cc: Introduce BeginFrame sequence numbers and acknowledgements.

cc/scheduler/begin_frame_source.h

 // 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.
 
 #ifndef CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_
 #define CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_
 
 #include <stddef.h>
 #include <stdint.h>
 
 #include <set>
 #include <string>
+#include <unordered_set>
 
 #include "base/logging.h"
 #include "base/macros.h"
 #include "base/trace_event/trace_event.h"
 #include "cc/output/begin_frame_args.h"
 #include "cc/scheduler/delay_based_time_source.h"
 
 namespace cc {
 
 // (Pure) Interface for observing BeginFrame messages from BeginFrameSource
 // objects.
 class CC_EXPORT BeginFrameObserver {
  public:
   virtual ~BeginFrameObserver() {}
 
   // The |args| given to OnBeginFrame is guaranteed to have
-  // |args|.IsValid()==true and have |args|.frame_time
-  // field be strictly greater than the previous call.
+  // |args|.IsValid()==true. If |args|.source_id did not change between
+  // invocations, |args|.sequence_number is guaranteed to be be strictly greater
+  // than the previous call. Further, |args|.frame_time is guaranteed to be
+  // greater than or equal to the previous call.
   //
   // Side effects: This function can (and most of the time *will*) change the
   // return value of the LastUsedBeginFrameArgs method. See the documentation
   // on that method for more information.
   virtual void OnBeginFrame(const BeginFrameArgs& args) = 0;
 
   // Returns the last BeginFrameArgs used by the observer. This method's return
   // value is affected by the OnBeginFrame method!
   //
   //  - Before the first call of OnBeginFrame, this method should return a
@@ skipping 51 matching lines @@
 // BeginFrameObserver.
 //
 // BeginFrame calls *normally* occur just after a vsync interrupt when input
 // processing has been finished and provide information about the time values
 // of the vsync times. *However*, these values can be heavily modified or even
 // plain made up (when no vsync signal is available or vsync throttling is
 // turned off). See the BeginFrameObserver for information about the guarantees
 // all BeginFrameSources *must* provide.
 class CC_EXPORT BeginFrameSource {
  public:
+  BeginFrameSource();
   virtual ~BeginFrameSource() {}
 
-  // DidFinishFrame provides back pressure to a frame source about frame
-  // processing (rather than toggling SetNeedsBeginFrames every frame). It is
-  // used by systems like the BackToBackFrameSource to make sure only one frame
-  // is pending at a time.
+  // BeginFrameObservers use DidFinishFrame to acknowledge that they have
+  // completed handling a BeginFrame.
+  //
+  // The DisplayScheduler uses these acknowledgments to trigger an early
+  // deadline once all BeginFrameObservers have completed a frame.
+  //
+  // They also provide back pressure to a frame source about frame processing
+  // (rather than toggling SetNeedsBeginFrames every frame). For example, the
+  // BackToBackFrameSource uses them to make sure only one frame is pending at a
+  // time.
   virtual void DidFinishFrame(BeginFrameObserver* obs,

[Sami, 2016/12/07 16:59:40]

Idle thought: should we have some debug safeguard that makes sure this is
actually called? Of course it's possible that the renderer crashes and we never
hear back.

[Eric Seckler, 2016/12/08 17:54:28]

Good question. Not sure if a check is easily enforceable, since it could take a
while for the ack to come back (even post the next BeginFrame).

-                              size_t remaining_frames) = 0;
+                              const BeginFrameAck& ack) = 0;
 
   // Add/Remove an observer from the source. When no observers are added the BFS
   // should shut down its timers, disable vsync, etc.
   virtual void AddObserver(BeginFrameObserver* obs) = 0;
   virtual void RemoveObserver(BeginFrameObserver* obs) = 0;
 
   // Returns false if the begin frame source will just continue to produce
   // begin frames without waiting.
   virtual bool IsThrottled() const = 0;
+
+  // Returns an identifier for this BeginFrameSource. Guaranteed unique within a
+  // process, and very likely unique across processes.
+  virtual uint64_t source_id() const;
+
+ private:
+  uint64_t source_id_;
 };
 
 // A BeginFrameSource that does nothing.
 class CC_EXPORT StubBeginFrameSource : public BeginFrameSource {
  public:
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const BeginFrameAck& ack) override {}
   void AddObserver(BeginFrameObserver* obs) override {}
   void RemoveObserver(BeginFrameObserver* obs) override {}
   bool IsThrottled() const override;
 };
 
 // A frame source which ticks itself independently.
 class CC_EXPORT SyntheticBeginFrameSource : public BeginFrameSource {
  public:
   ~SyntheticBeginFrameSource() override;
 
   virtual void OnUpdateVSyncParameters(base::TimeTicks timebase,
                                        base::TimeDelta interval) = 0;
   // This overrides any past or future interval from updating vsync parameters.
   virtual void SetAuthoritativeVSyncInterval(base::TimeDelta interval) = 0;
 };
 
 // A frame source which calls BeginFrame (at the next possible time) as soon as
 // remaining frames reaches zero.
 class CC_EXPORT BackToBackBeginFrameSource : public SyntheticBeginFrameSource,
                                              public DelayBasedTimeSourceClient {
  public:
   explicit BackToBackBeginFrameSource(
       std::unique_ptr<DelayBasedTimeSource> time_source);
   ~BackToBackBeginFrameSource() override;
 
   // BeginFrameSource implementation.
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override;
+                      const BeginFrameAck& ack) override;
   bool IsThrottled() const override;
 
   // SyntheticBeginFrameSource implementation.
   void OnUpdateVSyncParameters(base::TimeTicks timebase,
                                base::TimeDelta interval) override {}
   void SetAuthoritativeVSyncInterval(base::TimeDelta interval) override {}
 
   // DelayBasedTimeSourceClient implementation.
   void OnTimerTick() override;
 
  private:
   std::unique_ptr<DelayBasedTimeSource> time_source_;
   std::unordered_set<BeginFrameObserver*> observers_;
   std::unordered_set<BeginFrameObserver*> pending_begin_frame_observers_;
+  uint64_t next_sequence_number_;
   base::WeakPtrFactory<BackToBackBeginFrameSource> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(BackToBackBeginFrameSource);
 };
 
 // A frame source which is locked to an external parameters provides from a
 // vsync source and generates BeginFrameArgs for it.
 class CC_EXPORT DelayBasedBeginFrameSource : public SyntheticBeginFrameSource,
                                              public DelayBasedTimeSourceClient {
  public:
   explicit DelayBasedBeginFrameSource(
       std::unique_ptr<DelayBasedTimeSource> time_source);
   ~DelayBasedBeginFrameSource() override;
 
   // BeginFrameSource implementation.
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const BeginFrameAck& ack) override {}
   bool IsThrottled() const override;
 
   // SyntheticBeginFrameSource implementation.
   void OnUpdateVSyncParameters(base::TimeTicks timebase,
                                base::TimeDelta interval) override;
   void SetAuthoritativeVSyncInterval(base::TimeDelta interval) override;
 
   // DelayBasedTimeSourceClient implementation.
   void OnTimerTick() override;
 
  private:
-  BeginFrameArgs CreateBeginFrameArgs(base::TimeTicks frame_time,
+  BeginFrameArgs CreateBeginFrameArgs(uint64_t sequence_number,
+                                      base::TimeTicks frame_time,
                                       BeginFrameArgs::BeginFrameArgsType type);
 
   std::unique_ptr<DelayBasedTimeSource> time_source_;
   std::unordered_set<BeginFrameObserver*> observers_;
   base::TimeTicks last_timebase_;
   base::TimeDelta authoritative_interval_;
+  uint64_t current_sequence_number_;
+  base::TimeTicks current_begin_frame_time_;
 
   DISALLOW_COPY_AND_ASSIGN(DelayBasedBeginFrameSource);
 };
 
+// Helper class that tracks outstanding acknowledgments from
+// BeginFrameObservers.
+class CC_EXPORT BeginFrameObserverAckTracker {
+ public:
+  BeginFrameObserverAckTracker();
+  virtual ~BeginFrameObserverAckTracker();
+
+  // The BeginFrameSource uses these methods to notify us when a BeginFrame was
+  // started and sent to a specific observer, an observer finished a frame, or
+  // an observer was added/removed.
+  void OnBeginFrame(const BeginFrameArgs& args);
+  void OnObserverBeginFrame(BeginFrameObserver* obs,
+                            const BeginFrameArgs& args);
+  void OnObserverFinishedFrame(BeginFrameObserver* obs,
+                               const BeginFrameAck& ack);
+  void OnObserverAdded(BeginFrameObserver* obs);
+  void OnObserverRemoved(BeginFrameObserver* obs);
+
+  // Returns |true| if all the source's observers completed the current frame.
+  bool AllObserversFinishedFrame() const;
+
+  // Returns |true| if any observer had updates during the current frame.
+  bool AnyObserversHadUpdates() const;
+
+  // Returns the sequence number of the oldest_incorporated_frame that any of
+  // the observers have contributed for the latest frame.
+  uint64_t OldestIncorporatedFrame() const;
+
+ private:
+  void SourceChanged(const BeginFrameArgs& args);
+
+  uint64_t current_source_id_;
+  uint64_t current_sequence_number_;
+  std::set<BeginFrameObserver*> observers_;
+  std::set<BeginFrameObserver*> finished_observers_;
+  bool observers_had_updates_;
+  std::unordered_map<BeginFrameObserver*, uint64_t> oldest_incorporated_frames_;
+};
+
 class CC_EXPORT ExternalBeginFrameSourceClient {
  public:
   // Only called when changed.  Assumed false by default.
   virtual void OnNeedsBeginFrames(bool needs_begin_frames) = 0;
+  // Called when all observers have completed a frame.
+  virtual void OnDidFinishFrame(const BeginFrameAck& ack) = 0;
 };
 
 // A BeginFrameSource that is only ticked manually.  Usually the endpoint
 // of messages from some other thread/process that send OnBeginFrame and
 // receive SetNeedsBeginFrame messages.  This turns such messages back into
 // an observable BeginFrameSource.
 class CC_EXPORT ExternalBeginFrameSource : public BeginFrameSource {
  public:
   // Client lifetime must be preserved by owner past the lifetime of this class.
   explicit ExternalBeginFrameSource(ExternalBeginFrameSourceClient* client);
   ~ExternalBeginFrameSource() override;
 
   // BeginFrameSource implementation.
   void AddObserver(BeginFrameObserver* obs) override;
   void RemoveObserver(BeginFrameObserver* obs) override;
   void DidFinishFrame(BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const BeginFrameAck& ack) override;
   bool IsThrottled() const override;
 
   void OnSetBeginFrameSourcePaused(bool paused);
   void OnBeginFrame(const BeginFrameArgs& args);
 
  protected:
+  void MaybeFinishFrame();
+  void FinishFrame();
+
   BeginFrameArgs missed_begin_frame_args_;
   std::unordered_set<BeginFrameObserver*> observers_;
   ExternalBeginFrameSourceClient* client_;
   bool paused_ = false;
+  bool frame_active_ = false;
+  BeginFrameObserverAckTracker ack_tracker_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(ExternalBeginFrameSource);
 };
 
+// A BeginFrameSource that delegates to another given source.
+class CC_EXPORT DelegatingBeginFrameSource : public BeginFrameSource {
+ public:
+  // |delegate| lifetime must be preserved past the lifetime of this class.
+  explicit DelegatingBeginFrameSource(BeginFrameSource* delegate);
+  ~DelegatingBeginFrameSource() override {}
+
+  // BeginFrameSource implementation.
+  void DidFinishFrame(BeginFrameObserver* obs,
+                      const BeginFrameAck& ack) override;
+  void AddObserver(BeginFrameObserver* obs) override;
+  void RemoveObserver(BeginFrameObserver* obs) override;
+  bool IsThrottled() const override;
+  uint64_t source_id() const override;
+
+ private:
+  BeginFrameSource* delegate_;  // Not owned.
+};
+
 }  // namespace cc
 
 #endif  // CC_SCHEDULER_BEGIN_FRAME_SOURCE_H_