/* 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/. */

const DEFAULT_CAPTURE_TIMEOUT = 30000; // ms
// For testing, the above timeout is excessive, and makes our tests overlong.
const TESTING_CAPTURE_TIMEOUT = 5000; // ms

const DESTROY_BROWSER_TIMEOUT = 60000; // ms

// Let the page settle for this amount of milliseconds before capturing to allow
// for any in-page changes or redirects.
const SETTLE_WAIT_TIME = 2500;
// For testing, the above timeout is excessive, and makes our tests overlong.
const TESTING_SETTLE_WAIT_TIME = 0;

const ABOUT_NEWTAB_SEGREGATION_PREF =
  "privacy.usercontext.about_newtab_segregation.enabled";

import {
  PageThumbs,
  PageThumbsStorage,
} from "resource://gre/modules/PageThumbs.sys.mjs";

// possible FX_THUMBNAILS_BG_CAPTURE_DONE_REASON_2 telemetry values
const TEL_CAPTURE_DONE_OK = 0;
const TEL_CAPTURE_DONE_TIMEOUT = 1;
// 2 and 3 were used when we had special handling for private-browsing.
const TEL_CAPTURE_DONE_CRASHED = 4;
const TEL_CAPTURE_DONE_BAD_URI = 5;
const TEL_CAPTURE_DONE_LOAD_FAILED = 6;
const TEL_CAPTURE_DONE_IMAGE_ZERO_DIMENSION = 7;

const lazy = {};

ChromeUtils.defineESModuleGetters(lazy, {
  ContextualIdentityService:
    "resource://gre/modules/ContextualIdentityService.sys.mjs",
});

export const BackgroundPageThumbs = {
  /**
   * Asynchronously captures a thumbnail of the given URL.
   *
   * The page is loaded anonymously, and plug-ins are disabled.
   *
   * @param url
   *   The URL to capture.
   * @param {object} [options]
   *   An optional object that configures the capture.
   * @param {Function} [options.onDone]
   *   A function that will be asynchronously called when the capture is
   *   complete or times out.  It's called as onDone(url), where `url` is the
   *   captured URL.
   * @param {number} [options.timeout]
   *   The capture will time out after this many milliseconds have elapsed after
   *   the capture has progressed to the head of the queue and started. Defaults
   *   to 30000 (30 seconds).
   * @param {boolean} [options.isImage]
   *   If true, backgroundPageThumbsContent will attempt to render the url
   *   directly to canvas. Note that images will mostly get detected and rendered
   *   as such anyway, but this will ensure it.
   * @param {number} [options.targetWidth]
   *   The target width when capturing an image.
   * @param {string} [options.backgroundColor]
   *   The background colour when capturing an image.
   * @param {boolean} [options.dontStore]
   *   If set to true, the image blob won't be stored to disk, an object will
   *   instead be passed as third argument to onDone:
   *   ```
   *   {
   *     data: an ArrayBuffer containing the data
   *     contentType: the data content-type
   *     originalUrl: the originally requested url
   *     currentUrl: the final url after redirects
   *   }
   *   ```
   * @param {string} [options.contentType]
   *   Can be set to an image contentType for the capture, defaults to
   *   PageThumbs.contentType.
   * @param {number} [options.settleWaitTime]
   *   Milliseconds to wait after the page reports loaded before capturing, to
   *   allow for in-page redirects. Defaults to 2500. Set to 0 when capturing a
   *   direct image URL where no settle is needed.
   */
  capture(url, options = {}) {
    if (!PageThumbs._prefEnabled()) {
      if (options.onDone) {
        schedule(() => options.onDone(url));
      }
      return;
    }
    this._captureQueue = this._captureQueue || [];
    this._capturesByURL = this._capturesByURL || new Map();

    Glean.thumbnails.queueSizeOnCapture.accumulateSingleSample(
      this._captureQueue.length
    );

    // We want to avoid duplicate captures for the same URL.  If there is an
    // existing one, we just add the callback to that one and we are done.
    let existing = this._capturesByURL.get(url);
    if (existing) {
      if (options.onDone) {
        existing.doneCallbacks.push(options.onDone);
      }
      // The queue is already being processed, so nothing else to do...
      return;
    }
    let cap = new Capture(url, this._onCaptureOrTimeout.bind(this), options);
    this._captureQueue.push(cap);
    this._capturesByURL.set(url, cap);
    this._processCaptureQueue();
  },

  /**
   * Asynchronously captures a thumbnail of the given URL if one does not
   * already exist.  Otherwise does nothing.
   *
   * @param url      The URL to capture.
   * @param options  An optional object that configures the capture.  See
   *                 capture() for description.
   *   unloadingPromise This option is resolved when the calling context is
   *                    unloading, so things can be cleaned up to avoid leak.
   * @return {Promise} A Promise that resolves when this task completes
   */
  async captureIfMissing(url, options = {}) {
    // Short circuit this function if pref is enabled, or else we leak observers.
    // See Bug 1400562
    if (!PageThumbs._prefEnabled()) {
      if (options.onDone) {
        options.onDone(url);
      }
      return url;
    }
    // The fileExistsForURL call is an optimization, potentially but unlikely
    // incorrect, and no big deal when it is.  After the capture is done, we
    // atomically test whether the file exists before writing it.
    let exists = await PageThumbsStorage.fileExistsForURL(url);
    if (exists) {
      if (options.onDone) {
        options.onDone(url);
      }
      return url;
    }
    let thumbPromise = new Promise((resolve, reject) => {
      let observe = (subject, topic, data) => {
        if (data === url) {
          switch (topic) {
            case "page-thumbnail:create":
              resolve();
              break;
            case "page-thumbnail:error":
              reject(new Error("page-thumbnail:error"));
              break;
          }
          cleanup();
        }
      };
      Services.obs.addObserver(observe, "page-thumbnail:create");
      Services.obs.addObserver(observe, "page-thumbnail:error");

      // Make sure to clean up to avoid leaks by removing observers when
      // observed or when our caller is unloading
      function cleanup() {
        if (observe) {
          Services.obs.removeObserver(observe, "page-thumbnail:create");
          Services.obs.removeObserver(observe, "page-thumbnail:error");
          observe = null;
        }
      }
      if (options.unloadingPromise) {
        options.unloadingPromise.then(cleanup);
      }
    });
    try {
      this.capture(url, options);
      await thumbPromise;
    } catch (err) {
      if (options.onDone) {
        options.onDone(url);
      }
      throw err;
    }
    return url;
  },

  /**
   * Tell the service that the thumbnail browser should be recreated at next
   * call of _ensureBrowser().
   */
  renewThumbnailBrowser() {
    this._renewThumbBrowser = true;
  },

  get useFissionBrowser() {
    return Services.appinfo.fissionAutostart;
  },

  /**
   * Ensures that initialization of the thumbnail browser's parent window has
   * begun.
   *
   * @return  True if the parent window is completely initialized and can be
   *          used, and false if initialization has started but not completed.
   */
  _ensureParentWindowReady() {
    if (this._parentWin) {
      // Already fully initialized.
      return true;
    }
    if (this._startedParentWinInit) {
      // Already started initializing.
      return false;
    }

    this._startedParentWinInit = true;

    // Create a windowless browser and load our hosting
    // (privileged) document in it.
    const flags = this.useFissionBrowser
      ? Ci.nsIWebBrowserChrome.CHROME_REMOTE_WINDOW |
        Ci.nsIWebBrowserChrome.CHROME_FISSION_WINDOW
      : 0;
    let wlBrowser = Services.appShell.createWindowlessBrowser(true, flags);
    wlBrowser.QueryInterface(Ci.nsIInterfaceRequestor);
    let webProgress = wlBrowser.getInterface(Ci.nsIWebProgress);
    this._listener = {
      QueryInterface: ChromeUtils.generateQI([
        "nsIWebProgressListener",
        "nsIWebProgressListener2",
        "nsISupportsWeakReference",
      ]),
    };
    this._listener.onStateChange = (wbp, request, stateFlags) => {
      if (!request) {
        return;
      }
      if (
        stateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
        stateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK
      ) {
        webProgress.removeProgressListener(this._listener);
        delete this._listener;
        // Get the window reference via the document.
        this._parentWin = wlBrowser.document.defaultView;
        this._processCaptureQueue();
      }
    };
    webProgress.addProgressListener(
      this._listener,
      Ci.nsIWebProgress.NOTIFY_STATE_ALL
    );
    let loadURIOptions = {
      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
    };
    wlBrowser.loadURI(
      Services.io.newURI("chrome://global/content/backgroundPageThumbs.xhtml"),
      loadURIOptions
    );
    this._windowlessContainer = wlBrowser;

    return false;
  },

  _init() {
    Services.prefs.addObserver(ABOUT_NEWTAB_SEGREGATION_PREF, this);
    Services.obs.addObserver(this, "profile-before-change");
  },

  observe(subject, topic, data) {
    if (topic == "profile-before-change") {
      this._destroy();
    } else if (
      topic == "nsPref:changed" &&
      data == ABOUT_NEWTAB_SEGREGATION_PREF
    ) {
      BackgroundPageThumbs.renewThumbnailBrowser();
    }
  },

  /**
   * Destroys the service.  Queued and pending captures will never complete, and
   * their consumer callbacks will never be called.
   */
  _destroy() {
    if (this._captureQueue) {
      this._captureQueue.forEach(cap => cap.destroy());
    }
    this._destroyBrowser();
    if (this._windowlessContainer) {
      this._windowlessContainer.close();
    }
    delete this._captureQueue;
    delete this._windowlessContainer;
    delete this._startedParentWinInit;
    delete this._parentWin;
    delete this._listener;
  },

  QueryInterface: ChromeUtils.generateQI([
    Ci.nsIWebProgressListener,
    Ci.nsIWebProgressListener2,
    Ci.nsISupportsWeakReference,
  ]),

  onStateChange(wbp, request, stateFlags, status) {
    if (!request || !wbp.isTopLevel) {
      return;
    }

    if (
      stateFlags & Ci.nsIWebProgressListener.STATE_STOP &&
      stateFlags & Ci.nsIWebProgressListener.STATE_IS_NETWORK
    ) {
      // If about:blank is being loaded after a capture, move on
      // to the next capture, otherwise ignore about:blank loads.
      if (
        request instanceof Ci.nsIChannel &&
        request.URI.spec == "about:blank"
      ) {
        if (this._expectingAboutBlank) {
          this._expectingAboutBlank = false;
          if (this._captureQueue.length) {
            this._processCaptureQueue();
          }
        }
        return;
      }

      if (!this._captureQueue.length) {
        return;
      }

      let currentCapture = this._captureQueue[0];
      if (
        Components.isSuccessCode(status) ||
        status === Cr.NS_BINDING_ABORTED
      ) {
        this._thumbBrowser.documentGlobal.requestIdleCallback(() => {
          currentCapture.pageLoaded(this._thumbBrowser);
        });
      } else {
        currentCapture._done(
          this._thumbBrowser,
          null,
          currentCapture.timedOut
            ? TEL_CAPTURE_DONE_TIMEOUT
            : TEL_CAPTURE_DONE_LOAD_FAILED
        );
      }
    }
  },

  /**
   * Creates the thumbnail browser if it doesn't already exist.
   */
  _ensureBrowser() {
    if (this._thumbBrowser && !this._renewThumbBrowser) {
      return;
    }

    this._destroyBrowser();
    this._renewThumbBrowser = false;

    let browser = this._parentWin.document.createXULElement("browser");
    browser.setAttribute("type", "content");
    browser.setAttribute("remote", "true");
    if (this.useFissionBrowser) {
      browser.setAttribute("maychangeremoteness", "true");
    }
    browser.setAttribute("disableglobalhistory", "true");
    browser.setAttribute("messagemanagergroup", "thumbnails");
    browser.setAttribute("manualactiveness", "true");

    if (Services.prefs.getBoolPref(ABOUT_NEWTAB_SEGREGATION_PREF)) {
      // Use the private container for thumbnails.
      let privateIdentity = lazy.ContextualIdentityService.getPrivateIdentity(
        "userContextIdInternal.thumbnail"
      );
      browser.setAttribute("usercontextid", privateIdentity.userContextId);
    }

    // Size the browser.  Make its aspect ratio the same as the canvases' that
    // the thumbnails are drawn into; the canvases' aspect ratio is the same as
    // the screen's, so use that.  Aim for a size in the ballpark of 1024x768.
    let [swidth, sheight] = [{}, {}];
    Cc["@mozilla.org/gfx/screenmanager;1"]
      .getService(Ci.nsIScreenManager)
      .primaryScreen.GetRectDisplayPix({}, {}, swidth, sheight);
    let bwidth = Math.min(1024, swidth.value);
    // Setting the width and height attributes doesn't work -- the resulting
    // thumbnails are blank and transparent -- but setting the style does.
    browser.style.width = bwidth + "px";
    browser.style.height = (bwidth * sheight.value) / swidth.value + "px";
    browser.style.colorScheme = "env(-moz-content-preferred-color-scheme)";

    this._parentWin.document.documentElement.appendChild(browser);

    browser.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_WINDOW);
    browser.mute();

    // an event that is sent if the remote process crashes - no need to remove
    // it as we want it to be there as long as the browser itself lives.
    browser.addEventListener("oop-browser-crashed", event => {
      if (!event.isTopFrame) {
        // It was a subframe that crashed. We'll ignore this.
        return;
      }

      console.error("BackgroundThumbnails remote process crashed - recovering");
      this._destroyBrowser();
      let curCapture = this._captureQueue.length ? this._captureQueue[0] : null;
      // we could retry the pending capture, but it's possible the crash
      // was due directly to it, so trying again might just crash again.
      // We could keep a flag to indicate if it previously crashed, but
      // "resetting" the capture requires more work - so for now, we just
      // discard it.
      if (curCapture) {
        // Continue queue processing by calling curCapture._done().  Do it after
        // this crashed listener returns, though.  A new browser will be created
        // immediately (on the same stack as the _done call stack) if there are
        // any more queued-up captures, and that seems to mess up the new
        // browser's message manager if it happens on the same stack as the
        // listener.  Trying to send a message to the manager in that case
        // throws NS_ERROR_NOT_INITIALIZED.
        Services.tm.dispatchToMainThread(() => {
          curCapture._done(browser, null, TEL_CAPTURE_DONE_CRASHED);
        });
      }
      // else: we must have been idle and not currently doing a capture (eg,
      // maybe a GC or similar crashed) - so there's no need to attempt a
      // queue restart - the next capture request will set everything up.
    });

    this._thumbBrowser = browser;
    browser.docShellIsActive = false;
  },

  _destroyBrowser() {
    if (!this._thumbBrowser) {
      return;
    }
    this._expectingAboutBlank = false;
    this._thumbBrowser.remove();
    delete this._thumbBrowser;
  },

  async _loadAboutBlank() {
    if (this._expectingAboutBlank) {
      return;
    }

    this._expectingAboutBlank = true;

    let loadURIOptions = {
      triggeringPrincipal: Services.scriptSecurityManager.getSystemPrincipal(),
      loadFlags: Ci.nsIWebNavigation.LOAD_FLAGS_STOP_CONTENT,
    };
    this._thumbBrowser.loadURI(
      Services.io.newURI("about:blank"),
      loadURIOptions
    );
  },

  /**
   * Starts the next capture if the queue is not empty and the service is fully
   * initialized.
   */
  _processCaptureQueue() {
    if (!this._captureQueue.length) {
      if (this._thumbBrowser) {
        BackgroundPageThumbs._loadAboutBlank();
      }
      return;
    }

    if (
      this._captureQueue[0].pending ||
      !this._ensureParentWindowReady() ||
      this._expectingAboutBlank
    ) {
      return;
    }

    // Ready to start the first capture in the queue.
    this._ensureBrowser();
    this._captureQueue[0].start(this._thumbBrowser);
    if (this._destroyBrowserTimer) {
      this._destroyBrowserTimer.cancel();
      delete this._destroyBrowserTimer;
    }
  },

  /**
   * Called when the current capture completes or fails (eg, times out, remote
   * process crashes.)
   */
  _onCaptureOrTimeout(capture, reason) {
    // Since timeouts start as an item is being processed, only the first
    // item in the queue can be passed to this method.
    if (capture !== this._captureQueue[0]) {
      throw new Error("The capture should be at the head of the queue.");
    }

    this._captureQueue.shift();
    this._capturesByURL.delete(capture.url);
    if (reason != TEL_CAPTURE_DONE_OK) {
      Services.obs.notifyObservers(null, "page-thumbnail:error", capture.url);
    }

    // Start the destroy-browser timer *before* processing the capture queue.
    let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
    timer.initWithCallback(
      this._destroyBrowser.bind(this),
      this._destroyBrowserTimeout,
      Ci.nsITimer.TYPE_ONE_SHOT
    );
    this._destroyBrowserTimer = timer;

    this._processCaptureQueue();
  },

  _destroyBrowserTimeout: DESTROY_BROWSER_TIMEOUT,
};

BackgroundPageThumbs._init();

/**
 * Represents a single capture request in the capture queue.
 *
 * @param url              The URL to capture.
 * @param captureCallback  A function you want called when the capture
 *                         completes.
 * @param options          The capture options.
 */
function Capture(url, captureCallback, options) {
  this.url = url;
  this.captureCallback = captureCallback;
  this.redirectTimer = null;
  this.timedOut = false;
  this.options = options;
  this.id = Capture.nextID++;
  this.creationDate = new Date();
  this.doneCallbacks = [];
  if (options.onDone) {
    this.doneCallbacks.push(options.onDone);
  }
}

Capture.prototype = {
  get pending() {
    return !!this._timeoutTimer;
  },

  /**
   * Sends a message to the content script to start the capture.
   *
   * @param browser  The thumbnail browser.
   */
  start(browser) {
    this.startDate = new Date();
    Glean.thumbnails.captureQueueTime.accumulateSingleSample(
      this.startDate - this.creationDate
    );

    let fallbackTimeout = Cu.isInAutomation
      ? TESTING_CAPTURE_TIMEOUT
      : DEFAULT_CAPTURE_TIMEOUT;

    // timeout timer
    let timeout =
      typeof this.options.timeout == "number"
        ? this.options.timeout
        : fallbackTimeout;
    this._timeoutTimer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
    this._timeoutTimer.initWithCallback(
      this,
      timeout,
      Ci.nsITimer.TYPE_ONE_SHOT
    );

    this._browser = browser;

    if (!browser.browsingContext) {
      return;
    }

    this._pageLoadStartTime = new Date();

    BackgroundPageThumbs._expectingAboutBlank = false;

    let thumbnailsActor = browser.browsingContext.currentWindowGlobal.getActor(
      "BackgroundThumbnails"
    );
    thumbnailsActor
      .sendQuery("Browser:Thumbnail:LoadURL", {
        url: this.url,
      })
      .then(
        success => {
          // If it failed, then this was likely a bad url. If successful,
          // BackgroundPageThumbs.onStateChange will call _done() after the
          // load has completed.
          if (!success) {
            this._done(browser, null, TEL_CAPTURE_DONE_BAD_URI);
          }
        },
        err => {
          console.error(err);
          // The query can fail when a crash occurs while loading. The error causes
          // thumbnail crash tests to fail with an uninteresting error message.
        }
      );
  },

  readBlob: function readBlob(blob) {
    return new Promise((resolve, reject) => {
      let reader = new FileReader();
      reader.onloadend = function onloadend() {
        if (reader.readyState != FileReader.DONE) {
          reject(reader.error);
        } else {
          resolve(reader.result);
        }
      };
      reader.readAsArrayBuffer(blob);
    });
  },

  async pageLoaded(aBrowser) {
    if (this.timedOut) {
      this._done(aBrowser, null, TEL_CAPTURE_DONE_TIMEOUT);
      return;
    }

    let waitTime;
    if (this.options.settleWaitTime !== undefined) {
      waitTime = this.options.settleWaitTime;
    } else if (Cu.isInAutomation) {
      waitTime = TESTING_SETTLE_WAIT_TIME;
    } else {
      waitTime = SETTLE_WAIT_TIME;
    }

    // There was additional activity, so restart the wait timer
    if (this.redirectTimer) {
      this.redirectTimer.delay = waitTime;
      return;
    }

    // The requested page has loaded or stopped/aborted, so capture the page
    // soon but first let it settle in case of in-page redirects
    await new Promise(resolve => {
      this.redirectTimer = Cc["@mozilla.org/timer;1"].createInstance(
        Ci.nsITimer
      );
      this.redirectTimer.init(resolve, waitTime, Ci.nsITimer.TYPE_ONE_SHOT);
    });

    this.redirectTimer = null;

    let pageLoadTime = new Date() - this._pageLoadStartTime;
    let canvasDrawStartTime = new Date();

    let canvas = PageThumbs.createCanvas(aBrowser.documentGlobal, 1, 1);
    try {
      await PageThumbs.captureToCanvas(
        aBrowser,
        canvas,
        {
          isBackgroundThumb: true,
          isImage: this.options.isImage,
          backgroundColor: this.options.backgroundColor,
        },
        true
      );
    } catch (ex) {
      this._done(
        aBrowser,
        null,
        ex == "IMAGE_ZERO_DIMENSION"
          ? TEL_CAPTURE_DONE_IMAGE_ZERO_DIMENSION
          : TEL_CAPTURE_DONE_LOAD_FAILED
      );
      return;
    }

    let canvasDrawTime = new Date() - canvasDrawStartTime;

    let contentType =
      (this.options.dontStore && this.options.contentType) ||
      PageThumbs.contentType;
    let imageData = await new Promise(resolve => {
      canvas.toBlob(blob => {
        resolve(blob, contentType);
      }, contentType);
    });

    this._done(aBrowser, imageData, TEL_CAPTURE_DONE_OK, {
      pageLoadTime,
      canvasDrawTime,
    });
  },

  /**
   * The only intended external use of this method is by the service when it's
   * uninitializing and doing things like destroying the thumbnail browser.  In
   * that case the consumer's completion callback will never be called.
   */
  destroy() {
    // This method may be called for captures that haven't started yet, so
    // guard against not yet having _timeoutTimer, _msgMan etc properties...
    if (this._timeoutTimer) {
      this._timeoutTimer.cancel();
      delete this._timeoutTimer;
    }
    delete this.captureCallback;
    delete this.doneCallbacks;
    delete this.options;
  },

  // Called when the timeout timer fires.
  notify() {
    this.timedOut = true;
    this._browser.stop();
  },

  _done(browser, imageData, reason, telemetry) {
    // Note that _done will be called only once, by either receiveMessage or
    // notify, since it calls destroy here, which cancels the timeout timer and
    // removes the didCapture message listener.
    let { captureCallback, doneCallbacks, options } = this;
    this.destroy();

    if (typeof reason != "number") {
      throw new Error("A done reason must be given.");
    }

    Glean.thumbnails.captureDoneReason2.accumulateSingleSample(reason);

    if (telemetry) {
      // Telemetry is currently disabled in the content process (bug 680508).
      Glean.thumbnails.capturePageLoadTime.accumulateSingleSample(
        telemetry.pageLoadTime
      );
      Glean.thumbnails.captureCanvasDrawTime.accumulateSingleSample(
        telemetry.canvasDrawTime
      );
    }

    let done = (info = null) => {
      captureCallback(this, reason);
      for (let callback of doneCallbacks) {
        try {
          callback.call(options, this.url, reason, info);
        } catch (err) {
          console.error(err);
        }
      }

      if (Services.prefs.getBoolPref(ABOUT_NEWTAB_SEGREGATION_PREF)) {
        // Clear the data in the private container for thumbnails.
        let privateIdentity = lazy.ContextualIdentityService.getPrivateIdentity(
          "userContextIdInternal.thumbnail"
        );
        if (privateIdentity) {
          Services.clearData.deleteDataFromOriginAttributesPattern({
            userContextId: privateIdentity.userContextId,
          });
        }
      }
    };

    if (!imageData) {
      done();
      return;
    }

    this.readBlob(imageData).then(buffer => {
      if (options.dontStore) {
        done({
          data: buffer,
          originalUrl: this.url,
          finalUrl: browser.currentURI.spec,
          contentType: options.contentType || PageThumbs.contentType,
        });
      } else {
        PageThumbs._store(this.url, browser.currentURI.spec, buffer, true).then(
          done,
          done
        );
      }
    });
  },
};

Capture.nextID = 0;

function schedule(callback) {
  Services.tm.dispatchToMainThread(callback);
}
