diff -r 000000000000 -r 6474c204b198 services/sync/tests/unit/test_utils_lock.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/services/sync/tests/unit/test_utils_lock.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,75 @@
+_("Make sure lock prevents calling with a shared lock");
+Cu.import("resource://services-sync/util.js");
+
+// Utility that we only use here.
+
+function do_check_begins(thing, startsWith) {
+  if (!(thing && thing.indexOf && (thing.indexOf(startsWith) == 0)))
+    do_throw(thing + " doesn't begin with " + startsWith);
+}
+
+function run_test() {
+  let ret, rightThis, didCall;
+  let state, lockState, lockedState, unlockState;
+  let obj = {
+    _lock: Utils.lock,
+    lock: function() {
+      lockState = ++state;
+      if (this._locked) {
+        lockedState = ++state;
+        return false;
+      }
+      this._locked = true;
+      return true;
+    },
+    unlock: function() {
+      unlockState = ++state;
+      this._locked = false;
+    },
+
+    func: function() this._lock("Test utils lock",
+    function() {
+      rightThis = this == obj;
+      didCall = true;
+      return 5;
+    })(),
+
+    throwy: function() this._lock("Test utils lock throwy",
+    function() {
+      rightThis = this == obj;
+      didCall = true;
+      this.throwy();
+    })()
+  };
+
+  _("Make sure a normal call will call and return");
+  rightThis = didCall = false;
+  state = 0;
+  ret = obj.func();
+  do_check_eq(ret, 5);
+  do_check_true(rightThis);
+  do_check_true(didCall);
+  do_check_eq(lockState, 1);
+  do_check_eq(unlockState, 2);
+  do_check_eq(state, 2);
+
+  _("Make sure code that calls locked code throws");
+  ret = null;
+  rightThis = didCall = false;
+  try {
+    ret = obj.throwy();
+    do_throw("throwy internal call should have thrown!");
+  }
+  catch(ex) {
+    // Should throw an Error, not a string.
+    do_check_begins(ex, "Could not acquire lock");
+  }
+  do_check_eq(ret, null);
+  do_check_true(rightThis);
+  do_check_true(didCall);
+  _("Lock should be called twice so state 3 is skipped");
+  do_check_eq(lockState, 4);
+  do_check_eq(lockedState, 5);
+  do_check_eq(unlockState, 6);
+  do_check_eq(state, 6);
+}