Bug 1772956 - only sync after tab changes if the user has more than one client r=markh

Differential Revision: https://phabricator.services.mozilla.com/D148467

services/sync/modules/engines/tabs.js

@@ -429,7 +429,7 @@ TabTracker.prototype = {
 
   callScheduleSync(scoreIncrement) {
     this.modified = true;
-
+    let { scheduler } = this.engine.service;
     const delayInMs = lazy.NimbusFeatures.syncAfterTabChange.getVariable(
       "syncDelayAfterTabChange"
     );
@@ -437,11 +437,14 @@ TabTracker.prototype = {
     // If we are part of the experiment don't use score here
     // and instead schedule a sync once we detect a tab change
     //  to ensure the server always has the most up to date tabs
-    if (delayInMs > 0) {
+    if (
+      delayInMs > 0 &&
+      scheduler.numClients > 1 // Don't constantly schedule syncs for single client users
+    ) {
       this._log.debug(
         "Detected a tab change: scheduling a sync in " + delayInMs + "ms"
       );
-      this.engine.service.scheduler.scheduleNextSync(delayInMs, {
+      scheduler.scheduleNextSync(delayInMs, {
         why: "tabschanged",
       });
     } else if (scoreIncrement) {

services/sync/tests/unit/test_tab_tracker.js

@@ -185,6 +185,12 @@ add_task(async function run_sync_on_tab_change_test() {
 
   // This is the fallback pref if we don't have a experiment running
   Svc.Prefs.set("syncedTabs.syncDelayAfterTabChange", testExperimentDelay);
+  // We should only be syncing on tab change if
+  // the user has > 1 client
+  Svc.Prefs.set("clients.devices.desktop", 1);
+  Svc.Prefs.set("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 2);
 
   let doEnrollmentCleanup = await ExperimentFakes.enrollWithFeatureConfig(
     {
@@ -281,12 +287,43 @@ add_task(async function run_sync_on_tab_change_test() {
     scheduler.nextSync - Date.now() <= testExperimentDelay,
     "about page should trigger a sync soon after we changed the pref"
   );
+
+  _("Test no sync after tab change for accounts with <= 1 clients");
+  // Pretend we just synced
+  await tracker.clearChangedIDs();
+  scheduler.nextSync = Date.now() + scheduler.idleInterval;
+  // Setting clients to only 1 so we don't sync after a tab change
+  Svc.Prefs.set("clients.devices.desktop", 1);
+  Svc.Prefs.set("clients.devices.mobile", 0);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 1);
+
+  tracker.onLocationChange(
+    { isTopLevel: true },
+    undefined,
+    Services.io.newURI("https://www.mozilla.org"),
+    Ci.nsIWebProgressListener.LOCATION_CHANGE_RELOAD
+  );
+  Assert.ok(
+    tracker.modified,
+    "location change for a new top-level document flagged as modified"
+  );
+  Assert.ok(
+    scheduler.nextSync - Date.now() > testExperimentDelay,
+    "We should NOT be syncing shortly because there is only one client"
+  );
+
   await doEnrollmentCleanup();
 
   _("If there is no experiment, fallback to the pref");
   let delayPref = Svc.Prefs.get("syncedTabs.syncDelayAfterTabChange");
   let evttype = "TabOpen";
   Assert.equal(delayPref, testExperimentDelay);
+  // Only have task continuity if we have more than 1 device
+  Svc.Prefs.set("clients.devices.desktop", 1);
+  Svc.Prefs.set("clients.devices.mobile", 1);
+  scheduler.updateClientMode();
+  Assert.equal(scheduler.numClients, 2);
   // Fire ontab event
   tracker.onTab({ type: evttype, originalTarget: evttype });