diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/src/SessionMigration.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/src/SessionMigration.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,119 @@
+/* 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.EXPORTED_SYMBOLS = ["SessionMigration"];
+
+const Cu = Components.utils;
+Cu.import("resource://gre/modules/XPCOMUtils.jsm", this);
+Cu.import("resource://gre/modules/Task.jsm", this);
+Cu.import("resource://gre/modules/osfile.jsm", this);
+
+// An encoder to UTF-8.
+XPCOMUtils.defineLazyGetter(this, "gEncoder", function () {
+  return new TextEncoder();
+});
+
+// A decoder.
+XPCOMUtils.defineLazyGetter(this, "gDecoder", function () {
+  return new TextDecoder();
+});
+
+let SessionMigrationInternal = {
+  /**
+   * Convert the original session restore state into a minimal state. It will
+   * only contain:
+   * - open windows
+   *   - with tabs
+   *     - with history entries with only title, url
+   *     - with pinned state
+   *     - with tab group info (hidden + group id)
+   *     - with selected tab info
+   *   - with selected window info
+   *   - with tabgroups info
+   *
+   * The complete state is then wrapped into the "about:welcomeback" page as
+   * form field info to be restored when restoring the state.
+   */
+  convertState: function(aStateObj) {
+    let state = {
+      selectedWindow: aStateObj.selectedWindow,
+      _closedWindows: []
+    };
+    state.windows = aStateObj.windows.map(function(oldWin) {
+      var win = {extData: {}};
+      win.tabs = oldWin.tabs.map(function(oldTab) {
+        var tab = {};
+        // Keep only titles and urls for history entries
+        tab.entries = oldTab.entries.map(function(entry) {
+          return {url: entry.url, title: entry.title};
+        });
+        tab.index = oldTab.index;
+        tab.hidden = oldTab.hidden;
+        tab.pinned = oldTab.pinned;
+        // The tabgroup info is in the extData, so we need to get it out.
+        if (oldTab.extData && "tabview-tab" in oldTab.extData) {
+          tab.extData = {"tabview-tab": oldTab.extData["tabview-tab"]};
+        }
+        return tab;
+      });
+      // There are various tabgroup-related attributes that we need to get out
+      // of the session restore data for the window, too.
+      if (oldWin.extData) {
+        for (let k of Object.keys(oldWin.extData)) {
+          if (k.startsWith("tabview-")) {
+            win.extData[k] = oldWin.extData[k];
+          }
+        }
+      }
+      win.selected = oldWin.selected;
+      win._closedTabs = [];
+      return win;
+    });
+    let wrappedState = {
+      url: "about:welcomeback",
+      formdata: {
+        id: {"sessionData": state},
+        xpath: {}
+      }
+    };
+    return {windows: [{tabs: [{entries: [wrappedState]}]}]};
+  },
+  /**
+   * Asynchronously read session restore state (JSON) from a path
+   */
+  readState: function(aPath) {
+    return Task.spawn(function() {
+      let bytes = yield OS.File.read(aPath);
+      let text = gDecoder.decode(bytes);
+      let state = JSON.parse(text);
+      throw new Task.Result(state);
+    });
+  },
+  /**
+   * Asynchronously write session restore state as JSON to a path
+   */
+  writeState: function(aPath, aState) {
+    let bytes = gEncoder.encode(JSON.stringify(aState));
+    return OS.File.writeAtomic(aPath, bytes, {tmpPath: aPath + ".tmp"});
+  }
+}
+
+let SessionMigration = {
+  /**
+   * Migrate a limited set of session data from one path to another.
+   */
+  migrate: function(aFromPath, aToPath) {
+    return Task.spawn(function() {
+      let inState = yield SessionMigrationInternal.readState(aFromPath);
+      let outState = SessionMigrationInternal.convertState(inState);
+      // Unfortunately, we can't use SessionStore's own SessionFile to
+      // write out the data because it has a dependency on the profile dir
+      // being known. When the migration runs, there is no guarantee that
+      // that's true.
+      yield SessionMigrationInternal.writeState(aToPath, outState);
+    });
+  }
+};