diff -r 000000000000 -r 6474c204b198 modules/libjar/zipwriter/test/unit/test_undochange.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/modules/libjar/zipwriter/test/unit/test_undochange.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,42 @@
+/* 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/.
+ */
+
+// Values taken from using zipinfo to list the test.zip contents
+var TESTS = [
+  "test.txt",
+  "test.png"
+];
+
+function run_test()
+{
+  zipW.open(tmpFile, PR_RDWR | PR_CREATE_FILE | PR_TRUNCATE);
+
+  for (var i = 0; i < TESTS.length; i++) {
+    var source = do_get_file(DATA_DIR + TESTS[i]);
+    zipW.addEntryFile(TESTS[i], Ci.nsIZipWriter.COMPRESSION_NONE, source,
+                      false);
+  }
+
+  try {
+    var source = do_get_file(DATA_DIR + TESTS[0]);
+    zipW.addEntryFile(TESTS[0], Ci.nsIZipWriter.COMPRESSION_NONE, source,
+                      false);
+    do_throw("Should not be able to add the same file twice");
+  }
+  catch (e) {
+    do_check_eq(e.result, Components.results.NS_ERROR_FILE_ALREADY_EXISTS);
+  }
+
+  // Remove all the tests and see if we are left with an empty zip
+  for (var i = 0; i < TESTS.length; i++) {
+    zipW.removeEntry(TESTS[i], false);
+  }
+
+  zipW.close();
+
+  // Empty zip file should just be the end of central directory marker
+  var newTmpFile = tmpFile.clone();
+  do_check_eq(newTmpFile.fileSize, ZIP_EOCDR_HEADER_SIZE);
+}