One-time reset of UMA client ids and low entropy source.

chrome/browser/metrics/metrics_service.cc

Index: chrome/browser/metrics/metrics_service.cc
===================================================================
--- chrome/browser/metrics/metrics_service.cc	(revision 260135)
+++ chrome/browser/metrics/metrics_service.cc	(working copy)
@@ -436,9 +436,9 @@
 void MetricsService::RegisterPrefs(PrefRegistrySimple* registry) {
   DCHECK(IsSingleThreaded());
   registry->RegisterStringPref(prefs::kMetricsClientID, std::string());
+  registry->RegisterInt64Pref(prefs::kMetricsReportingEnabledTimestamp, 0);
   registry->RegisterIntegerPref(prefs::kMetricsLowEntropySource,
                                 kLowEntropySourceNotSet);
-  registry->RegisterInt64Pref(prefs::kMetricsClientIDTimestamp, 0);
   registry->RegisterInt64Pref(prefs::kStabilityLaunchTimeSec, 0);
   registry->RegisterInt64Pref(prefs::kStabilityLastTimestampSec, 0);
   registry->RegisterStringPref(prefs::kStabilityStatsVersion, std::string());
@@ -483,6 +483,10 @@
   registry->RegisterInt64Pref(prefs::kUninstallLastLaunchTimeSec, 0);
   registry->RegisterInt64Pref(prefs::kUninstallLastObservedRunTimeSec, 0);
 
+  // TODO(asvitkine): Remove these once a couple of releases have passed.

[Ilya Sherman, 2014/03/28 18:23:06]

nit: Please add a specific milestone target to this TODO, and file a bug, so
that future readers of the code don't have to dig through history to figure out
when this TODO was added.

[Alexei Svitkine (slow), 2014/03/28 18:40:19]

Done. The milestone is on the bug.

+  registry->RegisterStringPref(prefs::kMetricsOldClientID, std::string());
+  registry->RegisterIntegerPref(prefs::kMetricsOldLowEntropySource, 0);
+
 #if defined(OS_ANDROID)
   RegisterPrefsAndroid(registry);
 #endif  // defined(OS_ANDROID)
@@ -641,9 +645,14 @@
   client_id_ = GenerateClientID();
   pref->SetString(prefs::kMetricsClientID, client_id_);
 
-  // Might as well make a note of how long this ID has existed
-  pref->SetString(prefs::kMetricsClientIDTimestamp,
-                  base::Int64ToString(Time::Now().ToTimeT()));
+  // Record the timestamp of when the user opted in to UMA.

[Ilya Sherman, 2014/03/28 18:23:06]

nit: Move this inside the if branch, since it doesn't apply to the else branch.

[Alexei Svitkine (slow), 2014/03/28 18:40:19]

Done.

+  if (pref->GetString(prefs::kMetricsOldClientID).empty()) {
+    pref->SetInt64(prefs::kMetricsReportingEnabledTimestamp,
+                   Time::Now().ToTimeT());
+  } else {
+    UMA_HISTOGRAM_BOOLEAN("UMA.MigratedClientID", true);

[Ilya Sherman, 2014/03/28 18:23:06]

You don't seem to ever emit a |false| value to this histogram.  What's the
purpose of tracking this as a histogram?

[Alexei Svitkine (slow), 2014/03/28 18:40:19]

This will allow analysis of when users migrated their ids. (We don't care about
the actual counts of this, since all UMA users on M35 will be affected by this.)
But we may want to know when a given "new" UMA client id we see is a result of
migration.

[Ilya Sherman, 2014/03/28 19:22:48]

Fair 'nuff.

+  }
+  pref->ClearPref(prefs::kMetricsOldClientID);
 }
 
 void MetricsService::EnableRecording() {
@@ -1198,12 +1207,6 @@
   // Otherwise, skip to generating a new value.
   if (!command_line->HasSwitch(switches::kResetVariationState)) {
     int value = local_state->GetInteger(prefs::kMetricsLowEntropySource);
-    // Old versions of the code would generate values in the range of [1, 8192],
-    // before the range was switched to [0, 8191] and then to [0, 7999]. Map
-    // 8192 to 0, so that the 0th bucket remains uniform, while re-generating
-    // the low entropy source for old values in the [8000, 8191] range.
-    if (value == 8192)
-      value = 0;
     // If the value is outside the [0, kMaxLowEntropySize) range, re-generate
     // it below.
     if (value >= 0 && value < kMaxLowEntropySize) {
@@ -1216,6 +1219,7 @@
   UMA_HISTOGRAM_BOOLEAN("UMA.GeneratedLowEntropySource", true);
   low_entropy_source_ = GenerateLowEntropySource();
   local_state->SetInteger(prefs::kMetricsLowEntropySource, low_entropy_source_);
+  local_state->ClearPref(prefs::kMetricsOldLowEntropySource);
   metrics::CachingPermutedEntropyProvider::ClearCache(local_state);
 
   return low_entropy_source_;