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);
+  for (auto it = base::StatisticsRecorder::begin(true),
+            end = base::StatisticsRecorder::end();
+       it != end; ++it) {
+    // kUmaStabilityHistogramFlag is actually a combination of two flags and
+    // so the result of the bitwise-and must be checked against the "flag"
+    // value or incorrect matches will occur.
+    if (((*it)->flags() & base::Histogram::kUmaStabilityHistogramFlag) ==
+        base::Histogram::kUmaStabilityHistogramFlag) {

[Alexei Svitkine (slow), 2016/05/10 19:55:42]

I would actually prefer we don't duplicate this logic that's already in
histogram_snapshot_manager_.PrepareDeltas().

Looks like the only difference is you don't want StartDeltas/FinishDeltas be
called before/after this and want to do it yourself. I suggest making another
version of PrepareDeltas() functions that doesn't call those.

For example, PrepareDeltasNoStartFinish().

Then, PrepareDeltas() can call that one surrounded by StartFinish() and this
file doesn't need to duplicate this logic.

[bcwhite, 2016/05/11 15:47:12]

Done.

+      histogram_snapshot_manager_.PrepareDelta(*it);
+    }
+  }
+  for (MetricsProvider* provider : metrics_providers_)
+    provider->RecordInitialHistogramSnapshots(&histogram_snapshot_manager_);
+  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