Reset some UMA stability stats on a version change.

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 540 matching lines @@
 
 //------------------------------------------------------------------------------
 // private methods
 //------------------------------------------------------------------------------
 
 
 //------------------------------------------------------------------------------
 // Initialization methods
 
 void MetricsService::InitializeMetricsState() {
-  local_state_->SetString(metrics::prefs::kStabilityStatsVersion,
-                          client_->GetVersionString());
-  local_state_->SetInt64(metrics::prefs::kStabilityStatsBuildTime,
-                         MetricsLog::GetBuildTime());
+  const int64 buildtime = MetricsLog::GetBuildTime();
+  const std::string version = client_->GetVersionString();
+  bool version_changed = false;
+  if (local_state_->GetInt64(prefs::kStabilityStatsBuildTime) != buildtime ||
+      local_state_->GetString(prefs::kStabilityStatsVersion) != version) {
+    local_state_->SetString(metrics::prefs::kStabilityStatsVersion, version);
+    local_state_->SetInt64(metrics::prefs::kStabilityStatsBuildTime, buildtime);
+    version_changed = true;
+  }
 
   log_manager_.LoadPersistedUnsentLogs();
 
   session_id_ = local_state_->GetInteger(metrics::prefs::kMetricsSessionID);
 
   if (!clean_exit_beacon_.exited_cleanly()) {
     IncrementPrefValue(metrics::prefs::kStabilityCrashCount);
     // Reset flag, and wait until we call LogNeedForCleanShutdown() before
     // monitoring.
     clean_exit_beacon_.WriteBeaconValue(true);
   }
 
   if (!clean_exit_beacon_.exited_cleanly() || ProvidersHaveStabilityMetrics()) {
     // TODO(rtenneti): On windows, consider saving/getting execution_phase from
     // the registry.
     int execution_phase =
         local_state_->GetInteger(metrics::prefs::kStabilityExecutionPhase);
     UMA_HISTOGRAM_SPARSE_SLOWLY("Chrome.Browser.CrashedExecutionPhase",
                                 execution_phase);
 
     // If the previous session didn't exit cleanly, or if any provider
     // explicitly requests it, prepare an initial stability log -
     // provided UMA is enabled.
     if (state_manager_->IsMetricsReportingEnabled())
       PrepareInitialStabilityLog();
   }
 
+  // If no initial stability log was generated and there was a version upgrade,
+  // reset stability stats from the previous version.

[Ilya Sherman, 2014/09/18 18:30:20]

nit: I'd go into a bit more detail here on why this might happen -- something
like "There is an edge case where the initial stability log can fail to be
generated even if the previous version crashed: It's possible that the previous
version crashed before it was able to save off a system profile. In this case,
we can't correctly identify what version the saved stability metrics correspond
to; but since we detected that the version changed, we at least know that it's
not the current version."

... but, you know, shorter would be better.  Please improve that further; I just
think the general idea is important to get across.

[Alexei Svitkine (slow), 2014/09/18 20:21:14]

Done.

+  if (!has_initial_stability_log_ && version_changed) {
+    for (size_t i = 0; i < metrics_providers_.size(); ++i)
+      metrics_providers_[i]->ClearSavedStabilityMetrics();
+
+    // Reset the prefs that are managed by MetricsService/MetricsLog directly.
+    local_state_->SetInteger(prefs::kStabilityCrashCount, 0);
+    local_state_->SetInteger(prefs::kStabilityExecutionPhase,
+                             UNINITIALIZED_PHASE);
+    local_state_->SetInteger(prefs::kStabilityIncompleteSessionEndCount, 0);
+    local_state_->SetInteger(prefs::kStabilityLaunchCount, 0);
+    local_state_->SetBoolean(prefs::kStabilitySessionEndCompleted, true);
+  }
+
   // Update session ID.
   ++session_id_;
   local_state_->SetInteger(metrics::prefs::kMetricsSessionID, session_id_);
 
   // Stability bookkeeping
   IncrementPrefValue(metrics::prefs::kStabilityLaunchCount);
 
   DCHECK_EQ(UNINITIALIZED_PHASE, execution_phase_);
   SetExecutionPhase(START_METRICS_RECORDING, local_state_);
 
@@ skipping 589 matching lines @@
   local_state_->SetBoolean(path, value);
   RecordCurrentState(local_state_);
 }
 
 void MetricsService::RecordCurrentState(PrefService* pref) {
   pref->SetInt64(metrics::prefs::kStabilityLastTimestampSec,
                  base::Time::Now().ToTimeT());
 }
 
 }  // namespace metrics