Bug 993084 - Delay initialization of telemetry experiments if there is no active experiment. r=bsmedberg

To sync experiment state with the addon manager, we need to start it early when there is a
active experiment.
However, initializing the telemetry experiments system too early can lead to performance
regressions, so we delay the initialization if we don't have an active experiment.

browser/experiments/Experiments.jsm

@@ -58,6 +58,7 @@ const MIN_EXPERIMENT_ACTIVE_SECONDS = 60;
 
 const PREF_BRANCH               = "experiments.";
 const PREF_ENABLED              = "enabled"; // experiments.enabled
+const PREF_ACTIVE_EXPERIMENT    = "activeExperiment"; // whether we have an active experiment
 const PREF_LOGGING              = "logging";
 const PREF_LOGGING_LEVEL        = PREF_LOGGING + ".level"; // experiments.logging.level
 const PREF_LOGGING_DUMP         = PREF_LOGGING + ".dump"; // experiments.logging.dump
@@ -975,6 +976,11 @@ Experiments.Experiments.prototype = {
     let activeChanged = false;
     let now = this._policy.now();
 
+    if (!activeExperiment) {
+      // Avoid this pref staying out of sync if there were e.g. crashes.
+      gPrefs.set(PREF_ACTIVE_EXPERIMENT, false);
+    }
+
     if (activeExperiment) {
       this._pendingUninstall = activeExperiment._addonId;
       try {
@@ -1484,6 +1490,7 @@ Experiments.ExperimentEntry.prototype = {
       }
 
       yield this._installAddon();
+      gPrefs.set(PREF_ACTIVE_EXPERIMENT, true);
     }.bind(this));
   },
 
@@ -1604,6 +1611,8 @@ Experiments.ExperimentEntry.prototype = {
     }
 
     this._enabled = false;
+    gPrefs.set(PREF_ACTIVE_EXPERIMENT, false);
+
     let deferred = Promise.defer();
     let updateDates = () => {
       let now = this._policy.now();
@@ -1851,8 +1860,6 @@ ExperimentsProvider.prototype = Object.freeze({
   ],
 
   postInit: function () {
-    this._experiments = Experiments.instance();
-
     for (let o of this._OBSERVERS) {
       Services.obs.addObserver(this, o, false);
     }
@@ -1881,6 +1888,14 @@ ExperimentsProvider.prototype = Object.freeze({
   },
 
   recordLastActiveExperiment: function () {
+    if (!gExperimentsEnabled) {
+      return;
+    }
+
+    if (!this._experiments) {
+      this._experiments = Experiments.instance();
+    }
+
     let m = this.getMeasurement(ExperimentsLastActiveMeasurement1.prototype.name,
                                 ExperimentsLastActiveMeasurement1.prototype.version);
 

browser/experiments/ExperimentsService.js

@@ -8,29 +8,41 @@ const {interfaces: Ci, utils: Cu} = Components;
 
 Cu.import("resource://gre/modules/XPCOMUtils.jsm");
 Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/Preferences.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "Experiments",
                                   "resource:///modules/experiments/Experiments.jsm");
 XPCOMUtils.defineLazyModuleGetter(this, "OS",
                                   "resource://gre/modules/osfile.jsm");
+XPCOMUtils.defineLazyModuleGetter(this, "CommonUtils",
+                                  "resource://services-common/utils.js");
 
 const PREF_EXPERIMENTS_ENABLED  = "experiments.enabled";
+const PREF_ACTIVE_EXPERIMENT    = "experiments.activeExperiment"; // whether we have an active experiment
 const PREF_HEALTHREPORT_ENABLED = "datareporting.healthreport.service.enabled";
 const PREF_TELEMETRY_ENABLED    = "toolkit.telemetry.enabled";
+const DELAY_INIT_MS             = 30 * 1000;
+
+XPCOMUtils.defineLazyGetter(
+  this, "gPrefs", () => {
+    return new Preferences();
+  });
 
 XPCOMUtils.defineLazyGetter(
   this, "gExperimentsEnabled", () => {
-    try {
-      let prefs = Services.prefs;
-      return prefs.getBoolPref(PREF_EXPERIMENTS_ENABLED) &&
-             prefs.getBoolPref(PREF_TELEMETRY_ENABLED) &&
-             prefs.getBoolPref(PREF_HEALTHREPORT_ENABLED);
-    } catch (e) {
-      return false;
-    }
+    return gPrefs.get(PREF_EXPERIMENTS_ENABLED, false) &&
+           gPrefs.get(PREF_TELEMETRY_ENABLED, false) &&
+           gPrefs.get(PREF_HEALTHREPORT_ENABLED, false);
+  });
+
+XPCOMUtils.defineLazyGetter(
+  this, "gActiveExperiment", () => {
+    return gPrefs.get(PREF_ACTIVE_EXPERIMENT);
   });
 
 function ExperimentsService() {
+  this._initialized = false;
+  this._delayedInitTimer = null;
 }
 
 ExperimentsService.prototype = {
@@ -47,11 +59,36 @@ ExperimentsService.prototype = {
     Experiments.instance().updateManifest();
   },
 
+  _delayedInit: function () {
+    if (!this._initialized) {
+      this._initialized = true;
+      Experiments.instance(); // for side effects
+    }
+  },
+
   observe: function (subject, topic, data) {
     switch (topic) {
       case "profile-after-change":
         if (gExperimentsEnabled) {
-          Experiments.instance(); // for side effects
+          Services.obs.addObserver(this, "quit-application", false);
+          Services.obs.addObserver(this, "sessionstore-state-finalized", false);
+
+          if (gActiveExperiment) {
+            this._initialized = true;
+            Experiments.instance(); // for side effects
+          }
+        }
+        break;
+      case "sessionstore-state-finalized":
+        if (!this._initialized) {
+          CommonUtils.namedTimer(this._delayedInit, DELAY_INIT_MS, this, "_delayedInitTimer");
+        }
+        break;
+      case "quit-application":
+        Services.obs.removeObserver(this, "quit-application");
+        Services.obs.removeObserver(this, "sessionstore-state-finalized");
+        if (this._delayedInitTimer) {
+          this._delayedInitTimer.clear();
         }
         break;
     }