diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/src/SessionWorker.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/src/SessionWorker.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,329 @@
+/* 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/. */
+
+/**
+ * A worker dedicated to handle I/O for Session Store.
+ */
+
+"use strict";
+
+importScripts("resource://gre/modules/osfile.jsm");
+
+let File = OS.File;
+let Encoder = new TextEncoder();
+let Decoder = new TextDecoder();
+
+/**
+ * Communications with the controller.
+ *
+ * Accepts messages:
+ * {fun:function_name, args:array_of_arguments_or_null, id: custom_id}
+ *
+ * Sends messages:
+ * {ok: result, id: custom_id, telemetry: {}} /
+ * {fail: serialized_form_of_OS.File.Error, id: custom_id}
+ */
+self.onmessage = function (msg) {
+  let data = msg.data;
+  if (!(data.fun in Agent)) {
+    throw new Error("Cannot find method " + data.fun);
+  }
+
+  let result;
+  let id = data.id;
+
+  try {
+    result = Agent[data.fun].apply(Agent, data.args) || {};
+  } catch (ex if ex instanceof OS.File.Error) {
+    // Instances of OS.File.Error know how to serialize themselves
+    // (deserialization ensures that we end up with OS-specific
+    // instances of |OS.File.Error|)
+    self.postMessage({fail: OS.File.Error.toMsg(ex), id: id});
+    return;
+  }
+
+  // Other exceptions do not, and should be propagated through DOM's
+  // built-in mechanism for uncaught errors, although this mechanism
+  // may lose interesting information.
+  self.postMessage({
+    ok: result.result,
+    id: id,
+    telemetry: result.telemetry || {}
+  });
+};
+
+let Agent = {
+  // Boolean that tells whether we already made a
+  // call to write(). We will only attempt to move
+  // sessionstore.js to sessionstore.bak on the
+  // first write.
+  hasWrittenState: false,
+
+  // The path to sessionstore.js
+  path: OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js"),
+
+  // The path to sessionstore.bak
+  backupPath: OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.bak"),
+
+  /**
+   * NO-OP to start the worker.
+   */
+  init: function () {
+    return {result: true};
+  },
+
+  /**
+   * Write the session to disk.
+   */
+  write: function (stateString) {
+    let exn;
+    let telemetry = {};
+
+    if (!this.hasWrittenState) {
+      try {
+        let startMs = Date.now();
+        File.move(this.path, this.backupPath);
+        telemetry.FX_SESSION_RESTORE_BACKUP_FILE_MS = Date.now() - startMs;
+      } catch (ex if isNoSuchFileEx(ex)) {
+        // Ignore exceptions about non-existent files.
+      } catch (ex) {
+        // Throw the exception after we wrote the state to disk
+        // so that the backup can't interfere with the actual write.
+        exn = ex;
+      }
+
+      this.hasWrittenState = true;
+    }
+
+    let ret = this._write(stateString, telemetry);
+
+    if (exn) {
+      throw exn;
+    }
+
+    return ret;
+  },
+
+  /**
+   * Extract all sorts of useful statistics from a state string,
+   * for use with Telemetry.
+   *
+   * @return {object}
+   */
+  gatherTelemetry: function (stateString) {
+    return Statistics.collect(stateString);
+  },
+
+  /**
+   * Write a stateString to disk
+   */
+  _write: function (stateString, telemetry = {}) {
+    let bytes = Encoder.encode(stateString);
+    let startMs = Date.now();
+    let result = File.writeAtomic(this.path, bytes, {tmpPath: this.path + ".tmp"});
+    telemetry.FX_SESSION_RESTORE_WRITE_FILE_MS = Date.now() - startMs;
+    telemetry.FX_SESSION_RESTORE_FILE_SIZE_BYTES = bytes.byteLength;
+    return {result: result, telemetry: telemetry};
+  },
+
+  /**
+   * Creates a copy of sessionstore.js.
+   */
+  createBackupCopy: function (ext) {
+    try {
+      return {result: File.copy(this.path, this.backupPath + ext)};
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+      return {result: true};
+    }
+  },
+
+  /**
+   * Removes a backup copy.
+   */
+  removeBackupCopy: function (ext) {
+    try {
+      return {result: File.remove(this.backupPath + ext)};
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+      return {result: true};
+    }
+  },
+
+  /**
+   * Wipes all files holding session data from disk.
+   */
+  wipe: function () {
+    let exn;
+
+    // Erase session state file
+    try {
+      File.remove(this.path);
+    } catch (ex if isNoSuchFileEx(ex)) {
+      // Ignore exceptions about non-existent files.
+    } catch (ex) {
+      // Don't stop immediately.
+      exn = ex;
+    }
+
+    // Erase any backup, any file named "sessionstore.bak[-buildID]".
+    let iter = new File.DirectoryIterator(OS.Constants.Path.profileDir);
+    for (let entry in iter) {
+      if (!entry.isDir && entry.path.startsWith(this.backupPath)) {
+        try {
+          File.remove(entry.path);
+        } catch (ex) {
+          // Don't stop immediately.
+          exn = exn || ex;
+        }
+      }
+    }
+
+    if (exn) {
+      throw exn;
+    }
+
+    return {result: true};
+  }
+};
+
+function isNoSuchFileEx(aReason) {
+  return aReason instanceof OS.File.Error && aReason.becauseNoSuchFile;
+}
+
+/**
+ * Estimate the number of bytes that a data structure will use on disk
+ * once serialized.
+ */
+function getByteLength(str) {
+  return Encoder.encode(JSON.stringify(str)).byteLength;
+}
+
+/**
+ * Tools for gathering statistics on a state string.
+ */
+let Statistics = {
+  collect: function(stateString) {
+    let start = Date.now();
+    let TOTAL_PREFIX = "FX_SESSION_RESTORE_TOTAL_";
+    let INDIVIDUAL_PREFIX = "FX_SESSION_RESTORE_INDIVIDUAL_";
+    let SIZE_SUFFIX = "_SIZE_BYTES";
+
+    let state = JSON.parse(stateString);
+
+    // Gather all data
+    let subsets = {};
+    this.gatherSimpleData(state, subsets);
+    this.gatherComplexData(state, subsets);
+
+    // Extract telemetry
+    let telemetry = {};
+    for (let k of Object.keys(subsets)) {
+      let obj = subsets[k];
+      telemetry[TOTAL_PREFIX + k + SIZE_SUFFIX] = getByteLength(obj);
+
+      if (Array.isArray(obj)) {
+        let size = obj.map(getByteLength);
+        telemetry[INDIVIDUAL_PREFIX + k + SIZE_SUFFIX] = size;
+      }
+    }
+
+    let stop = Date.now();
+    telemetry["FX_SESSION_RESTORE_EXTRACTING_STATISTICS_DURATION_MS"] = stop - start;
+    return {
+      telemetry: telemetry
+    };
+  },
+
+  /**
+   * Collect data that doesn't require a recursive walk through the
+   * data structure.
+   */
+  gatherSimpleData: function(state, subsets) {
+    // The subset of sessionstore.js dealing with open windows
+    subsets.OPEN_WINDOWS = state.windows;
+
+    // The subset of sessionstore.js dealing with closed windows
+    subsets.CLOSED_WINDOWS = state._closedWindows;
+
+    // The subset of sessionstore.js dealing with closed tabs
+    // in open windows
+    subsets.CLOSED_TABS_IN_OPEN_WINDOWS = [];
+
+    // The subset of sessionstore.js dealing with cookies
+    // in both open and closed windows
+    subsets.COOKIES = [];
+
+    for (let winData of state.windows) {
+      let closedTabs = winData._closedTabs || [];
+      subsets.CLOSED_TABS_IN_OPEN_WINDOWS.push(...closedTabs);
+
+      let cookies = winData.cookies || [];
+      subsets.COOKIES.push(...cookies);
+    }
+
+    for (let winData of state._closedWindows) {
+      let cookies = winData.cookies || [];
+      subsets.COOKIES.push(...cookies);
+    }
+  },
+
+  /**
+   * Walk through a data structure, recursively.
+   *
+   * @param {object} root The object from which to start walking.
+   * @param {function(key, value)} cb Callback, called for each
+   * item except the root. Returns |true| to walk the subtree rooted
+   * at |value|, |false| otherwise   */
+  walk: function(root, cb) {
+    if (!root || typeof root !== "object") {
+      return;
+    }
+    for (let k of Object.keys(root)) {
+      let obj = root[k];
+      let stepIn = cb(k, obj);
+      if (stepIn) {
+        this.walk(obj, cb);
+      }
+    }
+  },
+
+  /**
+   * Collect data that requires walking through the data structure
+   */
+  gatherComplexData: function(state, subsets) {
+    // The subset of sessionstore.js dealing with DOM storage
+    subsets.DOM_STORAGE = [];
+    // The subset of sessionstore.js storing form data
+    subsets.FORMDATA = [];
+    // The subset of sessionstore.js storing history
+    subsets.HISTORY = [];
+
+
+    this.walk(state, function(k, value) {
+      let dest;
+      switch (k) {
+        case "entries":
+          subsets.HISTORY.push(value);
+          return true;
+        case "storage":
+          subsets.DOM_STORAGE.push(value);
+          // Never visit storage, it's full of weird stuff
+          return false;
+        case "formdata":
+          subsets.FORMDATA.push(value);
+          // Never visit formdata, it's full of weird stuff
+          return false;
+        case "cookies": // Don't visit these places, they are full of weird stuff
+        case "extData":
+          return false;
+        default:
+          return true;
+      }
+    });
+
+    return subsets;
+  },
+
+};