diff -r 000000000000 -r 6474c204b198 services/sync/tests/unit/test_service_sync_locked.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/services/sync/tests/unit/test_service_sync_locked.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,36 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+Cu.import("resource://services-sync/service.js");
+Cu.import("resource://services-sync/util.js");
+
+function run_test() {
+  let debug = [];
+  let info  = [];
+
+  function augmentLogger(old) {
+    let d = old.debug;
+    let i = old.info;
+    old.debug = function(m) { debug.push(m); d.call(old, m); }
+    old.info  = function(m) { info.push(m);  i.call(old, m); }
+    return old;
+  }
+
+  Log.repository.rootLogger.addAppender(new Log.DumpAppender());
+
+  augmentLogger(Service._log);
+
+  // Avoid daily ping
+  Svc.Prefs.set("lastPing", Math.floor(Date.now() / 1000));
+
+  _("Check that sync will log appropriately if already in 'progress'.");
+  Service._locked = true;
+  Service.sync();
+  Service._locked = false;
+
+  do_check_eq(debug[debug.length - 2],
+              "Exception: Could not acquire lock. Label: \"service.js: login\". No traceback available");
+  do_check_eq(info[info.length - 1],
+              "Cannot start sync: already syncing?");
+}
+