Support saving browser metrics to disk and reading them during next run.

components/metrics/metrics_service.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.
 
 //------------------------------------------------------------------------------
 // Description of the life cycle of a instance of MetricsService.
 //
 //  OVERVIEW
 //
 // A MetricsService instance is typically created at application startup.  It is
@@ skipping 1137 matching lines @@
     if ((*it)->flags() & base::Histogram::kUmaTargetedHistogramFlag)
       histogram_snapshot_manager_.PrepareDelta(*it);
   }
   for (MetricsProvider* provider : metrics_providers_)
     provider->RecordHistogramSnapshots(&histogram_snapshot_manager_);
   histogram_snapshot_manager_.FinishDeltas();
 }
 
 void MetricsService::RecordCurrentStabilityHistograms() {
   DCHECK(log_manager_.current_log());
+  histogram_snapshot_manager_.StartDeltas();
   // "true" indicates that StatisticsRecorder should include histograms in
   // persistent storage.
-  histogram_snapshot_manager_.PrepareDeltas(
-      base::StatisticsRecorder::begin(true), base::StatisticsRecorder::end(),
-      base::Histogram::kNoFlags, base::Histogram::kUmaStabilityHistogramFlag);
+  auto end = base::StatisticsRecorder::end();

[Ilya Sherman, 2016/05/05 07:22:37]

nit: Please move this so that it's scoped to within the for loop.

[bcwhite, 2016/05/05 16:01:17]

Done.

+  for (auto it = base::StatisticsRecorder::begin(true); it != end; ++it) {
+    if (((*it)->flags() & base::Histogram::kUmaStabilityHistogramFlag) ==
+        base::Histogram::kUmaStabilityHistogramFlag) {
+      histogram_snapshot_manager_.PrepareDelta(*it);
+    }
+  }
+  for (MetricsProvider* provider : metrics_providers_)
+    provider->RecordInitialHistogramSnapshots(&histogram_snapshot_manager_);

[Ilya Sherman, 2016/05/05 07:22:37]

Providers might choose to include metrics that aren't stability metrics, in case
of a crash, right?  So, what's the purpose of checking the
kUmaStabilityHistogramFlag above?

[bcwhite, 2016/05/05 16:01:17]

You mean on line 1163?  That's always been there and means that only a small
subset of all the histograms known to the StatisticsRecorder (basically
everything created by this particular process) will get reported here.

In the case of the providers, it's up to them what metrics to include.  The
different calls of RecordInitialHsitogramSnapshots() vs
RecordHistogramSnapshots() allows providers to choose.

[Ilya Sherman, 2016/05/05 22:22:52]

Ah, I was misreading the code.  Thanks for explaining that in more detail :)

+  histogram_snapshot_manager_.FinishDeltas();
 }
 
 void MetricsService::LogCleanShutdown() {
   // Redundant setting to assure that we always reset this value at shutdown
   // (and that we don't use some alternate path, and not call LogCleanShutdown).
   clean_shutdown_status_ = CLEANLY_SHUTDOWN;
 
   clean_exit_beacon_.WriteBeaconValue(true);
   RecordCurrentState(local_state_);
   local_state_->SetInteger(prefs::kStabilityExecutionPhase,
@@ skipping 18 matching lines @@
                  base::Time::Now().ToTimeT());
 }
 
 void MetricsService::SkipAndDiscardUpload() {
   log_manager_.DiscardStagedLog();
   scheduler_->UploadCancelled();
   log_upload_in_progress_ = false;
 }
 
 }  // namespace metrics