YAML to Mux: unwind the handle_control_path_using utility function

The function where this utility method lives is really big and
complex.  Let's break it down for the sake of code readability,
while adding better docstrings.

I've timed the performance changes caused by this, and on most
trials they presented similar performance.

This also adds new tests for this specific unit of code.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

optional_plugins/varianter_yaml_to_mux/avocado_varianter_yaml_to_mux/__init__.py

@@ -125,22 +125,33 @@ def _handle_control_tag(path, cls_node, node, value):
             node.filters[1].append(new_value)
 
 
+def _handle_control_tag_using(path, name, using, value):
+    """
+    Handling of the "!using" YAML control tag
+
+    :param path: path on the YAML
+    :type path: str
+    :param name: name to be applied in the "!using" tag
+    :type name: str
+    :param using: wether using is already being used
+    :type using: bool
+    :param value: the value of the node
+    """
+    if using:
+        raise ValueError("!using can be used only once per "
+                         "node! (%s:%s)" % (path, name))
+    using = value
+    if using[0] == '/':
+        using = using[1:]
+    if using[-1] == '/':
+        using = using[:-1]
+    return using
+
+
 def _create_from_yaml(path, cls_node=mux.MuxTreeNode):
     """Create tree structure from yaml stream"""
     def tree_node_from_values(name, values):
         """Create `name` node and add values"""
-        def handle_control_tag_using(name, using, value):
-            """Handling of the !using tag"""
-            if using:
-                raise ValueError("!using can be used only once per "
-                                 "node! (%s:%s)" % (path, name))
-            using = value
-            if using[0] == '/':
-                using = using[1:]
-            if using[-1] == '/':
-                using = using[:-1]
-            return using
-
         def node_content_from_node(node, values, using):
             """Processes node values into the current node content"""
             for value in values:
@@ -148,7 +159,7 @@ def _create_from_yaml(path, cls_node=mux.MuxTreeNode):
                     node.add_child(value)
                 elif isinstance(value[0], mux.Control):
                     if value[0].code == YAML_USING:
-                        using = handle_control_tag_using(name, using, value[1])
+                        using = _handle_control_tag_using(path, name, using, value[1])
                     else:
                         _handle_control_tag(path, cls_node, node, value)
                 elif isinstance(value[1], collections.OrderedDict):
@@ -163,7 +174,7 @@ def _create_from_yaml(path, cls_node=mux.MuxTreeNode):
             for key, value in iteritems(values):
                 if isinstance(key, mux.Control):
                     if key.code == YAML_USING:
-                        using = handle_control_tag_using(name, using, value)
+                        using = _handle_control_tag_using(path, name, using, value)
                     else:
                         _handle_control_tag(path, cls_node, node, [key, value])
                 elif (isinstance(value, collections.OrderedDict) or

optional_plugins/varianter_yaml_to_mux/tests/test_mux.py

@@ -560,6 +560,18 @@ class TestCreateFromYaml(unittest.TestCase):
         self.assertEqual(control.value, to_be_removed)
         self.assertIn(control, node.ctrl)
 
+    def test_handle_control_tag_using_multiple(self):
+        with self.assertRaises(ValueError):
+            yaml_to_mux._handle_control_tag_using('original_fake_file.yaml',
+                                                  'name', True, 'using')
+
+    def test_handle_control_tag_using(self):
+        using = yaml_to_mux._handle_control_tag_using('fake_path',
+                                                      'name',
+                                                      False,
+                                                      '/using/path/')
+        self.assertEqual(using, 'using/path')
+
 
 class TestFingerprint(unittest.TestCase):