Avoid unnecessarily reopening the welcome page because of idle background process termination

doc/permissions.md

@@ -26,3 +26,6 @@ These permissions allow Privacy Badger to use the WebRequest and WebRequestBlock
 
 ## Tabs
 Privacy Badger needs access to the tabs API so that the extension can detect which tab is active and which tabs are simply present in the background. The extension icon, badge and popup update to reflect the state of Privacy Badger. This often requires knowing the tab's URL. For example, updating the icon requires the URL in order to determine whether Privacy Badger should be shown as disabled on that tab. Privacy Badger also uses the tabs API for miscellaneous tasks such as opening or switching to the already open new user welcome page.
+
+## Alarms
+Privacy Badger uses the Alarms API to temporarily ensure the background process does not get terminated as idle when Privacy Badger needs it to perform some longer running asynchronous task. This workaround is used to help reopen the welcome page when it appears that the extension has been restarted because of interaction with the Private/Incognito browsing permision prompt, but not because of idle background process termination.

src/js/background.js

@@ -43,6 +43,13 @@ function Badger(from_qunit) {
   self.isFirstRun = false;
   self.isUpdate = false;
 
+  (function () {
+    let manifestJson = chrome.runtime.getManifest();
+    self.manifestVersion = manifestJson.manifest_version;
+    self.isEventPage = (utils.hasOwn(manifestJson.background, "persistent") &&
+      manifestJson.background.persistent === false);
+  }());
+
   self.firstPartyDomainPotentiallyRequired = testCookiesFirstPartyDomain();
 
   self.widgetList = [];
@@ -344,6 +351,79 @@ Badger.prototype = {
     });
   },
 
+  /**
+   * If the background process is an event page or a service worker,
+   * it can get terminated while the user is still on the welcome page.
+   *
+   * When the user spends >= 30s on the welcome page, the background process
+   * will get terminated and another welcome page will unexpectedly appear
+   * following any user action that restarts the background process.
+   *
+   * (We reopen the welcome page via firstRunTimerFinished, our workaround
+   * for restoring the welcome page when Firefox restarts the extension
+   * in response to interaction with the private browsing permission hanger.)
+   *
+   * Let's periodically call a low-overhead, no-side effects API to keep
+   * the background process running as long as the welcome page stays open.
+   *
+   * While extension alarm events reset the idle timer in both Firefox and
+   * Chrome, Chrome enforces a minimum resolution of one minute. However,
+   * since most extension API calls also reset the idle timer in Chrome,
+   * simply looking up whether the welcome page is still open is enough
+   * to reset the idle timer.
+  */
+  keepBackgroundAliveForWelcomePage: function () {
+    let ALARM_NAME = "welcome-page-keepalive",
+      INTERVAL = 10000; // 10 secs
+
+    if (badger.manifestVersion == 2 && !badger.isEventPage) {
+      return; // noop
+    }
+
+    function getWelcomeTab(callback) {
+      chrome.tabs.query({
+        url: chrome.runtime.getURL("/skin/firstRun.html")
+      }, function (tabs) {
+        callback(tabs[0]);
+      });
+    }
+
+    function workaroundForChrome() {
+      setTimeout(function () {
+        getWelcomeTab(function (tab) {
+          if (tab) {
+            workaroundForChrome();
+          }
+        });
+      }, INTERVAL);
+    }
+
+    if (!badger.isEventPage) {
+      workaroundForChrome();
+      return;
+    }
+
+    // create an alarm that will reset the idle timer in Firefox
+    chrome.alarms.create(ALARM_NAME, {
+      when: Date.now() + INTERVAL
+    });
+
+    chrome.alarms.onAlarm.addListener(alarm => {
+      if (alarm.name != ALARM_NAME) {
+        return;
+      }
+      getWelcomeTab(function (tab) {
+        // if the welcome page is still open
+        if (tab) {
+          // create another alarm
+          chrome.alarms.create(ALARM_NAME, {
+            when: Date.now() + INTERVAL
+          });
+        }
+      });
+    });
+  },
+
   initWelcomePage: function () {
     let self = this,
       privateStore = self.getPrivateSettings();
@@ -365,10 +445,14 @@ Badger.prototype = {
   },
 
   showWelcomePage: function () {
-    let settings = this.getSettings();
+    let self = this,
+      settings = self.getSettings();
+
     if (settings.getItem("showIntroPage")) {
       chrome.tabs.create({
         url: chrome.runtime.getURL("/skin/firstRun.html")
+      }, function () {
+        self.keepBackgroundAliveForWelcomePage();
       });
     } else {
       // don't remind users to look at the intro page either

src/manifest.json

@@ -11,6 +11,7 @@
   },
   "incognito": "spanning",
   "permissions": [
+    "alarms",
     "tabs",
     "http://*/*",
     "https://*/*",

src/skin/js/firstRun.js

@@ -4,14 +4,15 @@ $(function () {
   $(".scroll-it").smoothScroll();
 
   $(window).scroll(function () {
-    if (!already_set) {
-      if ($(window).scrollTop() > 400) {
-        already_set = true;
-        chrome.runtime.sendMessage({
-          type: "updateSettings",
-          data: { seenComic: true }
-        });
-      }
+    if (already_set) {
+      return;
+    }
+    if ($(window).scrollTop() > 400) {
+      already_set = true;
+      chrome.runtime.sendMessage({
+        type: "updateSettings",
+        data: { seenComic: true }
+      });
     }
   });
 });