hacking: force explicit import of python's mock

Since we dropped support for python 2 [1], we no longer need to use the
mock library, which existed to backport py3 functionality into py2.
Change Ib44b5bff657c8e76c4f701e14d51a4efda3f6d32 cut over to importing
the stock mock, which must be done by saying::

    from unittest import mock

...because if you say::

    import mock

...you will be using the third party mock library instead, which may or
may not be installed.

This commit adds hacking check N371 to enforce the former.

[1] https://review.opendev.org/#/c/687954/

Change-Id: I71439580e80d33cff62aba807df2b35164a47cbe

nova/hacking/checks.py

@@ -1046,3 +1046,35 @@ def check_six(logical_line):
     match = re.match(six_re, logical_line)
     if match:
         yield (0, "N370: Don't use or import six")
+
+
+@core.flake8ext
+def import_stock_mock(logical_line):
+    """Use python's mock, not the mock library.
+
+    Since we `dropped support for python 2`__, we no longer need to use the
+    mock library, which existed to backport py3 functionality into py2. Change
+    Ib44b5bff657c8e76c4f701e14d51a4efda3f6d32 cut over to importing the stock
+    mock, which must be done by saying::
+
+        from unittest import mock
+
+    ...because if you say::
+
+        import mock
+
+    ...you may be getting the stock mock; or, due to transitive dependencies in
+    the environment, the library mock. This check can be removed in the future
+    (and we can start saying ``import mock`` again) if we manage to purge these
+    transitive dependencies.
+
+    .. __: https://review.opendev.org/#/c/687954/
+
+    N371
+    """
+    if logical_line == 'import mock' or logical_line.startswith('from mock'):
+        yield (
+            0,
+            "N371: You must explicitly import python's mock: "
+            "``from unittest import mock``"
+        )

nova/tests/unit/test_hacking.py

@@ -1030,3 +1030,16 @@ class HackingTestCase(test.NoDBTestCase):
         """
         errors = [(x + 1, 0, 'N370') for x in range(4)]
         self._assert_has_errors(code, checks.check_six, expected_errors=errors)
+
+    def test_import_stock_mock(self):
+        self._assert_has_errors(
+            "import mock",
+            checks.import_stock_mock, expected_errors=[(1, 0, 'N371')])
+        self._assert_has_errors(
+            "from mock import patch",
+            checks.import_stock_mock, expected_errors=[(1, 0, 'N371')])
+        code = """
+                    from unittest import mock
+                    import unittest.mock
+               """
+        self._assert_has_no_errors(code, checks.import_stock_mock)

tox.ini

@@ -345,6 +345,7 @@ extension =
   N368 = checks:do_not_use_mock_class_as_new_mock_value
   N369 = checks:check_lockutils_rwlocks
   N370 = checks:check_six
+  N371 = checks:import_stock_mock
 paths =
   ./nova/hacking