Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

11 files changed, 2435 insertions, 0 deletions

diff --git a/toolkit/components/asyncshutdown/AsyncShutdown.sys.mjs b/toolkit/components/asyncshutdown/AsyncShutdown.sys.mjs
new file mode 100644
index 0000000000..4f0adc092a
--- /dev/null
+++ b/toolkit/components/asyncshutdown/AsyncShutdown.sys.mjs
@@ -0,0 +1,1122 @@
+/* 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/. */
+
+/**
+ * Managing safe shutdown of asynchronous services.
+ *
+ * Firefox shutdown is composed of phases that take place
+ * sequentially. Typically, each shutdown phase removes some
+ * capabilities from the application. For instance, at the end of
+ * phase profileBeforeChange, no service is permitted to write to the
+ * profile directory (with the exception of Telemetry). Consequently,
+ * if any service has requested I/O to the profile directory before or
+ * during phase profileBeforeChange, the system must be informed that
+ * these requests need to be completed before the end of phase
+ * profileBeforeChange. Failing to inform the system of this
+ * requirement can (and has been known to) cause data loss.
+ *
+ * Example: At some point during shutdown, the Add-On Manager needs to
+ * ensure that all add-ons have safely written their data to disk,
+ * before writing its own data. Since the data is saved to the
+ * profile, this must be completed during phase profileBeforeChange.
+ *
+ * AsyncShutdown.profileBeforeChange.addBlocker(
+ *   "Add-on manager: shutting down",
+ *   function condition() {
+ *     // Do things.
+ *     // Perform I/O that must take place during phase profile-before-change
+ *     return promise;
+ *   }
+ * });
+ *
+ * In this example, function |condition| will be called at some point
+ * during phase profileBeforeChange and phase profileBeforeChange
+ * itself is guaranteed to not terminate until |promise| is either
+ * resolved or rejected.
+ */
+
+import { XPCOMUtils } from "resource://gre/modules/XPCOMUtils.sys.mjs";
+
+const lazy = {};
+
+XPCOMUtils.defineLazyServiceGetter(
+  lazy,
+  "gDebug",
+  "@mozilla.org/xpcom/debug;1",
+  "nsIDebug2"
+);
+
+// `true` if this is a content process, `false` otherwise.
+// It would be nicer to go through `Services.appinfo`, but some tests need to be
+// able to replace that field with a custom implementation before it is first
+// called.
+const isContent =
+  // eslint-disable-next-line mozilla/use-services
+  Cc["@mozilla.org/xre/app-info;1"].getService(Ci.nsIXULRuntime).processType ==
+  Ci.nsIXULRuntime.PROCESS_TYPE_CONTENT;
+
+// Display timeout warnings after 10 seconds
+const DELAY_WARNING_MS = 10 * 1000;
+
+// Crash the process if shutdown is really too long
+// (allowing for sleep).
+const PREF_DELAY_CRASH_MS = "toolkit.asyncshutdown.crash_timeout";
+var DELAY_CRASH_MS = Services.prefs.getIntPref(PREF_DELAY_CRASH_MS, 60 * 1000); // One minute
+Services.prefs.addObserver(PREF_DELAY_CRASH_MS, function () {
+  DELAY_CRASH_MS = Services.prefs.getIntPref(PREF_DELAY_CRASH_MS);
+});
+
+/**
+ * Any addBlocker calls that failed. We add this into barrier wait
+ * crash annotations to help with debugging. When we fail to add
+ * shutdown blockers that can break shutdown. We track these globally
+ * rather than per barrier because one failure mode is when the
+ * barrier has already finished by the time addBlocker is invoked -
+ * but the failure to add the blocker may result in later barriers
+ * waiting indefinitely, so the debug information is still useful
+ * for those later barriers. See bug 1801674 for more context.
+ */
+let gBrokenAddBlockers = [];
+
+/**
+ * A set of Promise that supports waiting.
+ *
+ * Promise items may be added or removed during the wait. The wait will
+ * resolve once all Promise items have been resolved or removed.
+ */
+function PromiseSet() {
+  /**
+   * key: the Promise passed pass the client of the `PromiseSet`.
+   * value: an indirection on top of `key`, as an object with
+   *   the following fields:
+   *   - indirection: a Promise resolved if `key` is resolved or
+   *     if `resolve` is called
+   *   - resolve: a function used to resolve the indirection.
+   */
+  this._indirections = new Map();
+  // Once all the tracked promises have been resolved we are done. Once Wait()
+  // resolves, it should not be possible anymore to add further promises.
+  // This covers for a possibly rare case, where something may try to add a
+  // blocker after wait() is done, that would never be awaited for.
+  this._done = false;
+}
+PromiseSet.prototype = {
+  /**
+   * Wait until all Promise have been resolved or removed.
+   *
+   * Note that calling `wait()` causes Promise to be removed from the
+   * Set once they are resolved.
+   * @param {function} onDoneCb invoked synchronously once all the entries
+   * have been handled and no new entries will be accepted.
+   * @return {Promise} Resolved once all Promise have been resolved or removed,
+   * or rejected after at least one Promise has rejected.
+   */
+  wait(onDoneCb) {
+    // Pick an arbitrary element in the map, if any exists.
+    let entry = this._indirections.entries().next();
+    if (entry.done) {
+      // No indirections left, we are done.
+      this._done = true;
+      onDoneCb();
+      return Promise.resolve();
+    }
+
+    let [, indirection] = entry.value;
+    let promise = indirection.promise;
+    promise = promise.then(() =>
+      // At this stage, the entry has been cleaned up.
+      this.wait(onDoneCb)
+    );
+    return promise;
+  },
+
+  /**
+   * Add a new Promise to the set.
+   *
+   * Calls to wait (including ongoing calls) will only return once
+   * `key` has either resolved or been removed.
+   */
+  add(key) {
+    if (this._done) {
+      throw new Error("Wait is complete, cannot add further promises.");
+    }
+    this._ensurePromise(key);
+    let indirection = Promise.withResolvers();
+    key
+      .then(
+        x => {
+          // Clean up immediately.
+          // This needs to be done before the call to `resolve`, otherwise
+          // `wait()` may loop forever.
+          this._indirections.delete(key);
+          indirection.resolve(x);
+        },
+        err => {
+          this._indirections.delete(key);
+          indirection.reject(err);
+        }
+      )
+      .finally(() => {
+        this._indirections.delete(key);
+        // Normally the promise is resolved or rejected, but if its global
+        // goes away, only finally may be invoked. In all the other cases this
+        // is a no-op since the promise has been fulfilled already.
+        indirection.reject(
+          new Error("Promise not fulfilled, did it lost its global?")
+        );
+      });
+    this._indirections.set(key, indirection);
+  },
+
+  /**
+   * Remove a Promise from the set.
+   *
+   * Calls to wait (including ongoing calls) will ignore this promise,
+   * unless it is added again.
+   */
+  delete(key) {
+    this._ensurePromise(key);
+    let value = this._indirections.get(key);
+    if (!value) {
+      return false;
+    }
+    this._indirections.delete(key);
+    value.resolve();
+    return true;
+  },
+
+  _ensurePromise(key) {
+    if (!key || typeof key != "object") {
+      throw new Error("Expected an object");
+    }
+    if (!("then" in key) || typeof key.then != "function") {
+      throw new Error("Expected a Promise");
+    }
+  },
+};
+
+/**
+ * Display a warning.
+ *
+ * As this code is generally used during shutdown, there are chances
+ * that the UX will not be available to display warnings on the
+ * console. We therefore use dump() rather than Cu.reportError().
+ */
+function log(msg, prefix = "", error = null) {
+  try {
+    dump(prefix + msg + "\n");
+    if (error) {
+      dump(prefix + error + "\n");
+      if (typeof error == "object" && "stack" in error) {
+        dump(prefix + error.stack + "\n");
+      }
+    }
+  } catch (ex) {
+    dump("INTERNAL ERROR in AsyncShutdown: cannot log message.\n");
+  }
+}
+const PREF_DEBUG_LOG = "toolkit.asyncshutdown.log";
+var DEBUG_LOG = Services.prefs.getBoolPref(PREF_DEBUG_LOG, false);
+Services.prefs.addObserver(PREF_DEBUG_LOG, function () {
+  DEBUG_LOG = Services.prefs.getBoolPref(PREF_DEBUG_LOG);
+});
+
+function debug(msg, error = null) {
+  if (DEBUG_LOG) {
+    log(msg, "DEBUG: ", error);
+  }
+}
+function warn(msg, error = null) {
+  log(msg, "WARNING: ", error);
+}
+function fatalerr(msg, error = null) {
+  log(msg, "FATAL ERROR: ", error);
+}
+
+// Utility function designed to get the current state of execution
+// of a blocker.
+// We are a little paranoid here to ensure that in case of evaluation
+// error we do not block the AsyncShutdown.
+function safeGetState(fetchState) {
+  if (!fetchState) {
+    return "(none)";
+  }
+  let data, string;
+  try {
+    // Evaluate fetchState(), normalize the result into something that we can
+    // safely stringify or upload.
+    let state = fetchState();
+    if (!state) {
+      return "(none)";
+    }
+    string = JSON.stringify(state);
+    data = JSON.parse(string);
+    // Simplify the rest of the code by ensuring that we can simply
+    // concatenate the result to a message.
+    if (data && typeof data == "object") {
+      data.toString = function () {
+        return string;
+      };
+    }
+    return data;
+  } catch (ex) {
+    // Make sure that this causes test failures
+    Promise.reject(ex);
+
+    if (string) {
+      return string;
+    }
+    try {
+      return "Error getting state: " + ex + " at " + ex.stack;
+    } catch (ex2) {
+      return "Error getting state but could not display error";
+    }
+  }
+}
+
+/**
+ * Countdown for a given duration, skipping beats if the computer is too busy,
+ * sleeping or otherwise unavailable.
+ *
+ * @param {number} delay An approximate delay to wait in milliseconds (rounded
+ * up to the closest second).
+ *
+ * @return Deferred
+ */
+function looseTimer(delay) {
+  let DELAY_BEAT = 1000;
+  let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+  let beats = Math.ceil(delay / DELAY_BEAT);
+  let deferred = Promise.withResolvers();
+  timer.initWithCallback(
+    function () {
+      if (beats <= 0) {
+        deferred.resolve();
+      }
+      --beats;
+    },
+    DELAY_BEAT,
+    Ci.nsITimer.TYPE_REPEATING_PRECISE_CAN_SKIP
+  );
+  // Ensure that the timer is both canceled once we are done with it
+  // and not garbage-collected until then.
+  deferred.promise.then(
+    () => timer.cancel(),
+    () => timer.cancel()
+  );
+  return deferred;
+}
+
+/**
+ * Given an nsIStackFrame object, find the caller filename, line number,
+ * and stack if necessary, and return them as an object.
+ *
+ * @param {nsIStackFrame} topFrame Top frame of the call stack.
+ * @param {string} filename Pre-supplied filename or null if unknown.
+ * @param {number} lineNumber Pre-supplied line number or null if unknown.
+ * @param {string} stack Pre-supplied stack or null if unknown.
+ *
+ * @return object
+ */
+function getOrigin(topFrame, filename = null, lineNumber = null, stack = null) {
+  try {
+    // Determine the filename and line number of the caller.
+    let frame = topFrame;
+
+    for (; frame && frame.filename == topFrame.filename; frame = frame.caller) {
+      // Climb up the stack
+    }
+
+    if (filename == null) {
+      filename = frame ? frame.filename : "?";
+    }
+    if (lineNumber == null) {
+      lineNumber = frame ? frame.lineNumber : 0;
+    }
+    if (stack == null) {
+      // Now build the rest of the stack as a string, using Task.jsm's rewriting
+      // to ensure that we do not lose information at each call to `Task.spawn`.
+      stack = [];
+      while (frame != null) {
+        stack.push(frame.filename + ":" + frame.name + ":" + frame.lineNumber);
+        frame = frame.caller;
+      }
+    }
+
+    return {
+      filename,
+      lineNumber,
+      stack,
+    };
+  } catch (ex) {
+    return {
+      filename: "<internal error: could not get origin>",
+      lineNumber: -1,
+      stack: "<internal error: could not get origin>",
+    };
+  }
+}
+
+/**
+ * {string} topic -> phase
+ */
+var gPhases = new Map();
+
+export var AsyncShutdown = {
+  /**
+   * Access function getPhase. For testing purposes only.
+   */
+  get _getPhase() {
+    let accepted = Services.prefs.getBoolPref(
+      "toolkit.asyncshutdown.testing",
+      false
+    );
+    if (accepted) {
+      return getPhase;
+    }
+    return undefined;
+  },
+
+  /**
+   * This constant is used as the amount of milliseconds to allow shutdown to be
+   * blocked until we crash the process forcibly and is read from the
+   * 'toolkit.asyncshutdown.crash_timeout' pref.
+   */
+  get DELAY_CRASH_MS() {
+    return DELAY_CRASH_MS;
+  },
+};
+
+/**
+ * Register a new phase.
+ *
+ * @param {string} topic The notification topic for this Phase.
+ * @see {https://developer.mozilla.org/en-US/docs/Observer_Notifications}
+ */
+function getPhase(topic) {
+  let phase = gPhases.get(topic);
+  if (phase) {
+    return phase;
+  }
+  let spinner = new Spinner(topic);
+  phase = Object.freeze({
+    /**
+     * Register a blocker for the completion of a phase.
+     *
+     * @param {string} name The human-readable name of the blocker. Used
+     * for debugging/error reporting. Please make sure that the name
+     * respects the following model: "Some Service: some action in progress" -
+     * for instance "OS.File: flushing all pending I/O";
+     * @param {function|promise|*} condition A condition blocking the
+     * completion of the phase. Generally, this is a function
+     * returning a promise. This function is evaluated during the
+     * phase and the phase is guaranteed to not terminate until the
+     * resulting promise is either resolved or rejected. If
+     * |condition| is not a function but another value |v|, it behaves
+     * as if it were a function returning |v|.
+     * @param {object*} details Optionally, an object with details
+     * that may be useful for error reporting, as a subset of of the following
+     * fields:
+     * - fetchState (strongly recommended) A function returning
+     *    information about the current state of the blocker as an
+     *    object. Used for providing more details when logging errors or
+     *    crashing.
+     * - stack. A string containing stack information. This module can
+     *    generally infer stack information if it is not provided.
+     * - lineNumber A number containing the line number for the caller.
+     *    This module can generally infer this information if it is not
+     *    provided.
+     * - filename A string containing the filename for the caller. This
+     *    module can generally infer  the information if it is not provided.
+     *
+     * Examples:
+     * AsyncShutdown.profileBeforeChange.addBlocker("Module: just a promise",
+     *      promise); // profileBeforeChange will not complete until
+     *                // promise is resolved or rejected
+     *
+     * AsyncShutdown.profileBeforeChange.addBlocker("Module: a callback",
+     *     function callback() {
+     *       // ...
+     *       // Execute this code during profileBeforeChange
+     *       return promise;
+     *       // profileBeforeChange will not complete until promise
+     *       // is resolved or rejected
+     * });
+     *
+     * AsyncShutdown.profileBeforeChange.addBlocker("Module: trivial callback",
+     *     function callback() {
+     *       // ...
+     *       // Execute this code during profileBeforeChange
+     *       // No specific guarantee about completion of profileBeforeChange
+     * });
+     */
+    addBlocker(name, condition, details = null) {
+      spinner.addBlocker(name, condition, details);
+    },
+    /**
+     * Remove the blocker for a condition.
+     *
+     * If several blockers have been registered for the same
+     * condition, remove all these blockers. If no blocker has been
+     * registered for this condition, this is a noop.
+     *
+     * @return {boolean} true if a blocker has been removed, false
+     * otherwise. Note that a result of false may mean either that
+     * the blocker has never been installed or that the phase has
+     * completed and the blocker has already been resolved.
+     */
+    removeBlocker(condition) {
+      return spinner.removeBlocker(condition);
+    },
+
+    get name() {
+      return spinner.name;
+    },
+
+    get isClosed() {
+      return spinner.isClosed;
+    },
+
+    /**
+     * Trigger the phase without having to broadcast a
+     * notification. For testing purposes only.
+     */
+    get _trigger() {
+      let accepted = Services.prefs.getBoolPref(
+        "toolkit.asyncshutdown.testing",
+        false
+      );
+      if (accepted) {
+        return () => spinner.observe();
+      }
+      return undefined;
+    },
+  });
+  gPhases.set(topic, phase);
+  return phase;
+}
+
+/**
+ * Utility class used to spin the event loop until all blockers for a
+ * Phase are satisfied.
+ *
+ * @param {string} topic The xpcom notification for that phase.
+ */
+function Spinner(topic) {
+  this._barrier = new Barrier(topic);
+  this._topic = topic;
+  Services.obs.addObserver(this, topic);
+}
+
+Spinner.prototype = {
+  /**
+   * Register a new condition for this phase.
+   *
+   * See the documentation of `addBlocker` in property `client`
+   * of instances of `Barrier`.
+   */
+  addBlocker(name, condition, details) {
+    this._barrier.client.addBlocker(name, condition, details);
+  },
+  /**
+   * Remove the blocker for a condition.
+   *
+   * See the documentation of `removeBlocker` in rpoperty `client`
+   * of instances of `Barrier`
+   *
+   * @return {boolean} true if a blocker has been removed, false
+   * otherwise. Note that a result of false may mean either that
+   * the blocker has never been installed or that the phase has
+   * completed and the blocker has already been resolved.
+   */
+  removeBlocker(condition) {
+    return this._barrier.client.removeBlocker(condition);
+  },
+
+  get name() {
+    return this._barrier.client.name;
+  },
+
+  get isClosed() {
+    return this._barrier.client.isClosed;
+  },
+
+  // nsIObserver.observe
+  observe() {
+    let topic = this._topic;
+    debug(`Starting phase ${topic}`);
+    Services.obs.removeObserver(this, topic);
+
+    // Setup the promise that will signal our phase's end.
+    let isPhaseEnd = false;
+    try {
+      this._barrier
+        .wait({
+          warnAfterMS: DELAY_WARNING_MS,
+          crashAfterMS: DELAY_CRASH_MS,
+        })
+        .finally(() => {
+          isPhaseEnd = true;
+        });
+    } catch (ex) {
+      debug("Error waiting for notification");
+      throw ex;
+    }
+
+    // Now, spin the event loop. In case of a hang we will just crash without
+    // ever leaving this loop.
+    debug("Spinning the event loop");
+    Services.tm.spinEventLoopUntil(
+      `AsyncShutdown Spinner for ${topic}`,
+      () => isPhaseEnd
+    );
+
+    debug(`Finished phase ${topic}`);
+  },
+};
+
+/**
+ * A mechanism used to register blockers that prevent some action from
+ * happening.
+ *
+ * An instance of |Barrier| provides a capability |client| that
+ * clients can use to register blockers. The barrier is resolved once
+ * all registered blockers have been resolved. The owner of the
+ * |Barrier| may wait for the resolution of the barrier and obtain
+ * information on which blockers have not been resolved yet.
+ *
+ * @param {string} name The name of the blocker. Used mainly for error-
+ *     reporting.
+ */
+function Barrier(name) {
+  if (!name) {
+    throw new TypeError("Instances of Barrier need a (non-empty) name");
+  }
+
+  /**
+   * The set of all Promise for which we need to wait before the barrier
+   * is lifted. Note that this set may be changed while we are waiting.
+   *
+   * Set to `null` once the wait is complete.
+   */
+  this._waitForMe = new PromiseSet();
+
+  /**
+   * A map from conditions, as passed by users during the call to `addBlocker`,
+   * to `promise`, as present in `this._waitForMe`.
+   *
+   * Used to let users perform cleanup through `removeBlocker`.
+   * Set to `null` once the wait is complete.
+   *
+   * Key: condition (any, as passed by user)
+   * Value: promise used as a key in `this._waitForMe`. Note that there is
+   * no guarantee that the key is still present in `this._waitForMe`.
+   */
+  this._conditionToPromise = new Map();
+
+  /**
+   * A map from Promise, as present in `this._waitForMe` or
+   * `this._conditionToPromise`, to information on blockers.
+   *
+   * Key: Promise (as present in this._waitForMe or this._conditionToPromise).
+   * Value:  {
+   *  trigger: function,
+   *  promise,
+   *  name,
+   *  fetchState: function,
+   *  stack,
+   *  filename,
+   *  lineNumber
+   * };
+   */
+  this._promiseToBlocker = new Map();
+
+  /**
+   * The name of the barrier.
+   */
+  if (typeof name != "string") {
+    throw new TypeError("The name of the barrier must be a string");
+  }
+  this._name = name;
+
+  /**
+   * A cache for the promise returned by wait().
+   */
+  this._promise = null;
+
+  /**
+   * `true` once we have started waiting.
+   */
+  this._isStarted = false;
+
+  /**
+   * `true` once we're done and won't accept any new blockers.
+   */
+  this._isClosed = false;
+
+  /**
+   * The capability of adding blockers. This object may safely be returned
+   * or passed to clients.
+   */
+  this.client = {
+    /**
+     * The name of the barrier owning this client.
+     */
+    get name() {
+      return name;
+    },
+
+    /**
+     * Register a blocker for the completion of this barrier.
+     *
+     * @param {string} name The human-readable name of the blocker. Used
+     * for debugging/error reporting. Please make sure that the name
+     * respects the following model: "Some Service: some action in progress" -
+     * for instance "OS.File: flushing all pending I/O";
+     * @param {function|promise|*} condition A condition blocking the
+     * completion of the phase. Generally, this is a function
+     * returning a promise. This function is evaluated during the
+     * phase and the phase is guaranteed to not terminate until the
+     * resulting promise is either resolved or rejected. If
+     * |condition| is not a function but another value |v|, it behaves
+     * as if it were a function returning |v|.
+     * @param {object*} details Optionally, an object with details
+     * that may be useful for error reporting, as a subset of of the following
+     * fields:
+     * - fetchState (strongly recommended) A function returning
+     *    information about the current state of the blocker as an
+     *    object. Used for providing more details when logging errors or
+     *    crashing.
+     * - stack. A string containing stack information. This module can
+     *    generally infer stack information if it is not provided.
+     * - lineNumber A number containing the line number for the caller.
+     *    This module can generally infer this information if it is not
+     *    provided.
+     * - filename A string containing the filename for the caller. This
+     *    module can generally infer  the information if it is not provided.
+     */
+    addBlocker: (name, condition, details) => {
+      if (typeof name != "string") {
+        gBrokenAddBlockers.push("No-name blocker");
+        throw new TypeError("Expected a human-readable name as first argument");
+      }
+      if (details && typeof details == "function") {
+        details = {
+          fetchState: details,
+        };
+      } else if (!details) {
+        details = {};
+      }
+      if (typeof details != "object") {
+        gBrokenAddBlockers.push(`${name} - invalid details`);
+        throw new TypeError(
+          "Expected an object as third argument to `addBlocker`, got " + details
+        );
+      }
+      if (!this._waitForMe) {
+        gBrokenAddBlockers.push(`${name} - ${this._name} finished`);
+        throw new Error(
+          `Phase "${this._name}" is finished, it is too late to register completion condition "${name}"`
+        );
+      }
+      debug(`Adding blocker ${name} for phase ${this._name}`);
+
+      try {
+        this.client._internalAddBlocker(name, condition, details);
+      } catch (ex) {
+        gBrokenAddBlockers.push(`${name} - ${ex.message}`);
+        throw ex;
+      }
+    },
+
+    _internalAddBlocker: (
+      name,
+      condition,
+      { fetchState = null, filename = null, lineNumber = null, stack = null }
+    ) => {
+      if (fetchState != null && typeof fetchState != "function") {
+        throw new TypeError("Expected a function for option `fetchState`");
+      }
+
+      // Split the condition between a trigger function and a promise.
+
+      // The function to call to notify the blocker that we have started waiting.
+      // This function returns a promise resolved/rejected once the
+      // condition is complete, and never throws.
+      let trigger;
+
+      // A promise resolved once the condition is complete.
+      let promise;
+      if (typeof condition == "function") {
+        promise = new Promise((resolve, reject) => {
+          trigger = () => {
+            try {
+              resolve(condition());
+            } catch (ex) {
+              reject(ex);
+            }
+          };
+        });
+      } else {
+        // If `condition` is not a function, `trigger` is not particularly
+        // interesting, and `condition` needs to be normalized to a promise.
+        trigger = () => {};
+        promise = Promise.resolve(condition);
+      }
+
+      // Make sure that `promise` never rejects.
+      promise = promise
+        .catch(error => {
+          let msg = `A blocker encountered an error while we were waiting.
+          Blocker:  ${name}
+          Phase: ${this._name}
+          State: ${safeGetState(fetchState)}`;
+          warn(msg, error);
+
+          // The error should remain uncaught, to ensure that it
+          // still causes tests to fail.
+          Promise.reject(error);
+        })
+        // Added as a last line of defense, in case `warn`, `this._name` or
+        // `safeGetState` somehow throws an error.
+        .catch(() => {});
+
+      let topFrame = null;
+      if (filename == null || lineNumber == null || stack == null) {
+        topFrame = Components.stack;
+      }
+
+      let blocker = {
+        trigger,
+        promise,
+        name,
+        fetchState,
+        getOrigin: () => getOrigin(topFrame, filename, lineNumber, stack),
+      };
+
+      this._waitForMe.add(promise);
+      this._promiseToBlocker.set(promise, blocker);
+      this._conditionToPromise.set(condition, promise);
+
+      // As conditions may hold lots of memory, we attempt to cleanup
+      // as soon as we are done (which might be in the next tick, if
+      // we have been passed a resolved promise).
+      promise = promise.then(() => {
+        debug(`Completed blocker ${name} for phase ${this._name}`);
+        this._removeBlocker(condition);
+      });
+
+      if (this._isStarted) {
+        // The wait has already started. The blocker should be
+        // notified asap. We do it out of band as clients probably
+        // expect `addBlocker` to return immediately.
+        Promise.resolve().then(trigger);
+      }
+    },
+
+    /**
+     * Remove the blocker for a condition.
+     *
+     * If several blockers have been registered for the same
+     * condition, remove all these blockers. If no blocker has been
+     * registered for this condition, this is a noop.
+     *
+     * @return {boolean} true if at least one blocker has been
+     * removed, false otherwise.
+     */
+    removeBlocker: condition => {
+      return this._removeBlocker(condition);
+    },
+  };
+
+  /**
+   * Whether this client still accepts new blockers.
+   */
+  Object.defineProperty(this.client, "isClosed", {
+    get: () => {
+      return this._isClosed;
+    },
+  });
+}
+Barrier.prototype = Object.freeze({
+  /**
+   * The current state of the barrier, as a JSON-serializable object
+   * designed for error-reporting.
+   */
+  get state() {
+    if (!this._isStarted) {
+      return "Not started";
+    }
+    if (!this._waitForMe) {
+      return "Complete";
+    }
+    let frozen = [];
+    for (let blocker of this._promiseToBlocker.values()) {
+      let { name, fetchState } = blocker;
+      let { stack, filename, lineNumber } = blocker.getOrigin();
+      frozen.push({
+        name,
+        state: safeGetState(fetchState),
+        filename,
+        lineNumber,
+        stack,
+      });
+    }
+    return frozen;
+  },
+
+  /**
+   * Wait until all currently registered blockers are complete.
+   *
+   * Once this method has been called, any attempt to register a new blocker
+   * for this barrier will cause an error.
+   *
+   * Successive calls to this method always return the same value.
+   *
+   * @param {object=}  options Optionally, an object  that may contain
+   * the following fields:
+   *  {number} warnAfterMS If provided and > 0, print a warning if the barrier
+   *   has not been resolved after the given number of milliseconds.
+   *  {number} crashAfterMS If provided and > 0, crash the process if the barrier
+   *   has not been resolved after the give number of milliseconds (rounded up
+   *   to the next second). To avoid crashing simply because the computer is busy
+   *   or going to sleep, we actually wait for ceil(crashAfterMS/1000) successive
+   *   periods of at least one second. Upon crashing, if a crash reporter is present,
+   *   prepare a crash report with the state of this barrier.
+   *
+   *
+   * @return {Promise} A promise satisfied once all blockers are complete.
+   */
+  wait(options = {}) {
+    // This method only implements caching on top of _wait()
+    if (this._promise) {
+      return this._promise;
+    }
+    return (this._promise = this._wait(options));
+  },
+  _wait(options) {
+    // Sanity checks
+    if (this._isStarted) {
+      throw new TypeError("Internal error: already started " + this._name);
+    }
+    if (
+      !this._waitForMe ||
+      !this._conditionToPromise ||
+      !this._promiseToBlocker
+    ) {
+      throw new TypeError("Internal error: already finished " + this._name);
+    }
+
+    let topic = this._name;
+
+    // Notify blockers
+    for (let blocker of this._promiseToBlocker.values()) {
+      blocker.trigger(); // We have guarantees that this method will never throw
+    }
+
+    this._isStarted = true;
+
+    // Now, wait
+    let promise = this._waitForMe.wait(() => {
+      this._isClosed = true;
+    });
+
+    promise = promise.catch(function onError(error) {
+      // I don't think that this can happen.
+      // However, let's be overcautious with async/shutdown error reporting.
+      let msg =
+        "An uncaught error appeared while completing the phase." +
+        " Phase: " +
+        topic;
+      warn(msg, error);
+    });
+
+    promise = promise.then(() => {
+      // Cleanup memory
+      this._waitForMe = null;
+      this._promiseToBlocker = null;
+      this._conditionToPromise = null;
+    });
+
+    // Now handle warnings and crashes
+    let warnAfterMS = DELAY_WARNING_MS;
+    if (options && "warnAfterMS" in options) {
+      if (
+        typeof options.warnAfterMS == "number" ||
+        options.warnAfterMS == null
+      ) {
+        // Change the delay or deactivate warnAfterMS
+        warnAfterMS = options.warnAfterMS;
+      } else {
+        throw new TypeError("Wrong option value for warnAfterMS");
+      }
+    }
+
+    if (warnAfterMS && warnAfterMS > 0) {
+      // If the promise takes too long to be resolved/rejected,
+      // we need to notify the user.
+      let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+      timer.initWithCallback(
+        () => {
+          let msg =
+            "At least one completion condition is taking too long to complete." +
+            " Conditions: " +
+            JSON.stringify(this.state) +
+            " Barrier: " +
+            topic;
+          warn(msg);
+        },
+        warnAfterMS,
+        Ci.nsITimer.TYPE_ONE_SHOT
+      );
+
+      promise = promise.then(function onSuccess() {
+        timer.cancel();
+        // As a side-effect, this prevents |timer| from
+        // being garbage-collected too early.
+      });
+    }
+
+    let crashAfterMS = DELAY_CRASH_MS;
+    if (options && "crashAfterMS" in options) {
+      if (
+        typeof options.crashAfterMS == "number" ||
+        options.crashAfterMS == null
+      ) {
+        // Change the delay or deactivate crashAfterMS
+        crashAfterMS = options.crashAfterMS;
+      } else {
+        throw new TypeError("Wrong option value for crashAfterMS");
+      }
+    }
+
+    if (crashAfterMS > 0) {
+      let timeToCrash = null;
+
+      // If after |crashAfterMS| milliseconds (adjusted to take into
+      // account sleep and otherwise busy computer) we have not finished
+      // this shutdown phase, we assume that the shutdown is somehow
+      // frozen, presumably deadlocked. At this stage, the only thing we
+      // can do to avoid leaving the user's computer in an unstable (and
+      // battery-sucking) situation is report the issue and crash.
+      timeToCrash = looseTimer(crashAfterMS);
+      timeToCrash.promise.then(
+        () => {
+          // Report the problem as best as we can, then crash.
+          let state = this.state;
+
+          // If you change the following message, please make sure
+          // that any information on the topic and state appears
+          // within the first 200 characters of the message. This
+          // helps automatically sort oranges.
+          let msg =
+            "AsyncShutdown timeout in " +
+            topic +
+            " Conditions: " +
+            JSON.stringify(state) +
+            " At least one completion condition failed to complete" +
+            " within a reasonable amount of time. Causing a crash to" +
+            " ensure that we do not leave the user with an unresponsive" +
+            " process draining resources.";
+          fatalerr(msg);
+          if (gBrokenAddBlockers.length) {
+            fatalerr(
+              "Broken addBlocker calls: " + JSON.stringify(gBrokenAddBlockers)
+            );
+          }
+          if (Services.appinfo.crashReporterEnabled) {
+            Services.appinfo.annotateCrashReport(
+              "AsyncShutdownTimeout",
+              JSON.stringify(this._gatherCrashReportTimeoutData(topic, state))
+            );
+          } else {
+            warn("No crash reporter available");
+          }
+
+          // To help sorting out bugs, we want to make sure that the
+          // call to nsIDebug2.abort points to a guilty client, rather
+          // than to AsyncShutdown itself. We pick a client that is
+          // still blocking and use its filename/lineNumber,
+          // which have been determined during the call to `addBlocker`.
+          let filename = "?";
+          let lineNumber = -1;
+          for (let blocker of this._promiseToBlocker.values()) {
+            ({ filename, lineNumber } = blocker.getOrigin());
+            break;
+          }
+          lazy.gDebug.abort(filename, lineNumber);
+        },
+        function onSatisfied() {
+          // The promise has been rejected, which means that we have satisfied
+          // all completion conditions.
+        }
+      );
+
+      promise = promise.then(
+        function () {
+          timeToCrash.reject();
+        } /* No error is possible here*/
+      );
+    }
+
+    return promise;
+  },
+
+  _gatherCrashReportTimeoutData(phase, conditions) {
+    let data = { phase, conditions };
+    if (gBrokenAddBlockers.length) {
+      data.brokenAddBlockers = gBrokenAddBlockers;
+    }
+    return data;
+  },
+
+  _removeBlocker(condition) {
+    if (
+      !this._waitForMe ||
+      !this._promiseToBlocker ||
+      !this._conditionToPromise
+    ) {
+      // We have already cleaned up everything.
+      return false;
+    }
+
+    let promise = this._conditionToPromise.get(condition);
+    if (!promise) {
+      // The blocker has already been removed
+      return false;
+    }
+    this._conditionToPromise.delete(condition);
+    this._promiseToBlocker.delete(promise);
+    return this._waitForMe.delete(promise);
+  },
+});
+
+// List of well-known phases
+// Ideally, phases should be registered from the component that decides
+// when they start/stop. For compatibility with existing startup/shutdown
+// mechanisms, we register a few phases here.
+
+// Parent process
+if (!isContent) {
+  AsyncShutdown.profileChangeTeardown = getPhase("profile-change-teardown");
+  AsyncShutdown.profileBeforeChange = getPhase("profile-before-change");
+  AsyncShutdown.sendTelemetry = getPhase("profile-before-change-telemetry");
+}
+
+// Notifications that fire in the parent and content process, but should
+// only have phases in the parent process.
+if (!isContent) {
+  AsyncShutdown.quitApplicationGranted = getPhase("quit-application-granted");
+}
+
+// Don't add a barrier for content-child-shutdown because this
+// makes it easier to cause shutdown hangs.
+
+// All processes
+AsyncShutdown.webWorkersShutdown = getPhase("web-workers-shutdown");
+AsyncShutdown.xpcomWillShutdown = getPhase("xpcom-will-shutdown");
+
+AsyncShutdown.Barrier = Barrier;
+
+Object.freeze(AsyncShutdown);
diff --git a/toolkit/components/asyncshutdown/components.conf b/toolkit/components/asyncshutdown/components.conf
new file mode 100644
index 0000000000..d897553458
--- /dev/null
+++ b/toolkit/components/asyncshutdown/components.conf
@@ -0,0 +1,15 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+Classes = [
+    {
+        'name': 'AsyncShutdown',
+        'cid': '{35c496de-a115-475d-93b5-ffa3f3ae6fe3}',
+        'contract_ids': ['@mozilla.org/async-shutdown-service;1'],
+        'esModule': 'resource://gre/modules/nsAsyncShutdown.sys.mjs',
+        'constructor': 'nsAsyncShutdownService',
+    },
+]
diff --git a/toolkit/components/asyncshutdown/moz.build b/toolkit/components/asyncshutdown/moz.build
new file mode 100644
index 0000000000..fafa0c0566
--- /dev/null
+++ b/toolkit/components/asyncshutdown/moz.build
@@ -0,0 +1,25 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# 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/.
+
+XPCSHELL_TESTS_MANIFESTS += ["tests/xpcshell/xpcshell.toml"]
+
+XPIDL_MODULE = "toolkit_asyncshutdown"
+
+XPIDL_SOURCES += [
+    "nsIAsyncShutdown.idl",
+]
+
+EXTRA_JS_MODULES += [
+    "AsyncShutdown.sys.mjs",
+    "nsAsyncShutdown.sys.mjs",
+]
+
+XPCOM_MANIFESTS += [
+    "components.conf",
+]
+
+with Files("**"):
+    BUG_COMPONENT = ("Toolkit", "Async Tooling")
diff --git a/toolkit/components/asyncshutdown/nsAsyncShutdown.sys.mjs b/toolkit/components/asyncshutdown/nsAsyncShutdown.sys.mjs
new file mode 100644
index 0000000000..23336a40bf
--- /dev/null
+++ b/toolkit/components/asyncshutdown/nsAsyncShutdown.sys.mjs
@@ -0,0 +1,251 @@
+/* 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/. */
+
+/**
+ * An implementation of nsIAsyncShutdown* based on AsyncShutdown.sys.mjs
+ */
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  AsyncShutdown: "resource://gre/modules/AsyncShutdown.sys.mjs",
+});
+
+/**
+ * Conversion between nsIPropertyBags and JS values.
+ * This uses a conservative approach to avoid losing data and doesn't throw.
+ * Don't use this if you need perfect serialization and deserialization.
+ */
+class PropertyBagConverter {
+  /**
+   * When the js value to convert is a primitive, it is stored in the property
+   * bag under a key with this name.
+   */
+  get primitiveProperty() {
+    return "PropertyBagConverter_primitive";
+  }
+
+  /**
+   * Converts from a PropertyBag to a JS value.
+   * @param {nsIPropertyBag} bag The PropertyBag to convert.
+   * @returns {jsval} A JS value.
+   */
+  propertyBagToJsValue(bag) {
+    if (!(bag instanceof Ci.nsIPropertyBag)) {
+      return null;
+    }
+    let result = {};
+    for (let { name, value: property } of bag.enumerator) {
+      let value = this.#toValue(property);
+      if (name == this.primitiveProperty) {
+        return value;
+      }
+      result[name] = value;
+    }
+    return result;
+  }
+
+  #toValue(property) {
+    if (property instanceof Ci.nsIPropertyBag) {
+      return this.propertyBagToJsValue(property);
+    }
+    if (["number", "boolean"].includes(typeof property)) {
+      return property;
+    }
+    try {
+      return JSON.parse(property);
+    } catch (ex) {
+      // Not JSON.
+    }
+    return property;
+  }
+
+  /**
+   * Converts from a JS value to a PropertyBag.
+   * @param {jsval} val JS value to convert.
+   * @returns {nsIPropertyBag} A PropertyBag.
+   * @note function is converted to "(function)" and undefined to null.
+   */
+  jsValueToPropertyBag(val) {
+    let bag = Cc["@mozilla.org/hash-property-bag;1"].createInstance(
+      Ci.nsIWritablePropertyBag
+    );
+    if (val && typeof val == "object") {
+      for (let k of Object.keys(val)) {
+        bag.setProperty(k, this.#fromValue(val[k]));
+      }
+    } else {
+      bag.setProperty(this.primitiveProperty, this.#fromValue(val));
+    }
+    return bag;
+  }
+
+  #fromValue(value) {
+    if (typeof value == "function") {
+      return "(function)";
+    }
+    if (value === undefined) {
+      value = null;
+    }
+    if (["number", "boolean", "string"].includes(typeof value)) {
+      return value;
+    }
+    return JSON.stringify(value);
+  }
+}
+
+/**
+ * Construct an instance of nsIAsyncShutdownClient from a
+ * AsyncShutdown.Barrier client.
+ *
+ * @param {object} moduleClient A client, as returned from the `client`
+ * property of an instance of `AsyncShutdown.Barrier`. This client will
+ * serve as back-end for methods `addBlocker` and `removeBlocker`.
+ * @constructor
+ */
+function nsAsyncShutdownClient(moduleClient) {
+  if (!moduleClient) {
+    throw new TypeError("nsAsyncShutdownClient expects one argument");
+  }
+  this._moduleClient = moduleClient;
+  this._byXpcomBlocker = new Map();
+}
+nsAsyncShutdownClient.prototype = {
+  get jsclient() {
+    return this._moduleClient;
+  },
+  get name() {
+    return this._moduleClient.name;
+  },
+  get isClosed() {
+    return this._moduleClient.isClosed;
+  },
+  addBlocker(
+    /* nsIAsyncShutdownBlocker*/ xpcomBlocker,
+    fileName,
+    lineNumber,
+    stack
+  ) {
+    // We need a Promise-based function with the same behavior as
+    // `xpcomBlocker`. Furthermore, to support `removeBlocker`, we
+    // need to ensure that we always get the same Promise-based
+    // function if we call several `addBlocker`/`removeBlocker` several
+    // times with the same `xpcomBlocker`.
+
+    if (this._byXpcomBlocker.has(xpcomBlocker)) {
+      throw new Error(
+        `We have already registered the blocker (${xpcomBlocker.name})`
+      );
+    }
+
+    // Ideally, this should be done with a WeakMap() with xpcomBlocker
+    // as a key, but XPCWrappedNative objects cannot serve as WeakMap keys, see
+    // bug 1834365.
+    const moduleBlocker = () =>
+      new Promise(
+        // This promise is never resolved. By opposition to AsyncShutdown
+        // blockers, `nsIAsyncShutdownBlocker`s are always lifted by calling
+        // `removeBlocker`.
+        () => xpcomBlocker.blockShutdown(this)
+      );
+
+    this._byXpcomBlocker.set(xpcomBlocker, moduleBlocker);
+    this._moduleClient.addBlocker(xpcomBlocker.name, moduleBlocker, {
+      fetchState: () =>
+        new PropertyBagConverter().propertyBagToJsValue(xpcomBlocker.state),
+      filename: fileName,
+      lineNumber,
+      stack,
+    });
+  },
+
+  removeBlocker(xpcomBlocker) {
+    let moduleBlocker = this._byXpcomBlocker.get(xpcomBlocker);
+    if (!moduleBlocker) {
+      return false;
+    }
+    this._byXpcomBlocker.delete(xpcomBlocker);
+    return this._moduleClient.removeBlocker(moduleBlocker);
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsIAsyncShutdownClient"]),
+};
+
+/**
+ * Construct an instance of nsIAsyncShutdownBarrier from an instance
+ * of AsyncShutdown.Barrier.
+ *
+ * @param {object} moduleBarrier an instance if
+ * `AsyncShutdown.Barrier`. This instance will serve as back-end for
+ * all methods.
+ * @constructor
+ */
+function nsAsyncShutdownBarrier(moduleBarrier) {
+  this._client = new nsAsyncShutdownClient(moduleBarrier.client);
+  this._moduleBarrier = moduleBarrier;
+}
+nsAsyncShutdownBarrier.prototype = {
+  get state() {
+    return new PropertyBagConverter().jsValueToPropertyBag(
+      this._moduleBarrier.state
+    );
+  },
+  get client() {
+    return this._client;
+  },
+  wait(onReady) {
+    this._moduleBarrier.wait().then(() => {
+      onReady.done();
+    });
+    // By specification, _moduleBarrier.wait() cannot reject.
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsIAsyncShutdownBarrier"]),
+};
+
+export function nsAsyncShutdownService() {
+  // Cache for the getters
+
+  for (let _k of [
+    // Parent process
+    "profileBeforeChange",
+    "profileChangeTeardown",
+    "quitApplicationGranted",
+    "sendTelemetry",
+
+    // Child processes
+    "contentChildShutdown",
+
+    // All processes
+    "webWorkersShutdown",
+    "xpcomWillShutdown",
+  ]) {
+    let k = _k;
+    Object.defineProperty(this, k, {
+      configurable: true,
+      get() {
+        delete this[k];
+        let wrapped = lazy.AsyncShutdown[k]; // May be undefined, if we're on the wrong process.
+        let result = wrapped ? new nsAsyncShutdownClient(wrapped) : undefined;
+        Object.defineProperty(this, k, {
+          value: result,
+        });
+        return result;
+      },
+    });
+  }
+
+  // Hooks for testing purpose
+  this.wrappedJSObject = {
+    _propertyBagConverter: PropertyBagConverter,
+  };
+}
+
+nsAsyncShutdownService.prototype = {
+  makeBarrier(name) {
+    return new nsAsyncShutdownBarrier(new lazy.AsyncShutdown.Barrier(name));
+  },
+
+  QueryInterface: ChromeUtils.generateQI(["nsIAsyncShutdownService"]),
+};
diff --git a/toolkit/components/asyncshutdown/nsIAsyncShutdown.idl b/toolkit/components/asyncshutdown/nsIAsyncShutdown.idl
new file mode 100644
index 0000000000..945772bd51
--- /dev/null
+++ b/toolkit/components/asyncshutdown/nsIAsyncShutdown.idl
@@ -0,0 +1,229 @@
+/* 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/. */
+
+/**
+ * A mechanism for specifying shutdown dependencies between
+ * asynchronous services.
+ *
+ * Note that this XPCOM component is designed primarily for C++
+ * clients. JavaScript clients should rather use AsyncShutdown.sys.mjs,
+ * which provides a better API and better error reporting for them.
+ */
+
+
+#include "nsISupports.idl"
+#include "nsIPropertyBag.idl"
+#include "nsIVariant.idl"
+
+interface nsIAsyncShutdownClient;
+
+/**
+ * A blocker installed by a client to be informed during some stage of
+ * shutdown and block shutdown asynchronously until some condition is
+ * complete.
+ *
+ * If you wish to use AsyncShutdown, you will need to implement this
+ * interface (and only this interface).
+ */
+[scriptable, uuid(4ef43f29-6715-4b57-a750-2ff83695ddce)]
+interface nsIAsyncShutdownBlocker: nsISupports {
+  /**
+   * The *unique* name of the blocker.
+   *
+   * By convention, it should respect the following format:
+   * "MyModuleName: Doing something while it's time"
+   * e.g.
+   * "OS.File: Flushing before profile-before-change"
+   *
+   * This attribute is uploaded as part of crash reports.
+   */
+  readonly attribute AString name;
+
+  /**
+   * Inform the blocker that the stage of shutdown has started.
+   * Shutdown will NOT proceed until `aBarrierClient.removeBlocker(this)`
+   * has been called.
+   */
+  void blockShutdown(in nsIAsyncShutdownClient aBarrierClient);
+
+  /**
+   * The current state of the blocker.
+   *
+   * In case of crash, this is converted to JSON and attached to
+   * the crash report.
+   *
+   * This field may be used to provide JSON-style data structures.
+   * For this purpose, use
+   * - nsIPropertyBag to represent objects;
+   * - nsIVariant to represent field values (which may hold nsIPropertyBag
+   * themselves).
+   */
+  readonly attribute nsIPropertyBag state;
+};
+
+/**
+ * A client for a nsIAsyncShutdownBarrier.
+ */
+[scriptable, uuid(d2031049-b990-43a2-95be-59f8a3ca5954)]
+interface nsIAsyncShutdownClient: nsISupports {
+  /**
+   * The name of the barrier.
+   */
+  readonly attribute AString name;
+
+  /**
+   * Whether the client is still open for new blockers.
+   * When this is true it is too late to add new blockers and addBlocker will
+   * throw an exception.
+   */
+  readonly attribute boolean isClosed;
+
+  /**
+   * Add a blocker.
+   *
+   * After a `blocker` has been added with `addBlocker`, if it is not
+   * removed with `removeBlocker`, this will, by design, eventually
+   * CAUSE A CRASH.
+   *
+   * Calling `addBlocker` once nsIAsyncShutdownBarrier::wait() has been
+   * called on the owning barrier returns an error.
+   *
+   * @param aBlocker The blocker to add. Once
+   * nsIAsyncShutdownBarrier::wait() has been called, it will not
+   * call its `aOnReady` callback until all blockers have been
+   * removed, each  by a call to `removeBlocker`.
+   * @param aFileName The filename of the callsite, as given by `__FILE__`.
+   * @param aLineNumber The linenumber of the callsite, as given by `__LINE__`.
+   * @param aStack Information on the stack that lead to this call. Generally
+   * empty when called from C++.
+   * @throws If it's too late to add a blocker.
+   * @see isClosed.
+   */
+  void addBlocker(in nsIAsyncShutdownBlocker aBlocker,
+                  in AString aFileName,
+                  in long aLineNumber,
+                  in AString aStack);
+
+  /**
+   * Remove a blocker.
+   *
+   * @param aBlocker A blocker previously added to this client through
+   * `addBlocker`. Noop if the blocker has never been added or has been
+   * removed already.
+   */
+  void removeBlocker(in nsIAsyncShutdownBlocker aBlocker);
+
+  /**
+   * The JS implementation of the client.
+   *
+   * It is strongly recommended that JS clients of this API use
+   * `jsclient` instead of the `nsIAsyncShutdownClient`. See
+   * AsyncShutdown.sys.mjs for more information on the JS version of
+   * this API.
+   */
+  readonly attribute jsval jsclient;
+};
+
+/**
+ * Callback invoked once all blockers of a barrier have been removed.
+ */
+[scriptable, function, uuid(910c9309-1da0-4dd0-8bdb-a325a38c604e)]
+interface nsIAsyncShutdownCompletionCallback: nsISupports {
+  /**
+   * The operation has been completed.
+   */
+  void done();
+};
+
+/**
+ * A stage of shutdown that supports blocker registration.
+ */
+[scriptable, uuid(50fa8a86-9c91-4256-8389-17d310adec90)]
+interface nsIAsyncShutdownBarrier: nsISupports {
+
+  /**
+   * The blocker registration capability.  Most services may wish to
+   * publish this capability to let services that depend on it register
+   * blockers.
+   */
+  readonly attribute nsIAsyncShutdownClient client;
+
+  /**
+   * The state of all the blockers of the barrier.
+   *
+   * See the documentation of `nsIAsyncShutdownBlocker` for the
+   * format.
+   */
+  readonly attribute nsIPropertyBag state;
+
+  /**
+   * Wait for all blockers to complete.
+   *
+   * Method `aOnReady` will be called once all blockers have finished.
+   * The callback always receives NS_OK.
+   */
+  void wait(in nsIAsyncShutdownCompletionCallback aOnReady);
+};
+
+/**
+ * A service that allows registering shutdown-time dependencies.
+ */
+[scriptable, uuid(db365c78-c860-4e64-9a63-25b73f89a016)]
+interface nsIAsyncShutdownService: nsISupports {
+  /**
+   * Create a new barrier.
+   *
+   * By convention, the name should respect the following format:
+   * "MyModuleName: Doing something while it's time"
+   * e.g.
+   * "OS.File: Waiting for clients to flush before shutting down"
+   *
+   * This attribute is uploaded as part of crash reports.
+   */
+  nsIAsyncShutdownBarrier makeBarrier(in AString aName);
+
+
+  // Barriers for global shutdown stages in the parent process.
+
+  /**
+   * Barrier for notification profile-before-change.
+   */
+  readonly attribute nsIAsyncShutdownClient profileBeforeChange;
+
+  /**
+   * Barrier for notification profile-change-teardown.
+   */
+  readonly attribute nsIAsyncShutdownClient profileChangeTeardown;
+
+  /**
+   * Barrier for notification quit-application-granted.
+   */
+  readonly attribute nsIAsyncShutdownClient quitApplicationGranted;
+
+  /**
+   * Barrier for notification profile-before-change-telemetry.
+   */
+  readonly attribute nsIAsyncShutdownClient sendTelemetry;
+
+
+  // Barriers for global shutdown stages in all processes.
+
+  /**
+   * Barrier for notification web-workers-shutdown.
+   */
+  readonly attribute nsIAsyncShutdownClient webWorkersShutdown;
+
+  /**
+   * Barrier for notification xpcom-will-shutdown.
+   */
+  readonly attribute nsIAsyncShutdownClient xpcomWillShutdown;
+
+  // Don't add a barrier for content-child-shutdown because this
+  // makes it easier to cause shutdown hangs.
+
+};
+
+%{C++
+#define NS_ASYNCSHUTDOWNSERVICE_CONTRACTID "@mozilla.org/async-shutdown-service;1"
+%}
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/head.js b/toolkit/components/asyncshutdown/tests/xpcshell/head.js
new file mode 100644
index 0000000000..6b8c756122
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/head.js
@@ -0,0 +1,180 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+var { AsyncShutdown } = ChromeUtils.importESModule(
+  "resource://gre/modules/AsyncShutdown.sys.mjs"
+);
+
+var asyncShutdownService = Cc[
+  "@mozilla.org/async-shutdown-service;1"
+].getService(Ci.nsIAsyncShutdownService);
+
+Services.prefs.setBoolPref("toolkit.asyncshutdown.testing", true);
+
+/**
+ * Utility function used to provide the same API for various sources
+ * of async shutdown barriers.
+ *
+ * @param {string} kind One of
+ * - "phase" to test an AsyncShutdown phase;
+ * - "barrier" to test an instance of AsyncShutdown.Barrier;
+ * - "xpcom-barrier" to test an instance of nsIAsyncShutdownBarrier;
+ * - "xpcom-barrier-unwrapped" to test the field `jsclient` of a nsIAsyncShutdownClient.
+ *
+ * @return An object with the following methods:
+ *   - addBlocker() - the same method as AsyncShutdown phases and barrier clients
+ *   - wait() - trigger the resolution of the lock
+ */
+function makeLock(kind) {
+  if (kind == "phase") {
+    let topic = "test-Phase-" + ++makeLock.counter;
+    let phase = AsyncShutdown._getPhase(topic);
+    return {
+      addBlocker(...args) {
+        return phase.addBlocker(...args);
+      },
+      removeBlocker(blocker) {
+        return phase.removeBlocker(blocker);
+      },
+      wait() {
+        Services.obs.notifyObservers(null, topic);
+        return Promise.resolve();
+      },
+      get isClosed() {
+        return phase.isClosed;
+      },
+    };
+  } else if (kind == "barrier") {
+    let name = "test-Barrier-" + ++makeLock.counter;
+    let barrier = new AsyncShutdown.Barrier(name);
+    return {
+      addBlocker: barrier.client.addBlocker,
+      removeBlocker: barrier.client.removeBlocker,
+      wait() {
+        return barrier.wait();
+      },
+      get isClosed() {
+        return barrier.client.isClosed;
+      },
+    };
+  } else if (kind == "xpcom-barrier") {
+    let name = "test-xpcom-Barrier-" + ++makeLock.counter;
+    let barrier = asyncShutdownService.makeBarrier(name);
+    return {
+      addBlocker(blockerName, condition, state) {
+        if (condition == null) {
+          // Slight trick as `null` or `undefined` cannot be used as keys
+          // for `xpcomMap`. Note that this has no incidence on the result
+          // of the test as the XPCOM interface imposes that the condition
+          // is a method, so it cannot be `null`/`undefined`.
+          condition = "<this case can't happen with the xpcom interface>";
+        }
+        let blocker = makeLock.xpcomMap.get(condition);
+        if (!blocker) {
+          blocker = {
+            name: blockerName,
+            state,
+            blockShutdown(aBarrierClient) {
+              return (async function () {
+                try {
+                  if (typeof condition == "function") {
+                    await Promise.resolve(condition());
+                  } else {
+                    await Promise.resolve(condition);
+                  }
+                } finally {
+                  aBarrierClient.removeBlocker(blocker);
+                }
+              })();
+            },
+          };
+          makeLock.xpcomMap.set(condition, blocker);
+        }
+        let { fileName, lineNumber, stack } = new Error();
+        return barrier.client.addBlocker(blocker, fileName, lineNumber, stack);
+      },
+      removeBlocker(condition) {
+        let blocker = makeLock.xpcomMap.get(condition);
+        if (!blocker) {
+          return;
+        }
+        barrier.client.removeBlocker(blocker);
+      },
+      wait() {
+        return new Promise(resolve => {
+          barrier.wait(resolve);
+        });
+      },
+      get isClosed() {
+        return barrier.client.isClosed;
+      },
+    };
+  } else if ("unwrapped-xpcom-barrier") {
+    let name = "unwrapped-xpcom-barrier-" + ++makeLock.counter;
+    let barrier = asyncShutdownService.makeBarrier(name);
+    let client = barrier.client.jsclient;
+    return {
+      addBlocker: client.addBlocker,
+      removeBlocker: client.removeBlocker,
+      wait() {
+        return new Promise(resolve => {
+          barrier.wait(resolve);
+        });
+      },
+      get isClosed() {
+        return client.isClosed;
+      },
+    };
+  }
+  throw new TypeError("Unknown kind " + kind);
+}
+makeLock.counter = 0;
+makeLock.xpcomMap = new Map(); // Note: Not a WeakMap as we wish to handle non-gc-able keys (e.g. strings)
+
+/**
+ * An asynchronous task that takes several ticks to complete.
+ *
+ * @param {*=} resolution The value with which the resulting promise will be
+ * resolved once the task is complete. This may be a rejected promise,
+ * in which case the resulting promise will itself be rejected.
+ * @param {object=} outResult An object modified by side-effect during the task.
+ * Initially, its field |isFinished| is set to |false|. Once the task is
+ * complete, its field |isFinished| is set to |true|.
+ *
+ * @return {promise} A promise fulfilled once the task is complete
+ */
+function longRunningAsyncTask(resolution = undefined, outResult = {}) {
+  outResult.isFinished = false;
+  if (!("countFinished" in outResult)) {
+    outResult.countFinished = 0;
+  }
+  return new Promise(resolve => {
+    do_timeout(100, function () {
+      ++outResult.countFinished;
+      outResult.isFinished = true;
+      resolve(resolution);
+    });
+  });
+}
+
+function get_exn(f) {
+  try {
+    f();
+    return null;
+  } catch (ex) {
+    return ex;
+  }
+}
+
+function do_check_exn(exn, constructor) {
+  Assert.notEqual(exn, null);
+  if (exn.name == constructor) {
+    Assert.equal(exn.constructor.name, constructor);
+    return;
+  }
+  info("Wrong error constructor");
+  info(exn.constructor.name);
+  info(exn.stack);
+  Assert.ok(false);
+}
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown.js b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown.js
new file mode 100644
index 0000000000..27c3a26985
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown.js
@@ -0,0 +1,348 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+add_task(async function test_no_condition() {
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    info("Testing a barrier with no condition (" + kind + ")");
+    let lock = makeLock(kind);
+    await lock.wait();
+    info("Barrier with no condition didn't lock");
+  }
+});
+
+add_task(async function test_phase_various_failures() {
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    info("Kind: " + kind);
+    // Testing with wrong arguments
+    let lock = makeLock(kind);
+
+    Assert.throws(
+      () => lock.addBlocker(),
+      /TypeError|NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS/
+    );
+    Assert.throws(
+      () => lock.addBlocker(null, true),
+      /TypeError|NS_ERROR_XPC_JAVASCRIPT_ERROR_WITH_DETAILS/
+    );
+
+    if (kind != "xpcom-barrier") {
+      // xpcom-barrier actually expects a string in that position
+      Assert.throws(
+        () => lock.addBlocker("Test 2", () => true, "not a function"),
+        /TypeError/
+      );
+    }
+
+    if (kind == "xpcom-barrier") {
+      const blocker = () => true;
+      lock.addBlocker("Test 3", blocker);
+      Assert.throws(
+        () => lock.addBlocker("Test 3", blocker),
+        /We have already registered the blocker \(Test 3\)/
+      );
+    }
+
+    // Attempting to add a blocker after we are done waiting
+    Assert.ok(!lock.isClosed, "Barrier is open");
+    await lock.wait();
+    Assert.throws(() => lock.addBlocker("Test 4", () => true), /is finished/);
+    Assert.ok(lock.isClosed, "Barrier is closed");
+  }
+});
+
+add_task(async function test_reentrant() {
+  info("Ensure that we can call addBlocker from within a blocker");
+
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    info("Kind: " + kind);
+    let lock = makeLock(kind);
+
+    let deferredOuter = Promise.withResolvers();
+    let deferredInner = Promise.withResolvers();
+    let deferredBlockInner = Promise.withResolvers();
+
+    lock.addBlocker("Outer blocker", () => {
+      info("Entering outer blocker");
+      deferredOuter.resolve();
+      lock.addBlocker("Inner blocker", () => {
+        info("Entering inner blocker");
+        deferredInner.resolve();
+        return deferredBlockInner.promise;
+      });
+    });
+
+    // Note that phase-style locks spin the event loop and do not return from
+    // `lock.wait()` until after all blockers have been resolved. Therefore,
+    // to be able to test them, we need to dispatch the following steps to the
+    // event loop before calling `lock.wait()`, which we do by forcing
+    // a Promise.resolve().
+    //
+    let promiseSteps = (async function () {
+      await Promise.resolve();
+
+      info("Waiting until we have entered the outer blocker");
+      await deferredOuter.promise;
+
+      info("Waiting until we have entered the inner blocker");
+      await deferredInner.promise;
+
+      info("Allowing the lock to resolve");
+      deferredBlockInner.resolve();
+    })();
+
+    info("Starting wait");
+    await lock.wait();
+
+    info("Waiting until all steps have been walked");
+    await promiseSteps;
+  }
+});
+
+add_task(async function test_phase_removeBlocker() {
+  info(
+    "Testing that we can call removeBlocker before, during and after the call to wait()"
+  );
+
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    info("Switching to kind " + kind);
+    info("Attempt to add then remove a blocker before wait()");
+    let lock = makeLock(kind);
+    let blocker = () => {
+      info("This promise will never be resolved");
+      return Promise.withResolvers().promise;
+    };
+
+    lock.addBlocker("Wait forever", blocker);
+    let do_remove_blocker = function (aLock, aBlocker, aShouldRemove) {
+      info(
+        "Attempting to remove blocker " +
+          aBlocker +
+          ", expecting result " +
+          aShouldRemove
+      );
+      if (kind == "xpcom-barrier") {
+        // The xpcom variant always returns `undefined`, so we can't
+        // check its result.
+        aLock.removeBlocker(aBlocker);
+        return;
+      }
+      Assert.equal(aLock.removeBlocker(aBlocker), aShouldRemove);
+    };
+    do_remove_blocker(lock, blocker, true);
+    do_remove_blocker(lock, blocker, false);
+    info("Attempt to remove non-registered blockers before wait()");
+    do_remove_blocker(lock, "foo", false);
+    do_remove_blocker(lock, null, false);
+    info("Waiting (should lift immediately)");
+    await lock.wait();
+
+    info("Attempt to add a blocker then remove it during wait()");
+    lock = makeLock(kind);
+    let blockers = [
+      () => {
+        info("This blocker will self-destruct");
+        do_remove_blocker(lock, blockers[0], true);
+        return Promise.withResolvers().promise;
+      },
+      () => {
+        info("This blocker will self-destruct twice");
+        do_remove_blocker(lock, blockers[1], true);
+        do_remove_blocker(lock, blockers[1], false);
+        return Promise.withResolvers().promise;
+      },
+      () => {
+        info("Attempt to remove non-registered blockers during wait()");
+        do_remove_blocker(lock, "foo", false);
+        do_remove_blocker(lock, null, false);
+      },
+    ];
+    for (let i in blockers) {
+      lock.addBlocker("Wait forever again: " + i, blockers[i]);
+    }
+    info("Waiting (should lift very quickly)");
+    await lock.wait();
+    do_remove_blocker(lock, blockers[0], false);
+
+    info("Attempt to remove a blocker after wait");
+    lock = makeLock(kind);
+    blocker = Promise.resolve.bind(Promise);
+    await lock.wait();
+    do_remove_blocker(lock, blocker, false);
+
+    info("Attempt to remove non-registered blocker after wait()");
+    do_remove_blocker(lock, "foo", false);
+    do_remove_blocker(lock, null, false);
+  }
+});
+
+add_task(async function test_addBlocker_noDistinctNamesConstraint() {
+  info("Testing that we can add two distinct blockers with identical names");
+
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    info("Switching to kind " + kind);
+    let lock = makeLock(kind);
+    let deferred1 = Promise.withResolvers();
+    let resolved1 = false;
+    let deferred2 = Promise.withResolvers();
+    let resolved2 = false;
+    let blocker1 = () => {
+      info("Entering blocker1");
+      return deferred1.promise;
+    };
+    let blocker2 = () => {
+      info("Entering blocker2");
+      return deferred2.promise;
+    };
+
+    info("Attempt to add two distinct blockers with identical names");
+    lock.addBlocker("Blocker", blocker1);
+    lock.addBlocker("Blocker", blocker2);
+
+    // Note that phase-style locks spin the event loop and do not return from
+    // `lock.wait()` until after all blockers have been resolved. Therefore,
+    // to be able to test them, we need to dispatch the following steps to the
+    // event loop before calling `lock.wait()`, which we do by forcing
+    // a Promise.resolve().
+    //
+    let promiseSteps = (async () => {
+      info("Waiting for an event-loop spin");
+      await Promise.resolve();
+
+      info("Resolving blocker1");
+      deferred1.resolve();
+      resolved1 = true;
+
+      info("Waiting for an event-loop spin");
+      await Promise.resolve();
+
+      info("Resolving blocker2");
+      deferred2.resolve();
+      resolved2 = true;
+    })();
+
+    info("Waiting for lock");
+    await lock.wait();
+
+    Assert.ok(resolved1);
+    Assert.ok(resolved2);
+    await promiseSteps;
+  }
+});
+
+add_task(async function test_state() {
+  info("Testing information contained in `state`");
+
+  let BLOCKER_NAME = "test_state blocker " + Math.random();
+
+  // Set up the barrier. Note that we cannot test `barrier.state`
+  // immediately, as it initially contains "Not started"
+  let barrier = new AsyncShutdown.Barrier("test_filename");
+  let deferred = Promise.withResolvers();
+  let { filename, lineNumber } = Components.stack;
+  barrier.client.addBlocker(BLOCKER_NAME, function () {
+    return deferred.promise;
+  });
+
+  let promiseDone = barrier.wait();
+
+  // Now that we have called `wait()`, the state contains interesting things
+  info("State: " + JSON.stringify(barrier.state, null, "\t"));
+  let state = barrier.state[0];
+  Assert.equal(state.filename, filename);
+  Assert.equal(state.lineNumber, lineNumber + 1);
+  Assert.equal(state.name, BLOCKER_NAME);
+  Assert.ok(
+    state.stack.some(x => x.includes("test_state")),
+    "The stack contains the caller function's name"
+  );
+  Assert.ok(
+    state.stack.some(x => x.includes(filename)),
+    "The stack contains the calling file's name"
+  );
+
+  deferred.resolve();
+  await promiseDone;
+});
+
+add_task(async function test_multistate() {
+  info("Testing information contained in multiple `state`");
+
+  let BLOCKER_NAMES = [
+    "test_state blocker " + Math.random(),
+    "test_state blocker " + Math.random(),
+  ];
+
+  // Set up the barrier. Note that we cannot test `barrier.state`
+  // immediately, as it initially contains "Not started"
+  let barrier = asyncShutdownService.makeBarrier("test_filename");
+  let deferred = Promise.withResolvers();
+  let { filename, lineNumber } = Components.stack;
+  for (let name of BLOCKER_NAMES) {
+    barrier.client.jsclient.addBlocker(name, () => deferred.promise, {
+      fetchState: () => ({ progress: name }),
+    });
+  }
+
+  let promiseDone = new Promise(r => barrier.wait(r));
+
+  // Now that we have called `wait()`, the state contains interesting things.
+  Assert.ok(
+    barrier.state instanceof Ci.nsIPropertyBag,
+    "State is a PropertyBag"
+  );
+  for (let i = 0; i < BLOCKER_NAMES.length; ++i) {
+    let state = barrier.state.getProperty(i.toString());
+    Assert.equal(typeof state, "string", "state is a string");
+    info("State: " + state + "\t");
+    state = JSON.parse(state);
+    Assert.equal(state.filename, filename);
+    Assert.equal(state.lineNumber, lineNumber + 2);
+    Assert.equal(state.name, BLOCKER_NAMES[i]);
+    Assert.ok(
+      state.stack.some(x => x.includes("test_multistate")),
+      "The stack contains the caller function's name"
+    );
+    Assert.ok(
+      state.stack.some(x => x.includes(filename)),
+      "The stack contains the calling file's name"
+    );
+    Assert.equal(
+      state.state.progress,
+      BLOCKER_NAMES[i],
+      "The state contains the fetchState provided value"
+    );
+  }
+
+  deferred.resolve();
+  await promiseDone;
+});
+
+add_task(async function () {
+  Services.prefs.clearUserPref("toolkit.asyncshutdown.testing");
+});
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_blocker_error_annotations.js b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_blocker_error_annotations.js
new file mode 100644
index 0000000000..669164f68e
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_blocker_error_annotations.js
@@ -0,0 +1,55 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that when addBlocker fails, we store that failure internally
+ * and include its information in crash report annotation information.
+ */
+add_task(async function test_addBlockerFailureState() {
+  info("Testing addBlocker information reported to crash reporter");
+
+  let BLOCKER_NAME = "test_addBlocker_state blocker " + Math.random();
+
+  // Set up the barrier. Note that we cannot test `barrier.state`
+  // immediately, as it initially contains "Not started"
+  let barrier = new AsyncShutdown.Barrier("test_addBlocker_failure");
+  let deferred = Promise.withResolvers();
+  barrier.client.addBlocker(BLOCKER_NAME, function () {
+    return deferred.promise;
+  });
+
+  // Add a blocker and confirm that throws.
+  const THROWING_BLOCKER_NAME = "test_addBlocker_throws blocker";
+  Assert.throws(() => {
+    barrier.client.addBlocker(THROWING_BLOCKER_NAME, Promise.resolve(), 5);
+  }, /object as third argument/);
+
+  let promiseDone = barrier.wait();
+
+  // Now that we have called `wait()`, the state should match crash
+  // reporting info
+  let crashInfo = barrier._gatherCrashReportTimeoutData(
+    barrier._name,
+    barrier.state
+  );
+  Assert.deepEqual(
+    crashInfo.conditions,
+    barrier.state,
+    "Barrier state should match crash info."
+  );
+  Assert.equal(
+    crashInfo.brokenAddBlockers.length,
+    1,
+    "Should have registered the broken addblocker call."
+  );
+  Assert.stringMatches(
+    crashInfo.brokenAddBlockers?.[0] || "undefined",
+    THROWING_BLOCKER_NAME,
+    "Throwing call's blocker name should be listed in message."
+  );
+
+  deferred.resolve();
+  await promiseDone;
+});
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_leave_uncaught.js b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_leave_uncaught.js
new file mode 100644
index 0000000000..b37416d7a7
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/test_AsyncShutdown_leave_uncaught.js
@@ -0,0 +1,105 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+//
+// This file contains tests that need to leave uncaught asynchronous
+// errors. If your test catches all its asynchronous errors, please
+// put it in another file.
+//
+const { PromiseTestUtils } = ChromeUtils.importESModule(
+  "resource://testing-common/PromiseTestUtils.sys.mjs"
+);
+PromiseTestUtils.thisTestLeaksUncaughtRejectionsAndShouldBeFixed();
+
+add_task(async function test_phase_simple_async() {
+  info("Testing various combinations of a phase with a single condition");
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    for (let arg of [undefined, null, "foo", 100, new Error("BOOM")]) {
+      for (let resolution of [arg, Promise.reject(arg)]) {
+        for (let success of [false, true]) {
+          for (let state of [
+            [null],
+            [],
+            [() => "some state"],
+            [
+              function () {
+                throw new Error("State BOOM");
+              },
+            ],
+            [
+              function () {
+                return {
+                  toJSON() {
+                    throw new Error("State.toJSON BOOM");
+                  },
+                };
+              },
+            ],
+          ]) {
+            // Asynchronous phase
+            info(
+              "Asynchronous test with " + arg + ", " + resolution + ", " + kind
+            );
+            let lock = makeLock(kind);
+            let outParam = { isFinished: false };
+            lock.addBlocker(
+              "Async test",
+              function () {
+                if (success) {
+                  return longRunningAsyncTask(resolution, outParam);
+                }
+                throw resolution;
+              },
+              ...state
+            );
+            Assert.ok(!outParam.isFinished);
+            await lock.wait();
+            Assert.equal(outParam.isFinished, success);
+          }
+        }
+
+        // Synchronous phase - just test that we don't throw/freeze
+        info("Synchronous test with " + arg + ", " + resolution + ", " + kind);
+        let lock = makeLock(kind);
+        lock.addBlocker("Sync test", resolution);
+        await lock.wait();
+      }
+    }
+  }
+});
+
+add_task(async function test_phase_many() {
+  info("Testing various combinations of a phase with many conditions");
+  for (let kind of [
+    "phase",
+    "barrier",
+    "xpcom-barrier",
+    "xpcom-barrier-unwrapped",
+  ]) {
+    let lock = makeLock(kind);
+    let outParams = [];
+    for (let arg of [undefined, null, "foo", 100, new Error("BOOM")]) {
+      for (let resolve of [true, false]) {
+        info("Testing with " + kind + ", " + arg + ", " + resolve);
+        let resolution = resolve ? arg : Promise.reject(arg);
+        let outParam = { isFinished: false };
+        lock.addBlocker("Test " + Math.random(), () =>
+          longRunningAsyncTask(resolution, outParam)
+        );
+      }
+    }
+    Assert.ok(outParams.every(x => !x.isFinished));
+    await lock.wait();
+    Assert.ok(outParams.every(x => x.isFinished));
+  }
+});
+
+add_task(async function () {
+  Services.prefs.clearUserPref("toolkit.asyncshutdown.testing");
+});
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/test_converters.js b/toolkit/components/asyncshutdown/tests/xpcshell/test_converters.js
new file mode 100644
index 0000000000..1e4740ddc1
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/test_converters.js
@@ -0,0 +1,94 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+"use strict";
+
+/**
+ * Test conversion between nsIPropertyBag and JS values.
+ */
+
+var PropertyBagConverter =
+  new asyncShutdownService.wrappedJSObject._propertyBagConverter();
+
+function run_test() {
+  test_conversions();
+}
+
+function normalize(obj) {
+  if (obj === undefined) {
+    return null;
+  }
+  if (obj == null || typeof obj != "object") {
+    return obj;
+  }
+  if (Array.isArray(obj)) {
+    return obj.map(normalize);
+  }
+  let result = {};
+  for (let k of Object.keys(obj).sort()) {
+    result[k] = normalize(obj[k]);
+  }
+  return result;
+}
+
+function test_conversions() {
+  const SAMPLES = [
+    // Simple values
+    1,
+    true,
+    "string",
+    null,
+    undefined,
+    // Object
+    {
+      a: 1,
+      b: true,
+      c: "string",
+      d: 0.5,
+      e: [2, false, "another string", 0.3],
+      f: [],
+      g: {
+        a2: 1,
+        b2: true,
+        c2: "string",
+        d2: 0.5,
+        e2: [2, false, "another string", 0.3],
+        f2: [],
+        g2: [
+          {
+            a3: 1,
+            b3: true,
+            c3: "string",
+            d3: 0.5,
+            e3: [2, false, "another string", 0.3],
+            f3: [],
+            g3: {},
+          },
+        ],
+        h2: null,
+      },
+      h: null,
+    },
+    // Array
+    [1, 2, 3],
+    // Array of objects
+    [[1, 2], { a: 1, b: "string" }, null],
+  ];
+
+  for (let sample of SAMPLES) {
+    let stringified = JSON.stringify(normalize(sample), null, "\t");
+    info("Testing conversions of " + stringified);
+    let rewrites = [sample];
+    for (let i = 1; i < 3; ++i) {
+      let source = rewrites[i - 1];
+      let bag = PropertyBagConverter.jsValueToPropertyBag(source);
+      Assert.ok(bag instanceof Ci.nsIPropertyBag, "The bag is a property bag");
+      let dest = PropertyBagConverter.propertyBagToJsValue(bag);
+      let restringified = JSON.stringify(normalize(dest), null, "\t");
+      info("Comparing");
+      info(stringified);
+      info(restringified);
+      Assert.deepEqual(sample, dest, "Testing after " + i + " conversions");
+      rewrites.push(dest);
+    }
+  }
+}
diff --git a/toolkit/components/asyncshutdown/tests/xpcshell/xpcshell.toml b/toolkit/components/asyncshutdown/tests/xpcshell/xpcshell.toml
new file mode 100644
index 0000000000..365af63451
--- /dev/null
+++ b/toolkit/components/asyncshutdown/tests/xpcshell/xpcshell.toml
@@ -0,0 +1,11 @@
+[DEFAULT]
+head = "head.js"
+skip-if = ["os == 'android'"]
+
+["test_AsyncShutdown.js"]
+
+["test_AsyncShutdown_blocker_error_annotations.js"]
+
+["test_AsyncShutdown_leave_uncaught.js"]
+
+["test_converters.js"]