Avoid emptry tracing callbacks when possible.

base/trace_event/trace_event_impl.cc

 // Copyright (c) 2012 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 "base/trace_event/trace_event_impl.h"
 
 #include <algorithm>
 #include <cmath>
 
 #include "base/base_switches.h"
@@ skipping 1745 matching lines @@
 // Usually it runs on a different thread.
 void TraceLog::ConvertTraceEventsToTraceFormat(
     scoped_ptr<TraceBuffer> logged_events,
     const TraceLog::OutputCallback& flush_output_callback) {
 
   if (flush_output_callback.is_null())
     return;
 
   // The callback need to be called at least once even if there is no events
   // to let the caller know the completion of flush.
-  bool has_more_events = true;
-  do {
-    scoped_refptr<RefCountedString> json_events_str_ptr =
-        new RefCountedString();
-
-    while (json_events_str_ptr->size() < kTraceEventBufferSizeInBytes) {
-      const TraceBufferChunk* chunk = logged_events->NextChunk();
-      has_more_events = chunk != NULL;
-      if (!chunk)
-        break;
-      for (size_t j = 0; j < chunk->size(); ++j) {
-        if (json_events_str_ptr->size())
-          json_events_str_ptr->data().append(",\n");
-        chunk->GetEventAt(j)->AppendAsJSON(&(json_events_str_ptr->data()));
+  scoped_refptr<RefCountedString> json_events_str_ptr = new RefCountedString();
+  while (const TraceBufferChunk* chunk = logged_events->NextChunk()) {
+    for (size_t j = 0; j < chunk->size(); ++j) {
+      size_t size = json_events_str_ptr->size();
+      if (size > kTraceEventBufferSizeInBytes) {
+        flush_output_callback.Run(json_events_str_ptr, true);
+        json_events_str_ptr = new RefCountedString();

[Ken Russell (switch to Gerrit), 2015/05/08 18:54:34]

What happens if this is hit during the last iteration of the for loop, and there
are no more chunks? Now it looks to me like this has the same problem as the
previous code.

Maybe this isn't worth fixing, but instead rather should be documented.

[hubbe, 2015/05/08 18:58:14]

This flush happens before appending the current event to the string, so there
will always be at least one more event after this flush.

+      } else if (size) {
+        json_events_str_ptr->data().append(",\n");
       }
+      chunk->GetEventAt(j)->AppendAsJSON(&(json_events_str_ptr->data()));
     }
-    flush_output_callback.Run(json_events_str_ptr, has_more_events);
-  } while (has_more_events);
+  }
+  flush_output_callback.Run(json_events_str_ptr, false);
 }
 
 void TraceLog::FinishFlush(int generation) {
   scoped_ptr<TraceBuffer> previous_logged_events;
   OutputCallback flush_output_callback;
 
   if (!CheckGeneration(generation))
     return;
 
   {
@@ skipping 847 matching lines @@
 }
 
 ScopedTraceBinaryEfficient::~ScopedTraceBinaryEfficient() {
   if (*category_group_enabled_) {
     TRACE_EVENT_API_UPDATE_TRACE_EVENT_DURATION(category_group_enabled_,
                                                 name_, event_handle_);
   }
 }
 
 }  // namespace trace_event_internal