Add custom yaml class

- **duplicate key detection**
  yaml.load('{"a": 1, "a": 2}') == {'a': 2} error should be
  raised in case of duplicate keys.

- **load mappings as OrderedDict**. this allows to run
  workloads in the same order as they are in task file.

Change-Id: Iea05cd84e55c843e461c92c493ebb1342647ed3b

rally/common/yamlutils.py

@@ -0,0 +1,52 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+import collections
+
+import yaml
+from yaml import constructor
+from yaml import loader
+from yaml import nodes
+from yaml import parser
+from yaml import resolver
+
+ParserError = parser.ParserError
+
+
+def _construct_mapping(loader, node, deep=False):
+    keys = []
+    if isinstance(node, nodes.MappingNode):
+        for key_node, value_node in node.value:
+            key = loader.construct_object(key_node, deep=deep)
+            if key in keys:
+                raise constructor.ConstructorError(
+                    "while constructing a mapping",
+                    node.start_mark,
+                    "the key (%s) is redefined" % key,
+                    key_node.start_mark)
+            keys.append(key)
+    return collections.OrderedDict(loader.construct_pairs(node))
+
+
+class _SafeLoader(loader.SafeLoader):
+    pass
+
+
+def safe_load(stream):
+    """Load stream to create python object
+
+    :param stream: json/yaml stream.
+    :returns: dict object
+    """
+    _SafeLoader.add_constructor(resolver.BaseResolver.DEFAULT_MAPPING_TAG,
+                                _construct_mapping)
+    return yaml.load(stream, _SafeLoader)

tests/unit/common/test_yamlutils.py

@@ -0,0 +1,40 @@
+#    Licensed under the Apache License, Version 2.0 (the "License"); you may
+#    not use this file except in compliance with the License. You may obtain
+#    a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0
+#
+#    Unless required by applicable law or agreed to in writing, software
+#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+#    License for the specific language governing permissions and limitations
+#    under the License.
+
+from yaml import constructor
+
+from rally.common import yamlutils
+from tests.unit import test
+
+
+class YamlTestcase(test.TestCase):
+    """Test yaml loading method."""
+
+    def setUp(self):
+        super(YamlTestcase, self).setUp()
+
+    def test_safe_load(self):
+        stream = "{'a': 1, 'b': {'a': 2}}"
+        stream_obj = yamlutils.safe_load(stream)
+        self.assertEqual({"a": 1, "b": {"a": 2}},
+                         stream_obj)
+
+    def test_safe_load_duplicate_key(self):
+        stream = "{'a': 1, 'a': 2}"
+        self.assertRaises(constructor.ConstructorError,
+                          yamlutils.safe_load, stream)
+
+    def test_safe_load_order_key(self):
+        stream = "{'b': 1, 'a': 1, 'c': 1}"
+        stream_obj = yamlutils.safe_load(stream)
+        self.assertEqual({"a": 1, "b": 1, "c": 1}, stream_obj)
+        self.assertEqual(["b", "a", "c"], list(stream_obj))