hacking check for contextlib.nested for py34 support

Removed use of contextlib.nested call from codebase as
contextlib.nested is not compatible with Python 3.

Added hacking check to catch if any new instances are added
to the codebase.

Change-Id: Ib78102bc013e4cc91ba54d79aa2815f4cf9f446d

HACKING.rst

@@ -51,6 +51,7 @@ Nova Specific Commandments
   assertIn/NotIn(A, B)
 - [N339] Check common raise_feature_not_supported() is used for v2.1 HTTPNotImplemented response.
 - [N340] Check nova.utils.spawn() is used instead of greenthread.spawn() and eventlet.spawn()
+- [N341] contextlib.nested is deprecated
 
 Creating Unit Tests
 -------------------

nova/hacking/checks.py

@@ -98,6 +98,7 @@ decorator_re = re.compile(r"@.*")
 http_not_implemented_re = re.compile(r"raise .*HTTPNotImplemented\(")
 spawn_re = re.compile(
     r".*(eventlet|greenthread)\.(?P<spawn_part>spawn(_n)?)\(.*\)")
+contextlib_nested = re.compile(r"^with (contextlib\.)?nested\(")
 
 
 class BaseASTChecker(ast.NodeVisitor):
@@ -539,6 +540,16 @@ def check_greenthread_spawns(logical_line, physical_line, filename):
         yield (0, msg % {'spawn': match.group('spawn_part')})
 
 
+def check_no_contextlib_nested(logical_line, filename):
+    msg = ("N341: contextlib.nested is deprecated. With Python 2.7 and later "
+           "the with-statement supports multiple nested objects. See https://"
+           "docs.python.org/2/library/contextlib.html#contextlib.nested for "
+           "more information. nova.test.nested() is an alternative as well.")
+
+    if contextlib_nested.match(logical_line):
+        yield(0, msg)
+
+
 def factory(register):
     register(import_no_db_in_virt)
     register(no_db_session_in_public_api)
@@ -565,4 +576,5 @@ def factory(register):
     register(dict_constructor_with_list_copy)
     register(assert_equal_in)
     register(check_http_not_implemented)
+    register(check_no_contextlib_nested)
     register(check_greenthread_spawns)

nova/tests/unit/test_hacking.py

@@ -535,6 +535,31 @@ class HackingTestCase(test.NoDBTestCase):
         self._assert_has_no_errors(code, checks.check_http_not_implemented,
                                    filename=filename)
 
+    def test_check_contextlib_use(self):
+        code = """
+               with test.nested(
+                   mock.patch.object(network_model.NetworkInfo, 'hydrate'),
+                   mock.patch.object(objects.InstanceInfoCache, 'save'),
+               ) as (
+                   hydrate_mock, save_mock
+               )
+               """
+        filename = "nova/api/openstack/compute/v21/test.py"
+        self._assert_has_no_errors(code, checks.check_no_contextlib_nested,
+                                   filename=filename)
+        code = """
+               with contextlib.nested(
+                   mock.patch.object(network_model.NetworkInfo, 'hydrate'),
+                   mock.patch.object(objects.InstanceInfoCache, 'save'),
+               ) as (
+                   hydrate_mock, save_mock
+               )
+               """
+        filename = "nova/api/openstack/compute/legacy_v2/test.py"
+        errors = [(1, 0, 'N341')]
+        self._assert_has_errors(code, checks.check_no_contextlib_nested,
+                                expected_errors=errors, filename=filename)
+
     def test_check_greenthread_spawns(self):
         errors = [(1, 0, "N340")]