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diff --git a/toolkit/components/crashmonitor/CrashMonitor.jsm b/toolkit/components/crashmonitor/CrashMonitor.jsm
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+++ b/toolkit/components/crashmonitor/CrashMonitor.jsm
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+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* 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/. */
+
+/**
+ * Crash Monitor
+ *
+ * Monitors execution of a program to detect possible crashes. After
+ * program termination, the monitor can be queried during the next run
+ * to determine whether the last run exited cleanly or not.
+ *
+ * The monitoring is done by registering and listening for special
+ * notifications, or checkpoints, known to be sent by the monitored
+ * program as different stages in the execution are reached. As they
+ * are observed, these notifications are written asynchronously to a
+ * checkpoint file.
+ *
+ * During next program startup the crash monitor reads the checkpoint
+ * file from the last session. If notifications are missing, a crash
+ * has likely happened. By inspecting the notifications present, it is
+ * possible to determine what stages were reached in the program
+ * before the crash.
+ *
+ * Note that since the file is written asynchronously it is possible
+ * that a received notification is lost if the program crashes right
+ * after a checkpoint, but before crash monitor has been able to write
+ * it to disk. Thus, while the presence of a notification in the
+ * checkpoint file tells us that the corresponding stage was reached
+ * during the last run, the absence of a notification after a crash
+ * does not necessarily tell us that the checkpoint wasn't reached.
+ */
+
+var EXPORTED_SYMBOLS = ["CrashMonitor"];
+
+const { PrivateBrowsingUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PrivateBrowsingUtils.sys.mjs"
+);
+const { PromiseUtils } = ChromeUtils.importESModule(
+  "resource://gre/modules/PromiseUtils.sys.mjs"
+);
+
+const SESSIONSTORE_WINDOWS_RESTORED_TOPIC = "sessionstore-windows-restored";
+const SESSIONSTORE_FINAL_STATE_WRITE_COMPLETE_TOPIC =
+  "sessionstore-final-state-write-complete";
+
+const NOTIFICATIONS = [
+  "final-ui-startup",
+  SESSIONSTORE_WINDOWS_RESTORED_TOPIC,
+  "quit-application-granted",
+  "quit-application",
+  "profile-change-net-teardown",
+  "profile-change-teardown",
+  SESSIONSTORE_FINAL_STATE_WRITE_COMPLETE_TOPIC,
+];
+
+const SHUTDOWN_PHASES = ["profile-before-change"];
+
+var CrashMonitorInternal = {
+  /**
+   * Notifications received during the current session.
+   *
+   * Object where a property with a value of |true| means that the
+   * notification of the same name has been received at least once by
+   * the CrashMonitor during this session. Notifications that have not
+   * yet been received are not present as properties. |NOTIFICATIONS|
+   * lists the notifications tracked by the CrashMonitor.
+   */
+  checkpoints: {},
+
+  /**
+   * A deferred promise that resolves when all checkpoints have been written.
+   */
+  sessionStoreFinalWriteComplete: PromiseUtils.defer(),
+
+  /**
+   * Notifications received during previous session.
+   *
+   * Available after |loadPreviousCheckpoints|. Promise which resolves
+   * to an object containing a set of properties, where a property
+   * with a value of |true| means that the notification with the same
+   * name as the property name was received at least once last
+   * session.
+   */
+  previousCheckpoints: null,
+
+  /**
+   * Path to checkpoint file.
+   *
+   * Each time a new notification is received, this file is written to
+   * disc to reflect the information in |checkpoints|.
+   */
+  path: PathUtils.join(
+    Services.dirsvc.get("ProfD", Ci.nsIFile).path,
+    "sessionCheckpoints.json"
+  ),
+
+  /**
+   * Load checkpoints from previous session asynchronously.
+   *
+   * @return {Promise} A promise that resolves/rejects once loading is complete
+   */
+  loadPreviousCheckpoints() {
+    this.previousCheckpoints = (async function() {
+      let notifications;
+      try {
+        notifications = await IOUtils.readJSON(CrashMonitorInternal.path);
+      } catch (ex) {
+        // Ignore file not found errors, but report all others.
+        if (ex.name !== "NotFoundError") {
+          Cu.reportError(
+            `Error while loading crash monitor data: ${ex.message}`
+          );
+        }
+        return null;
+      }
+
+      // If `notifications` isn't an object, then the monitor data isn't valid.
+      if (Object(notifications) !== notifications) {
+        Cu.reportError(
+          "Error while parsing crash monitor data: invalid monitor data"
+        );
+        return null;
+      }
+
+      return Object.freeze(notifications);
+    })();
+
+    return this.previousCheckpoints;
+  },
+};
+
+var CrashMonitor = {
+  /**
+   * Notifications received during previous session.
+   *
+   * Return object containing the set of notifications received last
+   * session as keys with values set to |true|.
+   *
+   * @return {Promise} A promise resolving to previous checkpoints
+   */
+  get previousCheckpoints() {
+    if (!CrashMonitorInternal.initialized) {
+      throw new Error(
+        "CrashMonitor must be initialized before getting previous checkpoints"
+      );
+    }
+
+    return CrashMonitorInternal.previousCheckpoints;
+  },
+
+  /**
+   * Initialize CrashMonitor.
+   *
+   * Should only be called from the CrashMonitor XPCOM component.
+   *
+   * @return {Promise}
+   */
+  init() {
+    if (CrashMonitorInternal.initialized) {
+      throw new Error("CrashMonitor.init() must only be called once!");
+    }
+
+    let promise = CrashMonitorInternal.loadPreviousCheckpoints();
+    // Add "profile-after-change" to checkpoint as this method is
+    // called after receiving it
+    CrashMonitorInternal.checkpoints["profile-after-change"] = true;
+
+    NOTIFICATIONS.forEach(function(aTopic) {
+      Services.obs.addObserver(this, aTopic);
+    }, this);
+
+    // Add shutdown blocker for profile-before-change
+    IOUtils.profileBeforeChange.addBlocker(
+      "CrashMonitor: Writing notifications to file after receiving profile-before-change and awaiting all checkpoints written",
+      async () => {
+        await this.writeCheckpoint("profile-before-change");
+
+        // If SessionStore has not initialized, we don't want to wait for
+        // checkpoints that we won't hit, or we'll crash the browser during
+        // async shutdown.
+        if (
+          !PrivateBrowsingUtils.permanentPrivateBrowsing &&
+          CrashMonitorInternal.checkpoints[SESSIONSTORE_WINDOWS_RESTORED_TOPIC]
+        ) {
+          await CrashMonitorInternal.sessionStoreFinalWriteComplete.promise;
+        }
+      },
+      () => CrashMonitorInternal.checkpoints
+    );
+
+    CrashMonitorInternal.initialized = true;
+    return promise;
+  },
+
+  /**
+   * Handle registered notifications.
+   *
+   * Update checkpoint file for every new notification received.
+   */
+  observe(aSubject, aTopic, aData) {
+    this.writeCheckpoint(aTopic);
+
+    if (
+      NOTIFICATIONS.every(elem => elem in CrashMonitorInternal.checkpoints) &&
+      SHUTDOWN_PHASES.every(elem => elem in CrashMonitorInternal.checkpoints)
+    ) {
+      // All notifications received, unregister observers
+      NOTIFICATIONS.forEach(function(aTopic) {
+        Services.obs.removeObserver(this, aTopic);
+      }, this);
+    }
+
+    if (aTopic === SESSIONSTORE_FINAL_STATE_WRITE_COMPLETE_TOPIC) {
+      CrashMonitorInternal.sessionStoreFinalWriteComplete.resolve();
+    }
+  },
+
+  async writeCheckpoint(aCheckpoint) {
+    if (!(aCheckpoint in CrashMonitorInternal.checkpoints)) {
+      // If this is the first time this notification is received,
+      // remember it and write it to file
+      CrashMonitorInternal.checkpoints[aCheckpoint] = true;
+
+      /* Write to the checkpoint file asynchronously, off the main
+       * thread, for performance reasons. Note that this means
+       * that there's not a 100% guarantee that the file will be
+       * written by the time the notification completes. The
+       * exception is profile-before-change which has a shutdown
+       * blocker. */
+      await IOUtils.writeJSON(
+        CrashMonitorInternal.path,
+        CrashMonitorInternal.checkpoints,
+        {
+          tmpPath: CrashMonitorInternal.path + ".tmp",
+        }
+      );
+    }
+  },
+};
+Object.freeze(CrashMonitor);