[cc] Calculate the correct latest_confirmed_sequence_number in cc::Scheduler.

cc/scheduler/scheduler.cc

 // Copyright 2011 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 "cc/scheduler/scheduler.h"
 
 #include <algorithm>
 
 #include "base/auto_reset.h"
 #include "base/logging.h"
@@ skipping 243 matching lines @@
 // BeginFrame is the mechanism that tells us that now is a good time to start
 // making a frame. Usually this means that user input for the frame is complete.
 // If the scheduler is busy, we queue the BeginFrame to be handled later as
 // a BeginRetroFrame.
 bool Scheduler::OnBeginFrameDerivedImpl(const BeginFrameArgs& args) {
   TRACE_EVENT1("cc,benchmark", "Scheduler::BeginFrame", "args", args.AsValue());
 
   if (!state_machine_.BeginFrameNeeded()) {
     TRACE_EVENT_INSTANT0("cc", "Scheduler::BeginFrameDropped",
                          TRACE_EVENT_SCOPE_THREAD);
-    // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
-    BeginFrameAck ack(args.source_id, args.sequence_number,
-                      args.sequence_number, 0, false);
-    begin_frame_source_->DidFinishFrame(this, ack);
+    // Since we don't use the BeginFrame, we may later receive the same
+    // BeginFrame again. Thus, we can't confirm it at this point, even though we
+    // don't have any updates right now.
+    SendBeginFrameAck(args, kBeginFrameSkipped);

[Eric Seckler, 2017/02/23 12:58:00]

Since we require an ack here now, I've added a comment and test for this.

     return false;
   }
 
   // Trace this begin frame time through the Chrome stack
   TRACE_EVENT_FLOW_BEGIN0(
       TRACE_DISABLED_BY_DEFAULT("cc.debug.scheduler.frames"), "BeginFrameArgs",
       args.frame_time.ToInternalValue());
 
   if (settings_.using_synchronous_renderer_compositor) {
     BeginImplFrameSynchronous(args);
@@ skipping 41 matching lines @@
   BeginFrameArgs adjusted_args = args;
   // Cancel the missed begin frame task in case the BFS sends a begin frame
   // before the missed frame task runs.
   missed_begin_frame_task_.Cancel();
 
   base::TimeTicks now = Now();
 
   // Discard missed begin frames if they are too late.
   if (adjusted_args.type == BeginFrameArgs::MISSED &&
       now > adjusted_args.deadline) {
-    // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
-    BeginFrameAck ack(adjusted_args.source_id, adjusted_args.sequence_number,
-                      adjusted_args.sequence_number, 0, false);
-    begin_frame_source_->DidFinishFrame(this, ack);
+    SendBeginFrameAck(adjusted_args, kBeginFrameSkipped);
     return;
   }
 
   // Run the previous deadline if any.
   if (state_machine_.begin_impl_frame_state() ==
       SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_INSIDE_BEGIN_FRAME) {
     OnBeginImplFrameDeadline();
     // We may not need begin frames any longer.
     if (!observing_begin_frame_source_) {
-      // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
-      BeginFrameAck ack(adjusted_args.source_id, adjusted_args.sequence_number,
-                        adjusted_args.sequence_number, 0, false);
-      begin_frame_source_->DidFinishFrame(this, ack);
+      // We need to confirm the ignored BeginFrame, since we don't have updates.
+      // To persist the confirmation for future BeginFrameAcks, we let the state
+      // machine know about the BeginFrame.
+      state_machine_.OnBeginFrameDroppedNotObserving(args.source_id,
+                                                     args.sequence_number);
+      SendBeginFrameAck(adjusted_args, kBeginFrameSkipped);
       return;
     }
   }
   DCHECK_EQ(state_machine_.begin_impl_frame_state(),
             SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
 
   bool main_thread_is_in_high_latency_mode =
       state_machine_.main_thread_missed_last_deadline();
   TRACE_EVENT2("cc,benchmark", "Scheduler::BeginImplFrame", "args",
                adjusted_args.AsValue(), "main_thread_missed_last_deadline",
@@ skipping 38 matching lines @@
                                                  bmf_to_activate_estimate, now);
 
   if (ShouldRecoverMainLatency(adjusted_args, can_activate_before_deadline)) {
     TRACE_EVENT_INSTANT0("cc", "SkipBeginMainFrameToReduceLatency",
                          TRACE_EVENT_SCOPE_THREAD);
     state_machine_.SetSkipNextBeginMainFrameToReduceLatency();
   } else if (ShouldRecoverImplLatency(adjusted_args,
                                       can_activate_before_deadline)) {
     TRACE_EVENT_INSTANT0("cc", "SkipBeginImplFrameToReduceLatency",
                          TRACE_EVENT_SCOPE_THREAD);
-    if (begin_frame_source_) {
-      // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
-      BeginFrameAck ack(adjusted_args.source_id, adjusted_args.sequence_number,
-                        adjusted_args.sequence_number, 0, false);
-      begin_frame_source_->DidFinishFrame(this, ack);
-    }
+    SendBeginFrameAck(begin_main_frame_args_, kBeginFrameSkipped);
     return;
   }
 
   BeginImplFrame(adjusted_args);
 }
 
 void Scheduler::BeginImplFrameSynchronous(const BeginFrameArgs& args) {
   TRACE_EVENT1("cc,benchmark", "Scheduler::BeginImplFrame", "args",
                args.AsValue());
 
   // The main thread currently can't commit before we draw with the
   // synchronous compositor, so never consider the BeginMainFrame fast.
   state_machine_.SetCriticalBeginMainFrameToActivateIsFast(false);
   begin_main_frame_args_ = args;
   begin_main_frame_args_.on_critical_path = !ImplLatencyTakesPriority();
 
   BeginImplFrame(args);
   compositor_timing_history_->WillFinishImplFrame(
       state_machine_.needs_redraw());
   FinishImplFrame();
 }
 
 void Scheduler::FinishImplFrame() {
   state_machine_.OnBeginImplFrameIdle();
   ProcessScheduledActions();
 
   client_->DidFinishImplFrame();
-  if (begin_frame_source_) {
-    // TODO(eseckler): Determine and set correct |ack.latest_confirmed_frame|.
-    BeginFrameAck ack(begin_main_frame_args_.source_id,
-                      begin_main_frame_args_.sequence_number,
-                      begin_main_frame_args_.sequence_number, 0,
-                      state_machine_.did_submit_in_last_frame());
-    begin_frame_source_->DidFinishFrame(this, ack);
+  SendBeginFrameAck(begin_main_frame_args_, kBeginFrameFinished);
+  begin_impl_frame_tracker_.Finish();
+}
+
+void Scheduler::SendBeginFrameAck(const BeginFrameArgs& args,
+                                  BeginFrameResult result) {
+  if (!begin_frame_source_)
+    return;
+
+  uint64_t latest_confirmed_sequence_number =
+      BeginFrameArgs::kInvalidFrameNumber;
+  if (args.source_id == state_machine_.begin_frame_source_id()) {
+    latest_confirmed_sequence_number =
+        state_machine_
+            .last_begin_frame_sequence_number_compositor_frame_was_fresh();
   }
-  begin_impl_frame_tracker_.Finish();
+
+  bool did_submit = false;
+  if (result == kBeginFrameFinished) {
+    did_submit = state_machine_.did_submit_in_last_frame();
+  }
+
+  BeginFrameAck ack(args.source_id, args.sequence_number,
+                    latest_confirmed_sequence_number, 0, did_submit);
+  begin_frame_source_->DidFinishFrame(this, ack);
 }
 
 // BeginImplFrame starts a compositor frame that will wait up until a deadline
 // for a BeginMainFrame+activation to complete before it times out and draws
 // any asynchronous animation and scroll/pinch updates.
 void Scheduler::BeginImplFrame(const BeginFrameArgs& args) {
   DCHECK_EQ(state_machine_.begin_impl_frame_state(),
             SchedulerStateMachine::BEGIN_IMPL_FRAME_STATE_IDLE);
   DCHECK(begin_impl_frame_deadline_task_.IsCancelled());
   DCHECK(state_machine_.HasInitializedCompositorFrameSink());
 
   begin_impl_frame_tracker_.Start(args);
-  state_machine_.OnBeginImplFrame();
+  state_machine_.OnBeginImplFrame(args.source_id, args.sequence_number);
   devtools_instrumentation::DidBeginFrame(layer_tree_host_id_);
   compositor_timing_history_->WillBeginImplFrame(
       state_machine_.NewActiveTreeLikely());
   client_->WillBeginImplFrame(begin_impl_frame_tracker_.Current());
 
   ProcessScheduledActions();
 }
 
 void Scheduler::ScheduleBeginImplFrameDeadline() {
   // The synchronous compositor does not post a deadline task.
@@ skipping 302 matching lines @@
 }
 
 bool Scheduler::IsBeginMainFrameSentOrStarted() const {
   return (state_machine_.begin_main_frame_state() ==
               SchedulerStateMachine::BEGIN_MAIN_FRAME_STATE_SENT ||
           state_machine_.begin_main_frame_state() ==
               SchedulerStateMachine::BEGIN_MAIN_FRAME_STATE_STARTED);
 }
 
 }  // namespace cc