ExtensionCommon.jsm

/* -*- Mode: indent-tabs-mode: nil; js-indent-level: 2 -*- */
/* vim: set sts=2 sw=2 et tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
"use strict";

/**
 * This module contains utilities and base classes for logic which is
 * common between the parent and child process, and in particular
 * between ExtensionParent.jsm and ExtensionChild.jsm.
 */

/* exported ExtensionCommon */

this.EXPORTED_SYMBOLS = ["ExtensionCommon", "SchemaAPIManager"];

ChromeUtils.import("resource://gre/modules/Services.jsm");
ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm");

XPCOMUtils.defineLazyGlobalGetters(this, ["fetch"]);

XPCOMUtils.defineLazyModuleGetters(this, {
  AppConstants: "resource://gre/modules/AppConstants.jsm",
  ConsoleAPI: "resource://gre/modules/Console.jsm",
  MessageChannel: "resource://gre/modules/MessageChannel.jsm",
  PrivateBrowsingUtils: "resource://gre/modules/PrivateBrowsingUtils.jsm",
  Schemas: "resource://gre/modules/Schemas.jsm",
  SchemaRoot: "resource://gre/modules/Schemas.jsm",
});

XPCOMUtils.defineLazyServiceGetter(this, "styleSheetService",
                                   "@mozilla.org/content/style-sheet-service;1",
                                   "nsIStyleSheetService");

ChromeUtils.import("resource://gre/modules/ExtensionUtils.jsm");

var {
  DefaultMap,
  DefaultWeakMap,
  ExtensionError,
  filterStack,
  getInnerWindowID,
  getUniqueId,
  getWinUtils,
} = ExtensionUtils;

function getConsole() {
  return new ConsoleAPI({
    maxLogLevelPref: "extensions.webextensions.log.level",
    prefix: "WebExtensions",
  });
}

XPCOMUtils.defineLazyGetter(this, "console", getConsole);

XPCOMUtils.defineLazyPreferenceGetter(this, "DELAYED_BG_STARTUP",
                                      "extensions.webextensions.background-delayed-startup");

// Run a function and report exceptions.
function runSafeSyncWithoutClone(f, ...args) {
  try {
    return f(...args);
  } catch (e) {
    dump(`Extension error: ${e} ${e.fileName} ${e.lineNumber}\n[[Exception stack\n${filterStack(e)}Current stack\n${filterStack(Error())}]]\n`);
    Cu.reportError(e);
  }
}

// Return true if the given value is an instance of the given
// native type.
function instanceOf(value, type) {
  return (value && typeof value === "object" &&
          ChromeUtils.getClassName(value) === type);
}

/**
 * Convert any of several different representations of a date/time to a Date object.
 * Accepts several formats:
 * a Date object, an ISO8601 string, or a number of milliseconds since the epoch as
 * either a number or a string.
 *
 * @param {Date|string|number} date
 *      The date to convert.
 * @returns {Date}
 *      A Date object
 */
function normalizeTime(date) {
  // Of all the formats we accept the "number of milliseconds since the epoch as a string"
  // is an outlier, everything else can just be passed directly to the Date constructor.
  return new Date((typeof date == "string" && /^\d+$/.test(date))
                        ? parseInt(date, 10) : date);
}

function withHandlingUserInput(window, callable) {
  let handle = getWinUtils(window).setHandlingUserInput(true);
  try {
    return callable();
  } finally {
    handle.destruct();
  }
}

/**
 * Defines a lazy getter for the given property on the given object. The
 * first time the property is accessed, the return value of the getter
 * is defined on the current `this` object with the given property name.
 * Importantly, this means that a lazy getter defined on an object
 * prototype will be invoked separately for each object instance that
 * it's accessed on.
 *
 * @param {object} object
 *        The prototype object on which to define the getter.
 * @param {string|symbol} prop
 *        The property name for which to define the getter.
 * @param {function} getter
 *        The function to call in order to generate the final property
 *        value.
 */
function defineLazyGetter(object, prop, getter) {
  let redefine = (obj, value) => {
    Object.defineProperty(obj, prop, {
      enumerable: true,
      configurable: true,
      writable: true,
      value,
    });
    return value;
  };

  Object.defineProperty(object, prop, {
    enumerable: true,
    configurable: true,

    get() {
      return redefine(this, getter.call(this));
    },

    set(value) {
      redefine(this, value);
    },
  });
}

function checkLoadURL(url, principal, options) {
  let ssm = Services.scriptSecurityManager;

  let flags = ssm.STANDARD;
  if (!options.allowScript) {
    flags |= ssm.DISALLOW_SCRIPT;
  }
  if (!options.allowInheritsPrincipal) {
    flags |= ssm.DISALLOW_INHERIT_PRINCIPAL;
  }
  if (options.dontReportErrors) {
    flags |= ssm.DONT_REPORT_ERRORS;
  }

  try {
    ssm.checkLoadURIWithPrincipal(principal,
                                  Services.io.newURI(url),
                                  flags);
  } catch (e) {
    return false;
  }
  return true;
}

function makeWidgetId(id) {
  id = id.toLowerCase();
  // FIXME: This allows for collisions.
  return id.replace(/[^a-z0-9_-]/g, "_");
}

/**
 * A sentinel class to indicate that an array of values should be
 * treated as an array when used as a promise resolution value, but as a
 * spread expression (...args) when passed to a callback.
 */
class SpreadArgs extends Array {
  constructor(args) {
    super();
    this.push(...args);
  }
}

/**
 * Like SpreadArgs, but also indicates that the array values already
 * belong to the target compartment, and should not be cloned before
 * being passed.
 *
 * The `unwrappedValues` property contains an Array object which belongs
 * to the target compartment, and contains the same unwrapped values
 * passed the NoCloneSpreadArgs constructor.
 */
class NoCloneSpreadArgs {
  constructor(args) {
    this.unwrappedValues = args;
  }

  [Symbol.iterator]() {
    return this.unwrappedValues[Symbol.iterator]();
  }
}

const LISTENERS = Symbol("listeners");
const ONCE_MAP = Symbol("onceMap");

class EventEmitter {
  constructor() {
    this[LISTENERS] = new Map();
    this[ONCE_MAP] = new WeakMap();
  }

  /**
   * Adds the given function as a listener for the given event.
   *
   * The listener function may optionally return a Promise which
   * resolves when it has completed all operations which event
   * dispatchers may need to block on.
   *
   * @param {string} event
   *       The name of the event to listen for.
   * @param {function(string, ...any)} listener
   *        The listener to call when events are emitted.
   */
  on(event, listener) {
    let listeners = this[LISTENERS].get(event);
    if (!listeners) {
      listeners = new Set();
      this[LISTENERS].set(event, listeners);
    }

    listeners.add(listener);
  }

  /**
   * Removes the given function as a listener for the given event.
   *
   * @param {string} event
   *       The name of the event to stop listening for.
   * @param {function(string, ...any)} listener
   *        The listener function to remove.
   */
  off(event, listener) {
    let set = this[LISTENERS].get(event);
    if (set) {
      set.delete(listener);
      set.delete(this[ONCE_MAP].get(listener));
      if (!set.size) {
        this[LISTENERS].delete(event);
      }
    }
  }

  /**
   * Adds the given function as a listener for the given event once.
   *
   * @param {string} event
   *       The name of the event to listen for.
   * @param {function(string, ...any)} listener
   *        The listener to call when events are emitted.
   */
  once(event, listener) {
    let wrapper = (...args) => {
      this.off(event, wrapper);
      this[ONCE_MAP].delete(listener);

      // @ts-ignore
      return listener(...args);
    };
    this[ONCE_MAP].set(listener, wrapper);

    this.on(event, wrapper);
  }


  /**
   * Triggers all listeners for the given event. If any listeners return
   * a value, returns a promise which resolves when all returned
   * promises have resolved. Otherwise, returns undefined.
   *
   * @param {string} event
   *       The name of the event to emit.
   * @param {...any} args
   *        Arbitrary arguments to pass to the listener functions, after
   *        the event name.
   * @returns {Promise?}
   */
  emit(event, ...args) {
    let listeners = this[LISTENERS].get(event);

    if (listeners) {
      let promises = [];

      for (let listener of listeners) {
        try {
          let result = listener(event, ...args);
          if (result !== undefined) {
            promises.push(result);
          }
        } catch (e) {
          Cu.reportError(e);
        }
      }

      if (promises.length) {
        return Promise.all(promises);
      }
    }
  }
}

/**
 * Base class for WebExtension APIs.  Each API creates a new class
 * that inherits from this class, the derived class is instantiated
 * once for each extension that uses the API.
 */
class ExtensionAPI extends EventEmitter {
  constructor(extension) {
    super();

    this.extension = extension;
    this.onShutdown = null;

    extension.once("shutdown", () => {
      if (this.onShutdown) {
        this.onShutdown(extension.shutdownReason);
      }
      this.extension = null;
    });
  }

  destroy() {
  }

  onManifestEntry(entry) {
  }

  getAPI(context) {
    throw new Error("Not Implemented");
  }
}

/**
 * This class contains the information we have about an individual
 * extension.  It is never instantiated directly, instead subclasses
 * for each type of process extend this class and add members that are
 * relevant for that process.
 * @abstract
 */
class BaseContext {
  constructor(envType, extension) {
    this.envType = envType;
    this.onClose = new Set();
    this.checkedLastError = false;
    this._lastError = null;
    this.contextId = getUniqueId();
    this.unloaded = false;
    this.extension = extension;
    this.jsonSandbox = null;
    this.active = true;
    this.incognito = null;
    this.messageManager = null;
    this.docShell = null;
    this.contentWindow = null;
    this.innerWindowID = 0;
    this.childManager = null;
    this.viewType = null;
  }

  setContentWindow(contentWindow) {
    let {document, docShell} = contentWindow;

    this.innerWindowID = getInnerWindowID(contentWindow);
    this.messageManager = docShell.QueryInterface(Ci.nsIInterfaceRequestor)
                                  .getInterface(Ci.nsIContentFrameMessageManager);

    if (this.incognito == null) {
      this.incognito = PrivateBrowsingUtils.isContentWindowPrivate(contentWindow);
    }

    MessageChannel.setupMessageManagers([this.messageManager]);

    let onPageShow = event => {
      if (!event || event.target === document) {
        this.docShell = docShell;
        this.contentWindow = contentWindow;
        this.active = true;
      }
    };
    let onPageHide = event => {
      if (!event || event.target === document) {
        // Put this off until the next tick.
        Promise.resolve().then(() => {
          this.docShell = null;
          this.contentWindow = null;
          this.active = false;
        });
      }
    };

    onPageShow();
    contentWindow.addEventListener("pagehide", onPageHide, true);
    contentWindow.addEventListener("pageshow", onPageShow, true);
    this.callOnClose({
      close: () => {
        onPageHide();
        if (this.active) {
          contentWindow.removeEventListener("pagehide", onPageHide, true);
          contentWindow.removeEventListener("pageshow", onPageShow, true);
        }
      },
    });
  }

  /** @returns {any} */
  get cloneScope() {
    throw new Error("Not implemented");
  }

  /** @returns {any} */
  get principal() {
    throw new Error("Not implemented");
  }

  runSafe(callback, ...args) {
    return this.applySafe(callback, args);
  }

  runSafeWithoutClone(callback, ...args) {
    return this.applySafeWithoutClone(callback, args);
  }

  applySafe(callback, args, caller) {
    if (this.unloaded) {
      Cu.reportError("context.runSafe called after context unloaded",
                     caller);
    } else if (!this.active) {
      Cu.reportError("context.runSafe called while context is inactive",
                     caller);
    } else {
      try {
        let {cloneScope} = this;
        args = args.map(arg => Cu.cloneInto(arg, cloneScope));
      } catch (e) {
        Cu.reportError(e);
        dump(`runSafe failure: cloning into ${this.cloneScope}: ${e}\n\n${filterStack(Error())}`);
      }

      return this.applySafeWithoutClone(callback, args, caller);
    }
  }

  applySafeWithoutClone(callback, args, caller) {
    if (this.unloaded) {
      Cu.reportError("context.runSafeWithoutClone called after context unloaded",
                     caller);
    } else if (!this.active) {
      Cu.reportError("context.runSafeWithoutClone called while context is inactive",
                     caller);
    } else {
      try {
        return Reflect.apply(callback, null, args);
      } catch (e) {
        dump(`Extension error: ${e} ${e.fileName} ${e.lineNumber}\n[[Exception stack\n${filterStack(e)}Current stack\n${filterStack(Error())}]]\n`);
        Cu.reportError(e);
      }
    }
  }

  checkLoadURL(url, options = {}) {
    // As an optimization, f the URL starts with the extension's base URL,
    // don't do any further checks. It's always allowed to load it.
    if (url.startsWith(this.extension.baseURL)) {
      return true;
    }

    return checkLoadURL(url, this.principal, options);
  }

  /**
   * Safely call JSON.stringify() on an object that comes from an
   * extension.
   *
   * @param {Array<any>} args Arguments for JSON.stringify()
   * @returns {string} The stringified representation of obj
   */
  jsonStringify(...args) {
    if (!this.jsonSandbox) {
      this.jsonSandbox = Cu.Sandbox(this.principal, {
        sameZoneAs: this.cloneScope,
        wantXrays: false,
      });
    }

    return Cu.waiveXrays(this.jsonSandbox.JSON).stringify(...args);
  }

  callOnClose(obj) {
    this.onClose.add(obj);
  }

  forgetOnClose(obj) {
    this.onClose.delete(obj);
  }

  /**
   * A wrapper around MessageChannel.sendMessage which adds the extension ID
   * to the recipient object, and ensures replies are not processed after the
   * context has been unloaded.
   *
   * @param {nsIMessageManager} target
   * @param {string} messageName
   * @param {object} data
   * @param {object} [options]
   * @param {object} [options.sender]
   * @param {object} [options.recipient]
   *
   * @returns {Promise}
   */
  sendMessage(target, messageName, data, options = {}) {
    options.recipient = Object.assign({extensionId: this.extension.id}, options.recipient);
    options.sender = options.sender || {};

    options.sender.extensionId = this.extension.id;
    options.sender.contextId = this.contextId;

    return MessageChannel.sendMessage(target, messageName, data, options);
  }

  get lastError() {
    this.checkedLastError = true;
    return this._lastError;
  }

  set lastError(val) {
    this.checkedLastError = false;
    this._lastError = val;
  }

  /**
   * Normalizes the given error object for use by the target scope. If
   * the target is an error object which belongs to that scope, it is
   * returned as-is. If it is an ordinary object with a `message`
   * property, it is converted into an error belonging to the target
   * scope. If it is an Error object which does *not* belong to the
   * clone scope, it is reported, and converted to an unexpected
   * exception error.
   *
   * @param {Error|object} error
   * @param {SavedFrame?} [caller]
   * @returns {Error}
   */
  normalizeError(error, caller) {
    if (error instanceof this.cloneScope.Error) {
      return error;
    }
    let message, fileName;
    if (error && typeof error === "object") {
      const isPlain = ChromeUtils.getClassName(error) === "Object";
      if (isPlain && error.mozWebExtLocation) {
        caller = error.mozWebExtLocation;
      }
      if (isPlain && caller && (error.mozWebExtLocation || !error.fileName)) {
        caller = Cu.cloneInto(caller, this.cloneScope);
        return ChromeUtils.createError(error.message, caller);
      }

      if (isPlain ||
          error instanceof ExtensionError ||
          this.principal.subsumes(Cu.getObjectPrincipal(error))) {
        message = error.message;
        fileName = error.fileName;
      }
    }

    if (!message) {
      Cu.reportError(error);
      message = "An unexpected error occurred";
    }
    return new this.cloneScope.Error(message, fileName);
  }

  /**
   * Sets the value of `.lastError` to `error`, calls the given
   * callback, and reports an error if the value has not been checked
   * when the callback returns.
   *
   * @param {object} error An object with a `message` property. May
   *     optionally be an `Error` object belonging to the target scope.
   * @param {SavedFrame?} caller
   *        The optional caller frame which triggered this callback, to be used
   *        in error reporting.
   * @param {function} callback The callback to call.
   * @returns {*} The return value of callback.
   */
  withLastError(error, caller, callback) {
    this.lastError = this.normalizeError(error);
    try {
      return callback();
    } finally {
      if (!this.checkedLastError) {
        Cu.reportError(`Unchecked lastError value: ${this.lastError}`, caller);
      }
      this.lastError = null;
    }
  }

  /**
   * Captures the most recent stack frame which belongs to the extension.
   *
   * @returns {SavedFrame?}
   */
  getCaller() {
    return ChromeUtils.getCallerLocation(this.principal);
  }

  /**
   * Wraps the given promise so it can be safely returned to extension
   * code in this context.
   *
   * If `callback` is provided, however, it is used as a completion
   * function for the promise, and no promise is returned. In this case,
   * the callback is called when the promise resolves or rejects. In the
   * latter case, `lastError` is set to the rejection value, and the
   * callback function must check `browser.runtime.lastError` or
   * `extension.runtime.lastError` in order to prevent it being reported
   * to the console.
   *
   * @param {Promise} promise The promise with which to wrap the
   *     callback. May resolve to a `SpreadArgs` instance, in which case
   *     each element will be used as a separate argument.
   *
   *     Unless the promise object belongs to the cloneScope global, its
   *     resolution value is cloned into cloneScope prior to calling the
   *     `callback` function or resolving the wrapped promise.
   *
   * @param {function} [callback] The callback function to wrap
   *
   * @returns {Promise|undefined} If callback is null, a promise object
   *     belonging to the target scope. Otherwise, undefined.
   */
  wrapPromise(promise, callback = null) {
    let caller = this.getCaller();
    let applySafe = this.applySafe.bind(this);
    if (Cu.getGlobalForObject(promise) === this.cloneScope) {
      applySafe = this.applySafeWithoutClone.bind(this);
    }

    if (callback) {
      promise.then(
        args => {
          if (this.unloaded) {
            Cu.reportError(`Promise resolved after context unloaded\n`,
                           caller);
          } else if (!this.active) {
            Cu.reportError(`Promise resolved while context is inactive\n`,
                           caller);
          } else if (args instanceof NoCloneSpreadArgs) {
            this.applySafeWithoutClone(callback, args.unwrappedValues, caller);
          } else if (args instanceof SpreadArgs) {
            applySafe(callback, args, caller);
          } else {
            applySafe(callback, [args], caller);
          }
        },
        error => {
          this.withLastError(error, caller, () => {
            if (this.unloaded) {
              Cu.reportError(`Promise rejected after context unloaded\n`,
                             caller);
            } else if (!this.active) {
              Cu.reportError(`Promise rejected while context is inactive\n`,
                             caller);
            } else {
              this.applySafeWithoutClone(callback, [], caller);
            }
          });
        });
    } else {
      return new this.cloneScope.Promise((resolve, reject) => {
        promise.then(
          value => {
            if (this.unloaded) {
              Cu.reportError(`Promise resolved after context unloaded\n`,
                             caller);
            } else if (!this.active) {
              Cu.reportError(`Promise resolved while context is inactive\n`,
                             caller);
            } else if (value instanceof NoCloneSpreadArgs) {
              let values = value.unwrappedValues;
              this.applySafeWithoutClone(resolve, values.length == 1 ? [values[0]] : [values],
                                         caller);
            } else if (value instanceof SpreadArgs) {
              applySafe(resolve, value.length == 1 ? value : [value],
                        caller);
            } else {
              applySafe(resolve, [value], caller);
            }
          },
          value => {
            if (this.unloaded) {
              Cu.reportError(`Promise rejected after context unloaded: ${value && value.message}\n`,
                             caller);
            } else if (!this.active) {
              Cu.reportError(`Promise rejected while context is inactive: ${value && value.message}\n`,
                             caller);
            } else {
              this.applySafeWithoutClone(reject, [this.normalizeError(value, caller)],
                                         caller);
            }
          });
      });
    }
  }

  unload() {
    this.unloaded = true;

    MessageChannel.abortResponses({
      extensionId: this.extension.id,
      contextId: this.contextId,
    });

    for (let obj of this.onClose) {
      obj.close();
    }
  }

  /**
   * A simple proxy for unload(), for use with callOnClose().
   */
  close() {
    this.unload();
  }
}

/**
 * An object that runs the implementation of a schema API. Instantiations of
 * this interfaces are used by Schemas.jsm.
 *
 * @interface
 */
class SchemaAPIInterface {
  /**
   * Calls this as a function that returns its return value.
   *
   * @abstract
   * @param {Array} args The parameters for the function.
   * @returns {*} The return value of the invoked function.
   */
  callFunction(args) {
    throw new Error("Not implemented");
  }

  /**
   * Calls this as a function and ignores its return value.
   *
   * @abstract
   * @param {Array} args The parameters for the function.
   */
  callFunctionNoReturn(args) {
    throw new Error("Not implemented");
  }

  /**
   * Calls this as a function that completes asynchronously.
   *
   * @abstract
   * @param {Array} args The parameters for the function.
   * @param {function(*)} [callback] The callback to be called when the function
   *     completes.
   * @param {boolean} [requireUserInput=false] If true, the function should
   *                  fail if the browser is not currently handling user input.
   * @returns {Promise|undefined} Must be void if `callback` is set, and a
   *     promise otherwise. The promise is resolved when the function completes.
   */
  callAsyncFunction(args, callback, requireUserInput = false) {
    throw new Error("Not implemented");
  }

  /**
   * Retrieves the value of this as a property.
   *
   * @abstract
   * @returns {*} The value of the property.
   */
  getProperty() {
    throw new Error("Not implemented");
  }

  /**
   * Assigns the value to this as property.
   *
   * @abstract
   * @param {string} value The new value of the property.
   */
  setProperty(value) {
    throw new Error("Not implemented");
  }

  /**
   * Registers a `listener` to this as an event.
   *
   * @abstract
   * @param {function} listener The callback to be called when the event fires.
   * @param {Array} args Extra parameters for EventManager.addListener.
   * @see EventManager.addListener
   */
  addListener(listener, args) {
    throw new Error("Not implemented");
  }

  /**
   * Checks whether `listener` is listening to this as an event.
   *
   * @abstract
   * @param {function} listener The event listener.
   * @returns {boolean} Whether `listener` is registered with this as an event.
   * @see EventManager.hasListener
   */
  hasListener(listener) {
    throw new Error("Not implemented");
  }

  /**
   * Unregisters `listener` from this as an event.
   *
   * @abstract
   * @param {function} listener The event listener.
   * @see EventManager.removeListener
   */
  removeListener(listener) {
    throw new Error("Not implemented");
  }

  /**
   * Revokes the implementation object, and prevents any further method
   * calls from having external effects.
   *
   * @abstract
   */
  revoke() {
    throw new Error("Not implemented");
  }
}

/**
 * An object that runs a locally implemented API.
 */
class LocalAPIImplementation extends SchemaAPIInterface {
  /**
   * Constructs an implementation of the `name` method or property of `pathObj`.
   *
   * @param {object} pathObj The object containing the member with name `name`.
   * @param {string} name The name of the implemented member.
   * @param {BaseContext} context The context in which the schema is injected.
   */
  constructor(pathObj, name, context) {
    super();
    this.pathObj = pathObj;
    this.name = name;
    this.context = context;
  }

  revoke() {
    if (this.pathObj[this.name][Schemas.REVOKE]) {
      this.pathObj[this.name][Schemas.REVOKE]();
    }

    this.pathObj = null;
    this.name = null;
    this.context = null;
  }

  callFunction(args) {
    return this.pathObj[this.name](...args);
  }

  callFunctionNoReturn(args) {
    this.pathObj[this.name](...args);
  }

  callAsyncFunction(args, callback, requireUserInput) {
    let promise;
    try {
      if (requireUserInput) {
        if (!getWinUtils(this.context.contentWindow).isHandlingUserInput) {
          throw new ExtensionError(`${this.name} may only be called from a user input handler`);
        }
      }
      promise = this.pathObj[this.name](...args) || Promise.resolve();
    } catch (e) {
      promise = Promise.reject(e);
    }
    return this.context.wrapPromise(promise, callback);
  }

  getProperty() {
    return this.pathObj[this.name];
  }

  setProperty(value) {
    this.pathObj[this.name] = value;
  }

  addListener(listener, args) {
    try {
      this.pathObj[this.name].addListener.call(null, listener, ...args);
    } catch (e) {
      throw this.context.normalizeError(e);
    }
  }

  hasListener(listener) {
    return this.pathObj[this.name].hasListener.call(null, listener);
  }

  removeListener(listener) {
    this.pathObj[this.name].removeListener.call(null, listener);
  }
}

// Recursively copy properties from source to dest.
function deepCopy(dest, source) {
  for (let prop in source) {
    let desc = Object.getOwnPropertyDescriptor(source, prop);
    if (typeof(desc.value) == "object") {
      if (!(prop in dest)) {
        dest[prop] = {};
      }
      deepCopy(dest[prop], source[prop]);
    } else {
      Object.defineProperty(dest, prop, desc);
    }
  }
}

function getChild(map, key) {
  let child = map.children.get(key);
  if (!child) {
    child = {
      modules: new Set(),
      children: new Map(),
    };

    map.children.set(key, child);
  }
  return child;
}

function getPath(map, path) {
  for (let key of path) {
    map = getChild(map, key);
  }
  return map;
}

function mergePaths(dest, source) {
  for (let name of source.modules) {
    dest.modules.add(name);
  }

  for (let [name, child] of source.children.entries()) {
    mergePaths(getChild(dest, name),
               child);
  }
}

/**
 * Manages loading and accessing a set of APIs for a specific extension
 * context.
 */
class CanOfAPIs {
  /**
   * @param {BaseContext} context
   *        The context to manage APIs for.
   * @param {SchemaAPIManager} apiManager
   *        The API manager holding the APIs to manage.
   * @param {object} root
   *        The root object into which APIs will be injected.
   */
  constructor(context, apiManager, root) {
    this.context = context;
    this.scopeName = context.envType;
    this.apiManager = apiManager;
    this.root = root;

    this.apiPaths = new Map();

    this.apis = new Map();
  }

  /**
   * Synchronously loads and initializes an ExtensionAPI instance.
   *
   * @param {string} name
   *        The name of the API to load.
   */