diff -r 000000000000 -r 6474c204b198 toolkit/components/telemetry/tests/unit/test_TelemetryLockCount.js
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/telemetry/tests/unit/test_TelemetryLockCount.js	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,60 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ 
+*/
+/* A testcase to make sure reading the failed profile lock count works.  */
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+const Cu = Components.utils;
+const Cr = Components.results;
+
+Cu.import("resource://gre/modules/Services.jsm", this);
+
+const Telemetry = Cc["@mozilla.org/base/telemetry;1"].getService(Ci.nsITelemetry);
+
+const LOCK_FILE_NAME = "Telemetry.FailedProfileLocks.txt";
+const N_FAILED_LOCKS = 10;
+
+// Constants from prio.h for nsIFileOutputStream.init
+const PR_WRONLY = 0x2;
+const PR_CREATE_FILE = 0x8;
+const PR_TRUNCATE = 0x20;
+const RW_OWNER = 0600;
+
+function write_string_to_file(file, contents) {
+  let ostream = Cc["@mozilla.org/network/safe-file-output-stream;1"]
+                .createInstance(Ci.nsIFileOutputStream);
+  ostream.init(file, PR_WRONLY | PR_CREATE_FILE | PR_TRUNCATE,
+	       RW_OWNER, ostream.DEFER_OPEN);
+  ostream.write(contents, contents.length);
+  ostream.QueryInterface(Ci.nsISafeOutputStream).finish();
+  ostream.close();
+}
+
+function construct_file() {
+  let profileDirectory = Services.dirsvc.get("ProfD", Ci.nsIFile);
+  let file = profileDirectory.clone();
+  file.append(LOCK_FILE_NAME);
+  return file;
+}
+
+function run_test() {
+  do_get_profile();
+
+  do_check_eq(Telemetry.failedProfileLockCount, 0);
+
+  write_string_to_file(construct_file(), N_FAILED_LOCKS.toString());
+
+  // Make sure that we're not eagerly reading the count now that the
+  // file exists.
+  do_check_eq(Telemetry.failedProfileLockCount, 0);
+
+  do_test_pending();
+  Telemetry.asyncFetchTelemetryData(actual_test);
+}
+
+function actual_test() {
+  do_check_eq(Telemetry.failedProfileLockCount, N_FAILED_LOCKS);
+  do_check_false(construct_file().exists());
+  do_test_finished();
+}