Teach the mojo_shell --trace-startup flag to gather data from services

mojo/common/trace_controller_impl.cc

 // Copyright 2015 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 "mojo/common/trace_controller_impl.h"
 
 #include "base/logging.h"
 #include "base/trace_event/trace_event.h"
 #include "mojo/public/cpp/application/application_connection.h"
 #include "mojo/public/cpp/application/application_impl.h"
 
 namespace mojo {
 
 TraceControllerImpl::TraceControllerImpl(
     InterfaceRequest<tracing::TraceController> request)
-    : binding_(this, request.Pass()) {
+    : tracing_already_started_(false), binding_(this, request.Pass()) {
 }
 
 TraceControllerImpl::~TraceControllerImpl() {
 }
 
 void TraceControllerImpl::StartTracing(
     const String& categories,
     tracing::TraceDataCollectorPtr collector) {
   DCHECK(!collector_.get());
   collector_ = collector.Pass();
-  std::string categories_str = categories.To<std::string>();
-  base::trace_event::TraceLog::GetInstance()->SetEnabled(
-      base::trace_event::CategoryFilter(categories_str),
-      base::trace_event::TraceLog::RECORDING_MODE,
-      base::trace_event::TraceOptions(base::trace_event::RECORD_UNTIL_FULL));
+  if (!tracing_already_started_) {
+    std::string categories_str = categories.To<std::string>();
+    base::trace_event::TraceLog::GetInstance()->SetEnabled(
+        base::trace_event::CategoryFilter(categories_str),
+        base::trace_event::TraceLog::RECORDING_MODE,
+        base::trace_event::TraceOptions(base::trace_event::RECORD_UNTIL_FULL));
+  }
 }
 
 void TraceControllerImpl::StopTracing() {
   DCHECK(collector_);
   base::trace_event::TraceLog::GetInstance()->SetDisabled();
 
   base::trace_event::TraceLog::GetInstance()->Flush(
       base::Bind(&TraceControllerImpl::SendChunk, base::Unretained(this)));
 }
 
 void TraceControllerImpl::SendChunk(
     const scoped_refptr<base::RefCountedString>& events_str,
     bool has_more_events) {
   DCHECK(collector_);
   collector_->DataCollected(mojo::String(events_str->data()));
   if (!has_more_events) {
     collector_.reset();
   }
 }
 
 }  // namespace mojo