avocado.plugins.yaml_to_mux: Allow empty yaml files

This patch avoids avocado crash on empty multiplex yaml file. Empty file
contains basically nothing and that is how it's handled now.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/plugins/yaml_to_mux.py

@@ -170,6 +170,8 @@ def _create_from_yaml(path, cls_node=tree.TreeNode):
     # Load the tree
     with open(path) as stream:
         loaded_tree = yaml.load(stream, Loader)
+        if loaded_tree is None:
+            return
         loaded_tree = tree_node_from_values('', loaded_tree)
 
     # Add prefix
@@ -192,12 +194,16 @@ def create_from_yaml(paths, debug=False):
     """
     def _merge(data, path):
         """ Normal run """
-        data.merge(_create_from_yaml(path))
+        tmp = _create_from_yaml(path)
+        if tmp:
+            data.merge(tmp)
 
     def _merge_debug(data, path):
         """ Use NamedTreeNodeDebug magic """
         node_cls = tree.get_named_tree_cls(path)
-        data.merge(_create_from_yaml(path, node_cls))
+        tmp = _create_from_yaml(path, node_cls)
+        if tmp:
+            data.merge(tmp)
 
     if not debug:
         data = tree.TreeNode()

selftests/functional/test_multiplex.py

@@ -34,12 +34,17 @@ class MultiplexTests(unittest.TestCase):
     def setUp(self):
         self.tmpdir = tempfile.mkdtemp(prefix='avocado_' + __name__)
 
-    def run_and_check(self, cmd_line, expected_rc):
+    def run_and_check(self, cmd_line, expected_rc, tests=None):
         os.chdir(basedir)
         result = process.run(cmd_line, ignore_status=True)
         self.assertEqual(result.exit_status, expected_rc,
                          "Command %s did not return rc "
                          "%d:\n%s" % (cmd_line, expected_rc, result))
+        if tests:
+            exp = ("PASS %s | ERROR 0 | FAIL %s | SKIP 0 | WARN 0 | "
+                   "INTERRUPT 0" % tests)
+            self.assertIn(exp, result.stdout, "%s not in stdout:\n%s"
+                          % (exp, result))
         return result
 
     def test_mplex_plugin(self):
@@ -101,6 +106,12 @@ class MultiplexTests(unittest.TestCase):
         expected_rc = exit_codes.AVOCADO_ALL_OK
         self.run_and_check(cmd_line, expected_rc)
 
+    def test_empty_file(self):
+        cmd_line = ("./scripts/avocado run -m selftests/.data/empty_file "
+                    "-- passtest.py")
+        result = self.run_and_check(cmd_line, exit_codes.AVOCADO_ALL_OK,
+                                    (1, 0))
+
     def test_run_mplex_params(self):
         for variant_msg in (('/run/short', 'A'),
                             ('/run/medium', 'ASDFASDF'),