diff -r 000000000000 -r 6474c204b198 browser/components/sessionstore/test/browser_833286_atomic_backup.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/browser/components/sessionstore/test/browser_833286_atomic_backup.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,99 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// This tests are for a sessionstore.js atomic backup.
+// Each test will wait for a write to the Session Store
+// before executing.
+
+let tmp = {};
+Cu.import("resource://gre/modules/osfile.jsm", tmp);
+Cu.import("resource:///modules/sessionstore/SessionFile.jsm", tmp);
+
+const {OS, SessionFile} = tmp;
+
+const PREF_SS_INTERVAL = "browser.sessionstore.interval";
+// Full paths for sessionstore.js and sessionstore.bak.
+const path = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js");
+const backupPath = OS.Path.join(OS.Constants.Path.profileDir,
+  "sessionstore.bak");
+
+// A text decoder.
+let gDecoder = new TextDecoder();
+// Global variables that contain sessionstore.js and sessionstore.bak data for
+// comparison between tests.
+let gSSData;
+let gSSBakData;
+
+
+
+add_task(function* testAfterFirstWrite() {
+  // Ensure sessionstore.bak is not created. We start with a clean
+  // profile so there was nothing to move to sessionstore.bak before
+  // initially writing sessionstore.js
+  let ssExists = yield OS.File.exists(path);
+  let ssBackupExists = yield OS.File.exists(backupPath);
+  ok(ssExists, "sessionstore.js should exist.");
+  ok(!ssBackupExists, "sessionstore.bak should not have been created, yet");
+
+  // Save sessionstore.js data to compare to the sessionstore.bak data in the
+  // next test.
+  let array = yield OS.File.read(path);
+  gSSData = gDecoder.decode(array);
+
+  // Manually move to the backup since the first write has already happened
+  // and a backup would not be triggered again.
+  yield OS.File.move(path, backupPath);
+
+  yield forceSaveState();
+});
+
+add_task(function* testReadBackup() {
+  // Ensure sessionstore.bak is finally created.
+  let ssExists = yield OS.File.exists(path);
+  let ssBackupExists = yield OS.File.exists(backupPath);
+  ok(ssExists, "sessionstore.js exists.");
+  ok(ssBackupExists, "sessionstore.bak should now be created.");
+
+  // Read sessionstore.bak data.
+  let array = yield OS.File.read(backupPath);
+  gSSBakData = gDecoder.decode(array);
+
+  // Make sure that the sessionstore.bak is identical to the last
+  // sessionstore.js.
+  is(gSSBakData, gSSData, "sessionstore.js is backed up correctly.");
+
+  // Read latest sessionstore.js.
+  array = yield OS.File.read(path);
+  gSSData = gDecoder.decode(array);
+
+  // Read sessionstore.js with SessionFile.read.
+  let ssDataRead = yield SessionFile.read();
+  is(ssDataRead, gSSData, "SessionFile.read read sessionstore.js correctly.");
+
+  // Remove sessionstore.js to test fallback onto sessionstore.bak.
+  yield OS.File.remove(path);
+  ssExists = yield OS.File.exists(path);
+  ok(!ssExists, "sessionstore.js should be removed now.");
+
+  // Read sessionstore.bak with SessionFile.read.
+  ssDataRead = yield SessionFile.read();
+  is(ssDataRead, gSSBakData,
+    "SessionFile.read read sessionstore.bak correctly.");
+
+  yield forceSaveState();
+});
+
+add_task(function* testBackupUnchanged() {
+  // Ensure sessionstore.bak is backed up only once.
+
+  // Read sessionstore.bak data.
+  let array = yield OS.File.read(backupPath);
+  let ssBakData = gDecoder.decode(array);
+  // Ensure the sessionstore.bak did not change.
+  is(ssBakData, gSSBakData, "sessionstore.bak is unchanged.");
+});
+
+add_task(function* cleanup() {
+  // Cleaning up after the test: removing the sessionstore.bak file.
+  yield OS.File.remove(backupPath);
+});