UXP/toolkit/components/asyncshutdown/AsyncShutdown.jsm

/* 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. 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.
 */

"use strict";

const Cu = Components.utils;
const Cc = Components.classes;
const Ci = Components.interfaces;
Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
Cu.import("resource://gre/modules/Services.jsm", this);

XPCOMUtils.defineLazyModuleGetter(this, "Promise",
  "resource://gre/modules/Promise.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "PromiseUtils",
  "resource://gre/modules/PromiseUtils.jsm");
XPCOMUtils.defineLazyModuleGetter(this, "Task",
  "resource://gre/modules/Task.jsm");
XPCOMUtils.defineLazyServiceGetter(this, "gDebug",
  "@mozilla.org/xpcom/debug;1", "nsIDebug2");
Object.defineProperty(this, "gCrashReporter", {
  get: function() {
    delete this.gCrashReporter;
    try {
      let reporter = Cc["@mozilla.org/xre/app-info;1"].
            getService(Ci.nsICrashReporter);
      return this.gCrashReporter = reporter;
    } catch (ex) {
      return this.gCrashReporter = null;
    }
  },
  configurable: true
});

// `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 = 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);
}, false);

/**
 * 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();
}
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.
   *
   * @return {Promise} Resolved once all Promise have been resolved or removed,
   * or rejected after at least one Promise has rejected.
   */
  wait: function() {
    // 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.
      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()
    );
    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: function(key) {
    this._ensurePromise(key);
    let indirection = PromiseUtils.defer();
    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);
      });
    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: function(key) {
    this._ensurePromise(key);
    let value = this._indirections.get(key);
    if (!value) {
      return false;
    }
    this._indirections.delete(key);
    value.resolve();
    return true;
  },

  _ensurePromise: function(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);
}, false);

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.defer();
  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`.
      let frames = [];
      while (frame != null) {
        frames.push(frame.filename + ":" + frame.name + ":" + frame.lineNumber);
        frame = frame.caller;
      }
      stack = Task.Debugging.generateReadableStack(frames.join("\n")).split("\n");
    }

    return {
      filename: filename,
      lineNumber: lineNumber,
      stack: stack,
    };
  } catch (ex) {
    return {
      filename: "<internal error: could not get origin>",
      lineNumber: -1,
      stack: "<internal error: could not get origin>",
    }
  }
}

this.EXPORTED_SYMBOLS = ["AsyncShutdown"];

/**
 * {string} topic -> phase
 */
var gPhases = new Map();

this.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;
  }
};

/**
 * 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: function(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: function(condition) {
      return spinner.removeBlocker(condition);
    },

    get name() {
      return spinner.name;
    },

    /**
     * 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, false);
}

Spinner.prototype = {
  /**
   * Register a new condition for this phase.
   *
   * See the documentation of `addBlocker` in property `client`
   * of instances of `Barrier`.
   */
  addBlocker: function(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: function(condition) {
    return this._barrier.client.removeBlocker(condition);
  },

  get name() {
    return this._barrier.client.name;
  },

  // nsIObserver.observe
  observe: function() {
    let topic = this._topic;
    debug(`Starting phase ${ topic }`);
    Services.obs.removeObserver(this, topic);

    let satisfied = false; // |true| once we have satisfied all conditions
    let promise;
    try {
      promise = this._barrier.wait({
        warnAfterMS: DELAY_WARNING_MS,
        crashAfterMS: DELAY_CRASH_MS
      }).catch(
        // Additional precaution to be entirely sure that we cannot reject.
      );
    } catch (ex) {
      debug("Error waiting for notification");
      throw ex;
    }

    // Now, spin the event loop
    debug("Spinning the event loop");
    promise.then(() => satisfied = true); // This promise cannot reject
    let thread = Services.tm.mainThread;
    while (!satisfied) {
      try {
        thread.processNextEvent(true);
      } catch (ex) {
        // An uncaught error should not stop us, but it should still
        // be reported and cause tests to fail.
        Promise.reject(ex);
      }
    }
    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;

  /**
   * 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") {
        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") {
        throw new TypeError("Expected an object as third argument to `addBlocker`, got " + details);
      }
      if (!this._waitForMe) {
        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 }`);

      // Normalize the details

      let fetchState = details.fetchState || null;
      if (fetchState != null && typeof fetchState != "function") {
        throw new TypeError("Expected a function for option `fetchState`");
      }
      let filename = details.filename || null;
      let lineNumber = details.lineNumber || null;
      let stack = details.stack || null;

      // 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.then(null, 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);
      }).catch(
        // Added as a last line of defense, in case `warn`, `this._name` or
        // `safeGetState` somehow throws an error.
      );

      let topFrame = null;
      if (filename == null || lineNumber == null || stack == null) {
        topFrame = Components.stack;
      }

      let blocker = {
        trigger: trigger,
        promise: promise,
        name: name,
        fetchState: 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);
    }
  };
}
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: name,
        state: safeGetState(fetchState),
        filename: filename,
        lineNumber: lineNumber,
        stack: 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: function(options = {}) {
    // This method only implements caching on top of _wait()
    if (this._promise) {
      return this._promise;
    }
    return this._promise = this._wait(options);
  },
  _wait: function(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();

    promise = promise.then(null, 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(
        function onTimeout() {
          // 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 (gCrashReporter && gCrashReporter.enabled) {
            let data = {
              phase: topic,
              conditions: state
            };
            gCrashReporter.annotateCrashReport("AsyncShutdownTimeout",
              JSON.stringify(data));
          } 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;
          }
          gDebug.abort(filename, lineNumber);
        }.bind(this),
        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;
  },

  _removeBlocker: function(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) {
  this.AsyncShutdown.profileChangeTeardown = getPhase("profile-change-teardown");
  this.AsyncShutdown.profileBeforeChange = getPhase("profile-before-change");
  this.AsyncShutdown.placesClosingInternalConnection = getPhase("places-will-close-connection");
}

// Notifications that fire in the parent and content process, but should
// only have phases in the parent process.
if (!isContent) {
  this.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
this.AsyncShutdown.webWorkersShutdown = getPhase("web-workers-shutdown");
this.AsyncShutdown.xpcomWillShutdown = getPhase("xpcom-will-shutdown");

this.AsyncShutdown.Barrier = Barrier;

Object.freeze(this.AsyncShutdown);