Fix hangup on no-op sync

Fix #395

tests/cli/test_sync.py

@@ -408,3 +408,12 @@ def test_unknown_storage(tmpdir, runner, existing, missing):
         "These are the configured storages: ['{existing}']"
         .format(missing=missing, existing=existing)
     ) in result.output
+
+
+@pytest.mark.parametrize('cmd', ['sync', 'metasync'])
+def test_no_configured_pairs(tmpdir, runner, cmd):
+    runner.write_with_general('')
+
+    result = runner.invoke([cmd])
+    assert result.output == 'critical: Nothing to do.\n'
+    assert result.exception.code == 5

vdirsyncer/cli/utils.py

@@ -459,8 +459,17 @@ class WorkerQueue(object):
         assert self._workers or not self._queue.unfinished_tasks
         ui_worker = click_threading.UiWorker()
         self._shutdown_handlers.append(ui_worker.shutdown)
+        _echo = click.echo
+
         with ui_worker.patch_click():
             yield
+
+            if not self._workers:
+                # Ugly hack, needed because ui_worker is not running.
+                click.echo = _echo
+                cli_logger.critical('Nothing to do.')
+                sys.exit(5)
+
             ui_worker.run()
             self._queue.join()
             for worker in self._workers: