Check for BeginFrame time/seqnum continuity in ExternalBFS::OnBF.

cc/scheduler/begin_frame_source.cc

 // 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 "cc/scheduler/begin_frame_source.h"
 
 #include <stddef.h>
 
 #include "base/atomic_sequence_num.h"
 #include "base/auto_reset.h"
@@ skipping 243 matching lines @@
   DCHECK(observers_.find(obs) == observers_.end());
 
   bool observers_was_empty = observers_.empty();
   observers_.insert(obs);
   obs->OnBeginFrameSourcePausedChanged(paused_);
   if (observers_was_empty)
     client_->OnNeedsBeginFrames(true);
 
   // Send a MISSED begin frame if necessary.
   if (missed_begin_frame_args_.IsValid()) {
-    BeginFrameArgs last_args = obs->LastUsedBeginFrameArgs();
+    const BeginFrameArgs& last_args = obs->LastUsedBeginFrameArgs();
     if (!last_args.IsValid() ||
         (missed_begin_frame_args_.frame_time > last_args.frame_time)) {
       DCHECK((missed_begin_frame_args_.source_id != last_args.source_id) ||
              (missed_begin_frame_args_.sequence_number >
               last_args.sequence_number))
           << "current " << missed_begin_frame_args_.AsValue()->ToString()
           << ", last " << last_args.AsValue()->ToString();
       obs->OnBeginFrame(missed_begin_frame_args_);
     }
   }
@@ skipping 20 matching lines @@
   paused_ = paused;
   std::unordered_set<BeginFrameObserver*> observers(observers_);
   for (auto* obs : observers)
     obs->OnBeginFrameSourcePausedChanged(paused_);
 }
 
 void ExternalBeginFrameSource::OnBeginFrame(const BeginFrameArgs& args) {
   missed_begin_frame_args_ = args;
   missed_begin_frame_args_.type = BeginFrameArgs::MISSED;
   std::unordered_set<BeginFrameObserver*> observers(observers_);
-  for (auto* obs : observers)
-    obs->OnBeginFrame(args);
+  for (auto* obs : observers) {
+    // It is possible that the source in which |args| originate changes, or that
+    // our hookup to this source changes, so we have to check for continuity.
+    // See also https://crbug.com/690127 for what may happen without this check.
+    const BeginFrameArgs& last_args = obs->LastUsedBeginFrameArgs();
+    if (!last_args.IsValid() || (args.frame_time > last_args.frame_time)) {
+      DCHECK((args.source_id != last_args.source_id) ||
+             (args.sequence_number > last_args.sequence_number))
+          << "current " << args.AsValue()->ToString() << ", last "
+          << last_args.AsValue()->ToString();
+      obs->OnBeginFrame(args);
+    }
+  }
 }
 
 }  // namespace cc