diff -r 000000000000 -r 6474c204b198 dom/apps/src/FreeSpaceWatcher.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/apps/src/FreeSpaceWatcher.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,118 @@
+/* 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";
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/FileUtils.jsm");
+
+this.EXPORTED_SYMBOLS = ["FreeSpaceWatcher"];
+
+function debug(aMsg) {
+  //dump("-*-*- FreeSpaceWatcher.jsm : " + aMsg + "\n");
+}
+
+// Polling delay for free space, in ms.
+const DEFAULT_WATCHER_DELAY = 1000;
+
+this.FreeSpaceWatcher = {
+  timers: {},
+  id: 0,
+
+  /**
+   * This function  will call aOnStatusChange
+   * each time the free space for apps crosses aThreshold, checking
+   * every aDelay milliseconds, or every second by default.
+   * aOnStatusChange is called with either "free" or "full" and will
+   * always be called at least one to get the initial status.
+   * @param aThreshold      The amount of space in bytes to watch for.
+   * @param aOnStatusChange The function called when the state changes.
+   * @param aDelay          How often (in ms) we check free space. Defaults
+   *                        to DEFAULT_WATCHER_DELAY.
+   * @return                An opaque value to use with stop().
+   */
+  create: function spaceWatcher_create(aThreshold, aOnStatusChange, aDelay) {
+    let timer = Cc["@mozilla.org/timer;1"].createInstance(Ci.nsITimer);
+    debug("Creating new FreeSpaceWatcher");
+    let callback = {
+      currentStatus: null,
+      notify: function(aTimer) {
+        try {
+          let checkFreeSpace = function (freeBytes) {
+            debug("Free bytes: " + freeBytes);
+            let newStatus = freeBytes > aThreshold;
+            if (newStatus != callback.currentStatus) {
+              debug("New status: " + (newStatus ? "free" : "full"));
+              aOnStatusChange(newStatus ? "free" : "full");
+              callback.currentStatus = newStatus;
+            }
+          };
+
+          let navigator = Services.wm.getMostRecentWindow("navigator:browser")
+                                  .navigator;
+          let deviceStorage = null;
+
+          if (navigator.getDeviceStorage) {
+            deviceStorage = navigator.getDeviceStorage("apps");
+          }
+
+          if (deviceStorage) {
+            let req = deviceStorage.freeSpace();
+            req.onsuccess = req.onerror = function statResult(e) {
+              if (!e.target.result) {
+                return;
+              }
+
+              let freeBytes = e.target.result;
+              checkFreeSpace(freeBytes);
+            }
+          } else {
+            // deviceStorage isn't available, so use the webappsDir instead.
+            // This needs to be moved from a hardcoded string to DIRECTORY_NAME
+            // in AppsUtils. See bug 852685.
+            let dir = FileUtils.getDir("webappsDir", ["webapps"], true, true);
+            let freeBytes;
+            try {
+              freeBytes = dir.diskSpaceAvailable;
+            } catch(e) {
+              // If disk space information isn't available, we should assume
+              // that there is enough free space, and that we'll fail when
+              // we actually run out of disk space.
+              callback.currentStatus = true;
+            }
+            if (freeBytes) {
+              // We have disk space information. Call this here so that
+              // any exceptions are caught in the outer catch block.
+              checkFreeSpace(freeBytes);
+            }
+          }
+        } catch(e) {
+          // If the aOnStatusChange callback has errored we'll end up here.
+          debug(e);
+        }
+      }
+    }
+
+    timer.initWithCallback(callback, aDelay || DEFAULT_WATCHER_DELAY,
+                           Ci.nsITimer.TYPE_REPEATING_SLACK);
+    let id = "timer-" + this.id++;
+    this.timers[id] = timer;
+    return id;
+  },
+
+  /**
+   * This function stops a running watcher.
+   * @param aId The opaque timer id returned by create().
+   */
+  stop: function spaceWatcher_stop(aId) {
+    if (this.timers[aId]) {
+      this.timers[aId].cancel();
+      delete this.timers[aId];
+    }
+  }
+}