Bug 1852093 - Change 'allowUpdatesForUnelevatedInstallations' to use setPref, r=bytesized

and rename it to 'enableUpdatesForUnelevatedInstallations'.

Rationale: `defaultPref` worked in testing because the pref was manually
toggled. But in the wild, the Nimbus feature *did not* set the
preference, and therefore the background update task did not witness the
changed preference.  This meant the background update task was
_scheduled_ but, when run, immediately exited with
`SERVICE_REGISTRY_KEY_MISSING` because the pref was not set in the
background update task profile.

Differential Revision: https://phabricator.services.mozilla.com/D187865

toolkit/components/nimbus/FeatureManifest.yaml

@@ -1798,13 +1798,13 @@ backgroundUpdate:
     the feature can impact Firefox behaviour and the user experience.
   isEarlyStartup: false
   variables:
-    allowUpdatesForUnelevatedInstallations:
+    enableUpdatesForUnelevatedInstallations:
       description: >-
         Allow the background update process to download and apply updates when
         the Mozilla Maintenance Service is unavailable but the installation
         directory can be written.
       type: boolean
-      fallbackPref: app.update.background.allowUpdatesForUnelevatedInstallations
+      setPref: app.update.background.allowUpdatesForUnelevatedInstallations
 
 bookmarks:
   description: Prefs to control aspects of the bookmarks system.

toolkit/mozapps/update/BackgroundUpdate.sys.mjs

@@ -211,10 +211,13 @@ export var BackgroundUpdate = {
       }
 
       if (!serviceRegKeyExists) {
-        // The nimbus experiment allows users with unelevated installations
+        // A Nimbus rollout sets this preference and allows users with
-        // to update in the background.
+        // unelevated installations to update in the background. For that to
-        let allowUnelevated = lazy.NimbusFeatures.backgroundUpdate.getVariable(
+        // work we use the setPref function to toggle a preference, because the
-          "allowUpdatesForUnelevatedInstallations"
+        // value for Nimbus is currently not readable in a backgroundtask. The
+        // preference serves in that case as our communication channel.
+        let allowUnelevated = await Services.prefs.getBoolPref(
+          "app.update.background.allowUpdatesForUnelevatedInstallations"
         );
 
         if (!allowUnelevated) {

toolkit/mozapps/update/tests/unit_background_update/test_backgroundupdate_reason_update.js

@@ -6,12 +6,6 @@
 
 "use strict";
 
-const lazy = {};
-
-ChromeUtils.defineESModuleGetters(lazy, {
-  NimbusFeatures: "resource://nimbus/ExperimentAPI.sys.mjs",
-});
-
 const { BackgroundUpdate } = ChromeUtils.importESModule(
   "resource://gre/modules/BackgroundUpdate.sys.mjs"
 );
@@ -239,39 +233,11 @@ add_task(async function test_reasons_update_manual_update_only() {
   Assert.ok(!result.includes(REASON.MANUAL_UPDATE_ONLY));
 });
 
-// Ensure that we are starting on a clean machine without the service registry
-// key. Enable and disable the Nimbus feature that allows the background
-// updater to operate on unelevated installations, verifying that the
-// appropriate reason to not update is (respectively, is not) reported.
-add_task(
-  {
-    skip_if: () => AppConstants.platform != "win",
-  },
-  async function test_unelevated_nimbus_default() {
-    // Default is disabled.
-    Assert.equal(
-      false,
-      lazy.NimbusFeatures.backgroundUpdate.getVariable(
-        "allowUpdatesForUnelevatedInstallations"
-      ),
-      "default is disabled"
-    );
-
-    let r = await reasons();
-    Assert.ok(
-      r.includes(BackgroundUpdate.REASON.SERVICE_REGISTRY_KEY_MISSING),
-      `SERVICE_REGISTRY_KEY_MISSING in ${JSON.stringify(r)}`
-    );
-  }
-);
-
 add_task(
   {
     skip_if: () => AppConstants.platform != "win",
   },
   async function test_unelevated_nimbus_enabled() {
-    let r;
-
     // Enable feature.
     Services.prefs.setBoolPref(
       "app.update.background.allowUpdatesForUnelevatedInstallations",
@@ -284,7 +250,7 @@ add_task(
     });
 
     // execute!
-    r = await reasons();
+    let r = await reasons();
     Assert.ok(
       !r.includes(BackgroundUpdate.REASON.SERVICE_REGISTRY_KEY_MISSING),
       `no SERVICE_REGISTRY_KEY_MISSING in ${JSON.stringify(r)}`

toolkit/mozapps/update/tests/unit_background_update/xpcshell.ini

@@ -21,4 +21,6 @@ skip-if =
 [test_backgroundupdate_glean.js]
 [test_backgroundupdate_reason.js]
 [test_backgroundupdate_reason_update.js]
+run-sequentially = very high failure rate in parallel
+
 [test_backgroundupdate_reason_schedule.js]