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diff --git a/browser/components/newtab/AboutNewTabService.jsm b/browser/components/newtab/AboutNewTabService.jsm
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+++ b/browser/components/newtab/AboutNewTabService.jsm
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+/**
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/.
+ */
+
+"use strict";
+
+const EXPORTED_SYMBOLS = [
+  "AboutNewTabStubService",
+  "AboutHomeStartupCacheChild",
+];
+
+/**
+ * The nsIAboutNewTabService is accessed by the AboutRedirector anytime
+ * about:home, about:newtab or about:welcome are requested. The primary
+ * job of an nsIAboutNewTabService is to tell the AboutRedirector what
+ * resources to actually load for those requests.
+ *
+ * The nsIAboutNewTabService is not involved when the user has overridden
+ * the default about:home or about:newtab pages.
+ *
+ * There are two implementations of this service - one for the parent
+ * process, and one for content processes. Each one has some secondary
+ * responsibilties that are process-specific.
+ *
+ * The need for two implementations is an unfortunate consequence of how
+ * document loading and process redirection for about: pages currently
+ * works in Gecko. The commonalities between the two implementations has
+ * been put into an abstract base class.
+ */
+
+const { XPCOMUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/XPCOMUtils.sys.mjs"
+);
+const { AppConstants } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppConstants.sys.mjs"
+);
+const { E10SUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/E10SUtils.sys.mjs"
+);
+
+const lazy = {};
+
+XPCOMUtils.defineLazyModuleGetters(lazy, {
+  NimbusFeatures: "resource://nimbus/ExperimentAPI.jsm",
+});
+
+/**
+ * BEWARE: Do not add variables for holding state in the global scope.
+ * Any state variables should be properties of the appropriate class
+ * below. This is to avoid confusion where the state is set in one process,
+ * but not in another.
+ *
+ * Constants are fine in the global scope.
+ */
+
+const PREF_ABOUT_HOME_CACHE_TESTING =
+  "browser.startup.homepage.abouthome_cache.testing";
+const ABOUT_WELCOME_URL =
+  "resource://activity-stream/aboutwelcome/aboutwelcome.html";
+
+ChromeUtils.defineModuleGetter(
+  lazy,
+  "BasePromiseWorker",
+  "resource://gre/modules/PromiseWorker.jsm"
+);
+
+const CACHE_WORKER_URL = "resource://activity-stream/lib/cache-worker.js";
+
+const IS_PRIVILEGED_PROCESS =
+  Services.appinfo.remoteType === E10SUtils.PRIVILEGEDABOUT_REMOTE_TYPE;
+
+const PREF_SEPARATE_PRIVILEGEDABOUT_CONTENT_PROCESS =
+  "browser.tabs.remote.separatePrivilegedContentProcess";
+const PREF_ACTIVITY_STREAM_DEBUG = "browser.newtabpage.activity-stream.debug";
+
+/**
+ * The AboutHomeStartupCacheChild is responsible for connecting the
+ * nsIAboutNewTabService with a cached document and script for about:home
+ * if one happens to exist. The AboutHomeStartupCacheChild is only ever
+ * handed the streams for those caches when the "privileged about content
+ * process" first launches, so subsequent loads of about:home do not read
+ * from this cache.
+ *
+ * See https://firefox-source-docs.mozilla.org/browser/components/newtab/docs/v2-system-addon/about_home_startup_cache.html
+ * for further details.
+ */
+const AboutHomeStartupCacheChild = {
+  _initted: false,
+  CACHE_REQUEST_MESSAGE: "AboutHomeStartupCache:CacheRequest",
+  CACHE_RESPONSE_MESSAGE: "AboutHomeStartupCache:CacheResponse",
+  CACHE_USAGE_RESULT_MESSAGE: "AboutHomeStartupCache:UsageResult",
+  STATES: {
+    UNAVAILABLE: 0,
+    UNCONSUMED: 1,
+    PAGE_CONSUMED: 2,
+    PAGE_AND_SCRIPT_CONSUMED: 3,
+    FAILED: 4,
+  },
+  REQUEST_TYPE: {
+    PAGE: 0,
+    SCRIPT: 1,
+  },
+  _state: 0,
+  _consumerBCID: null,
+
+  /**
+   * Called via a process script very early on in the process lifetime. This
+   * prepares the AboutHomeStartupCacheChild to pass an nsIChannel back to
+   * the nsIAboutNewTabService when the initial about:home document is
+   * eventually requested.
+   *
+   * @param pageInputStream (nsIInputStream)
+   *   The stream for the cached page markup.
+   * @param scriptInputStream (nsIInputStream)
+   *   The stream for the cached script to run on the page.
+   */
+  init(pageInputStream, scriptInputStream) {
+    if (
+      !IS_PRIVILEGED_PROCESS &&
+      !Services.prefs.getBoolPref(PREF_ABOUT_HOME_CACHE_TESTING, false)
+    ) {
+      throw new Error(
+        "Can only instantiate in the privileged about content processes."
+      );
+    }
+
+    if (!lazy.NimbusFeatures.abouthomecache.getVariable("enabled")) {
+      return;
+    }
+
+    if (this._initted) {
+      throw new Error("AboutHomeStartupCacheChild already initted.");
+    }
+
+    Services.obs.addObserver(this, "memory-pressure");
+    Services.cpmm.addMessageListener(this.CACHE_REQUEST_MESSAGE, this);
+
+    this._pageInputStream = pageInputStream;
+    this._scriptInputStream = scriptInputStream;
+    this._initted = true;
+    this.setState(this.STATES.UNCONSUMED);
+  },
+
+  /**
+   * A function that lets us put the AboutHomeStartupCacheChild back into
+   * its initial state. This is used by tests to let us simulate the startup
+   * behaviour of the module without having to manually launch a new privileged
+   * about content process every time.
+   */
+  uninit() {
+    if (!Services.prefs.getBoolPref(PREF_ABOUT_HOME_CACHE_TESTING, false)) {
+      throw new Error(
+        "Cannot uninit AboutHomeStartupCacheChild unless testing."
+      );
+    }
+
+    if (!this._initted) {
+      return;
+    }
+
+    Services.obs.removeObserver(this, "memory-pressure");
+    Services.cpmm.removeMessageListener(this.CACHE_REQUEST_MESSAGE, this);
+
+    if (this._cacheWorker) {
+      this._cacheWorker.terminate();
+      this._cacheWorker = null;
+    }
+
+    this._pageInputStream = null;
+    this._scriptInputStream = null;
+    this._initted = false;
+    this._state = this.STATES.UNAVAILABLE;
+    this._consumerBCID = null;
+  },
+
+  /**
+   * A public method called from nsIAboutNewTabService that attempts
+   * return an nsIChannel for a cached about:home document that we
+   * were initialized with. If we failed to be initted with the
+   * cache, or the input streams that we were sent have no data
+   * yet available, this function returns null. The caller should
+   * fall back to generating the page dynamically.
+   *
+   * This function will be called when loading about:home, or
+   * about:home?jscache - the latter returns the cached script.
+   *
+   * It is expected that the same BrowsingContext that loads the cached
+   * page will also load the cached script.
+   *
+   * @param uri (nsIURI)
+   *   The URI for the requested page, as passed by nsIAboutNewTabService.
+   * @param loadInfo (nsILoadInfo)
+   *   The nsILoadInfo for the requested load, as passed by
+   *   nsIAboutNewWTabService.
+   * @return nsIChannel or null.
+   */
+  maybeGetCachedPageChannel(uri, loadInfo) {
+    if (!this._initted) {
+      return null;
+    }
+
+    if (this._state >= this.STATES.PAGE_AND_SCRIPT_CONSUMED) {
+      return null;
+    }
+
+    let requestType =
+      uri.query === "jscache"
+        ? this.REQUEST_TYPE.SCRIPT
+        : this.REQUEST_TYPE.PAGE;
+
+    // If this is a page request, then we need to be in the UNCONSUMED state,
+    // since we expect the page request to come first. If this is a script
+    // request, we expect to be in PAGE_CONSUMED state, since the page cache
+    // stream should he been consumed already.
+    if (
+      (requestType === this.REQUEST_TYPE.PAGE &&
+        this._state !== this.STATES.UNCONSUMED) ||
+      (requestType === this.REQUEST_TYPE_SCRIPT &&
+        this._state !== this.STATES.PAGE_CONSUMED)
+    ) {
+      return null;
+    }
+
+    // If by this point, we don't have anything in the streams,
+    // then either the cache was too slow to give us data, or the cache
+    // doesn't exist. The caller should fall back to generating the
+    // page dynamically.
+    //
+    // We only do this on the page request, because by the time
+    // we get to the script request, we should have already drained
+    // the page input stream.
+    if (requestType === this.REQUEST_TYPE.PAGE) {
+      try {
+        if (
+          !this._scriptInputStream.available() ||
+          !this._pageInputStream.available()
+        ) {
+          this.setState(this.STATES.FAILED);
+          this.reportUsageResult(false /* success */);
+          return null;
+        }
+      } catch (e) {
+        this.setState(this.STATES.FAILED);
+        if (e.result === Cr.NS_BASE_STREAM_CLOSED) {
+          this.reportUsageResult(false /* success */);
+          return null;
+        }
+        throw e;
+      }
+    }
+
+    if (
+      requestType === this.REQUEST_TYPE.SCRIPT &&
+      this._consumerBCID !== loadInfo.browsingContextID
+    ) {
+      // Some other document is somehow requesting the script - one
+      // that didn't originally request the page. This is not allowed.
+      this.setState(this.STATES.FAILED);
+      return null;
+    }
+
+    let channel = Cc[
+      "@mozilla.org/network/input-stream-channel;1"
+    ].createInstance(Ci.nsIInputStreamChannel);
+    channel.QueryInterface(Ci.nsIChannel);
+    channel.setURI(uri);
+    channel.loadInfo = loadInfo;
+    channel.contentStream =
+      requestType === this.REQUEST_TYPE.PAGE
+        ? this._pageInputStream
+        : this._scriptInputStream;
+
+    if (requestType === this.REQUEST_TYPE.SCRIPT) {
+      this.setState(this.STATES.PAGE_AND_SCRIPT_CONSUMED);
+      this.reportUsageResult(true /* success */);
+    } else {
+      this.setState(this.STATES.PAGE_CONSUMED);
+      // Stash the BrowsingContext ID so that when the script stream
+      // attempts to be consumed, we ensure that it's from the same
+      // BrowsingContext that loaded the page.
+      this._consumerBCID = loadInfo.browsingContextID;
+    }
+
+    return channel;
+  },
+
+  /**
+   * This function takes the state information required to generate
+   * the about:home cache markup and script, and then generates that
+   * markup in script asynchronously. Once that's done, a message
+   * is sent to the parent process with the nsIInputStream's for the
+   * markup and script contents.
+   *
+   * @param state (Object)
+   *   The Redux state of the about:home document to render.
+   * @return Promise
+   * @resolves undefined
+   *   After the message with the nsIInputStream's have been sent to
+   *   the parent.
+   */
+  async constructAndSendCache(state) {
+    if (!IS_PRIVILEGED_PROCESS) {
+      throw new Error("Wrong process type.");
+    }
+
+    let worker = this.getOrCreateWorker();
+
+    TelemetryStopwatch.start("FX_ABOUTHOME_CACHE_CONSTRUCTION");
+
+    let { page, script } = await worker
+      .post("construct", [state])
+      .finally(() => {
+        TelemetryStopwatch.finish("FX_ABOUTHOME_CACHE_CONSTRUCTION");
+      });
+
+    let pageInputStream = Cc[
+      "@mozilla.org/io/string-input-stream;1"
+    ].createInstance(Ci.nsIStringInputStream);
+
+    pageInputStream.setUTF8Data(page);
+
+    let scriptInputStream = Cc[
+      "@mozilla.org/io/string-input-stream;1"
+    ].createInstance(Ci.nsIStringInputStream);
+
+    scriptInputStream.setUTF8Data(script);
+
+    Services.cpmm.sendAsyncMessage(this.CACHE_RESPONSE_MESSAGE, {
+      pageInputStream,
+      scriptInputStream,
+    });
+  },
+
+  _cacheWorker: null,
+  getOrCreateWorker() {
+    if (this._cacheWorker) {
+      return this._cacheWorker;
+    }
+
+    this._cacheWorker = new lazy.BasePromiseWorker(CACHE_WORKER_URL);
+    return this._cacheWorker;
+  },
+
+  receiveMessage(message) {
+    if (message.name === this.CACHE_REQUEST_MESSAGE) {
+      let { state } = message.data;
+      this.constructAndSendCache(state);
+    }
+  },
+
+  reportUsageResult(success) {
+    Services.cpmm.sendAsyncMessage(this.CACHE_USAGE_RESULT_MESSAGE, {
+      success,
+    });
+  },
+
+  observe(subject, topic, data) {
+    if (topic === "memory-pressure" && this._cacheWorker) {
+      this._cacheWorker.terminate();
+      this._cacheWorker = null;
+    }
+  },
+
+  /**
+   * Transitions the AboutHomeStartupCacheChild from one state
+   * to the next, where each state is defined in this.STATES.
+   *
+   * States can only be transitioned in increasing order, otherwise
+   * an error is logged.
+   */
+  setState(state) {
+    if (state > this._state) {
+      this._state = state;
+    } else {
+      console.error(
+        "AboutHomeStartupCacheChild could not transition from state " +
+          `${this._state} to ${state}`,
+        new Error().stack
+      );
+    }
+  },
+};
+
+/**
+ * This is an abstract base class for the nsIAboutNewTabService
+ * implementations that has some common methods and properties.
+ */
+class BaseAboutNewTabService {
+  constructor() {
+    if (!AppConstants.RELEASE_OR_BETA) {
+      XPCOMUtils.defineLazyPreferenceGetter(
+        this,
+        "activityStreamDebug",
+        PREF_ACTIVITY_STREAM_DEBUG,
+        false
+      );
+    } else {
+      this.activityStreamDebug = false;
+    }
+
+    XPCOMUtils.defineLazyPreferenceGetter(
+      this,
+      "privilegedAboutProcessEnabled",
+      PREF_SEPARATE_PRIVILEGEDABOUT_CONTENT_PROCESS,
+      false
+    );
+
+    this.classID = Components.ID("{cb36c925-3adc-49b3-b720-a5cc49d8a40e}");
+    this.QueryInterface = ChromeUtils.generateQI([
+      "nsIAboutNewTabService",
+      "nsIObserver",
+    ]);
+  }
+
+  /**
+   * Returns the default URL.
+   *
+   * This URL depends on various activity stream prefs. Overriding
+   * the newtab page has no effect on the result of this function.
+   */
+  get defaultURL() {
+    // Generate the desired activity stream resource depending on state, e.g.,
+    // "resource://activity-stream/prerendered/activity-stream.html"
+    // "resource://activity-stream/prerendered/activity-stream-debug.html"
+    // "resource://activity-stream/prerendered/activity-stream-noscripts.html"
+    return [
+      "resource://activity-stream/prerendered/",
+      "activity-stream",
+      // Debug version loads dev scripts but noscripts separately loads scripts
+      this.activityStreamDebug && !this.privilegedAboutProcessEnabled
+        ? "-debug"
+        : "",
+      this.privilegedAboutProcessEnabled ? "-noscripts" : "",
+      ".html",
+    ].join("");
+  }
+
+  get welcomeURL() {
+    /*
+     * Returns the about:welcome URL
+     *
+     * This is calculated in the same way the default URL is.
+     */
+
+    lazy.NimbusFeatures.aboutwelcome.recordExposureEvent({ once: true });
+    if (lazy.NimbusFeatures.aboutwelcome.getVariable("enabled") ?? true) {
+      return ABOUT_WELCOME_URL;
+    }
+    return this.defaultURL;
+  }
+
+  aboutHomeChannel(uri, loadInfo) {
+    throw Components.Exception(
+      "AboutHomeChannel not implemented for this process.",
+      Cr.NS_ERROR_NOT_IMPLEMENTED
+    );
+  }
+}
+
+/**
+ * The child-process implementation of nsIAboutNewTabService,
+ * which also does the work of redirecting about:home loads to
+ * the about:home startup cache if its available.
+ */
+class AboutNewTabChildService extends BaseAboutNewTabService {
+  aboutHomeChannel(uri, loadInfo) {
+    if (IS_PRIVILEGED_PROCESS) {
+      let cacheChannel = AboutHomeStartupCacheChild.maybeGetCachedPageChannel(
+        uri,
+        loadInfo
+      );
+      if (cacheChannel) {
+        return cacheChannel;
+      }
+    }
+
+    let pageURI = Services.io.newURI(this.defaultURL);
+    let fileChannel = Services.io.newChannelFromURIWithLoadInfo(
+      pageURI,
+      loadInfo
+    );
+    fileChannel.originalURI = uri;
+    return fileChannel;
+  }
+}
+
+/**
+ * The AboutNewTabStubService is a function called in both the main and
+ * content processes when trying to get at the nsIAboutNewTabService. This
+ * function does the job of choosing the appropriate implementation of
+ * nsIAboutNewTabService for the process type.
+ */
+function AboutNewTabStubService() {
+  if (Services.appinfo.processType === Services.appinfo.PROCESS_TYPE_DEFAULT) {
+    return new BaseAboutNewTabService();
+  }
+  return new AboutNewTabChildService();
+}