diff -r 000000000000 -r 6474c204b198 dom/apps/src/AppDownloadManager.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/dom/apps/src/AppDownloadManager.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,104 @@
+/* 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/XPCOMUtils.jsm");
+
+XPCOMUtils.defineLazyModuleGetter(this, "FreeSpaceWatcher",
+                                  "resource://gre/modules/FreeSpaceWatcher.jsm");
+
+this.EXPORTED_SYMBOLS = ["AppDownloadManager"];
+
+function debug(aMsg) {
+  //dump("-*-*- AppDownloadManager.jsm : " + aMsg + "\n");
+}
+
+this.AppDownloadManager = {
+  // Minimum disk free space we want to keep, in bytes.
+  // Keep synchronized with Webapps.jsm
+  MIN_REMAINING_FREESPACE: 5 * 1024 * 1024,
+
+  downloads: {},
+  count: 0,
+  cancelFunc: null,
+  timer: null,
+
+  /**
+   * Registers the function called when we need to cancel a download.
+   * The function will be called with a single parameter being the
+   * manifest URL.
+   */
+  registerCancelFunction: function app_dlMgr_registerCancel(aFunction) {
+    this.cancelFunc = aFunction;
+  },
+
+  /**
+   * Adds a download to the list of current downloads.
+   * @param aManifestURL The manifest URL for the application being downloaded.
+   * @param aDownload    An opaque object representing the download.
+   */
+  add: function app_dlMgr_add(aManifestURL, aDownload) {
+    debug("Adding " + aManifestURL);
+    if (!(aManifestURL in this.downloads)) {
+      this.count++;
+      if (this.count == 1) {
+        this.timer = FreeSpaceWatcher.create(this.MIN_REMAINING_FREESPACE,
+                                             this._spaceWatcher.bind(this));
+      }
+    }
+    this.downloads[aManifestURL] = aDownload;
+  },
+
+  /**
+   * Retrieves a download from the list of current downloads.
+   * @param  aManifestURL The manifest URL for the application being retrieved.
+   * @return              The opaque object representing the download.
+   */
+  get: function app_dlMgr_get(aManifestURL) {
+    debug("Getting " + aManifestURL);
+    if (!this.downloads[aManifestURL]) {
+      return null;
+    }
+    return this.downloads[aManifestURL];
+  },
+
+  /**
+   * Removes a download of the list of current downloads.
+   * @param aManifestURL The manifest URL for the application being removed.
+   */
+  remove: function app_dlMgr_remove(aManifestURL) {
+    debug("Removing " + aManifestURL);
+    if (aManifestURL in this.downloads) {
+      this.count--;
+      delete this.downloads[aManifestURL];
+      if (this.count == 0) {
+        FreeSpaceWatcher.stop(this.timer);
+      }
+    }
+  },
+
+  /**
+   * Callback for the free space watcher. This will call cancel on downloads
+   * if needed.
+   */
+  _spaceWatcher: function app_dlMgr_watcher(aStatus) {
+    debug("Disk space is now " + aStatus);
+    if (aStatus == "free") {
+      // Nothing to do.
+      return;
+    }
+
+    // We cancel all downloads, because we don't know which ones we could
+    // keep running. We can improve that later if we have better heuristics,
+    // or when we'll support pause & resume we should just pause downloads.
+    for (let url in this.downloads) {
+      this.cancelFunc(url, "INSUFFICIENT_STORAGE");
+    }
+  }
+}