MODULES_MEDIASOURCE_MEDIA_SOURCE_TRACER_IMPL_H_