diff -r 000000000000 -r 6474c204b198 services/sync/tests/unit/test_utils_makeGUID.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/services/sync/tests/unit/test_utils_makeGUID.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,40 @@
+Cu.import("resource://services-sync/util.js");
+
+const base64url =
+  "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-_";
+
+function run_test() {
+  _("Make sure makeGUID makes guids of the right length/characters");
+  _("Create a bunch of guids to make sure they don't conflict");
+  let guids = [];
+  for (let i = 0; i < 1000; i++) {
+    let newGuid = Utils.makeGUID();
+    _("Generated " + newGuid);
+
+    // Verify that the GUID's length is correct, even when it's URL encoded.
+    do_check_eq(newGuid.length, 12);
+    do_check_eq(encodeURIComponent(newGuid).length, 12);
+
+    // Verify that the GUID only contains base64url characters
+    do_check_true(Array.every(newGuid, function(chr) {
+      return base64url.indexOf(chr) != -1;
+    }));
+
+    // Verify that Utils.checkGUID() correctly identifies them as valid.
+    do_check_true(Utils.checkGUID(newGuid));
+
+    // Verify uniqueness within our sample of 1000. This could cause random
+    // failures, but they should be extremely rare. Otherwise we'd have a
+    // problem with GUID collisions.
+    do_check_true(guids.every(function(g) { return g != newGuid; }));
+    guids.push(newGuid);
+  }
+
+  _("Make sure checkGUID fails for invalid GUIDs");
+  do_check_false(Utils.checkGUID(undefined));
+  do_check_false(Utils.checkGUID(null));
+  do_check_false(Utils.checkGUID(""));
+  do_check_false(Utils.checkGUID("blergh"));
+  do_check_false(Utils.checkGUID("ThisGUIDisWayTooLong"));
+  do_check_false(Utils.checkGUID("Invalid!!!!!"));
+}