Add hacking check to prevent assert_called_once

We've had a few patches now over the last few releases to fix invalid
mock assertions for assert_called_once. Rather than keep letting these
creep back in to the code, this adds a hacking check to watch for this
specific call and block it.

Change-Id: I3cf4df523ddba49f57dd1d4d23e95e6d62d03c9e

HACKING.rst

@@ -30,6 +30,7 @@ Cinder Specific Commandments
 - [C311] Check for proper naming and usage in option registration.
 - [C312] Check that assertIsNone(value) is used and not assertEqual(None, value).
 - [C313] Check that assertTrue(value) is used and not assertEqual(True, value).
+- [C314] Check for use of invalid mock.assert_called_once().
 
 General
 -------

cinder/hacking/checks.py

@@ -504,6 +504,15 @@ def validate_assertTrue(logical_line):
         yield(0, msg)
 
 
+def no_assert_called_once(logical_line, filename):
+    if "cinder/tests" not in filename or "test_hacking" in filename:
+        return
+    msg = ("C314: assert_called_once is not a valid mock assertion, "
+           "use assert_equal(1, mocked.call_count) instead.")
+    if 'assert_called_once(' in logical_line:
+        yield (0, msg)
+
+
 def factory(register):
     register(no_vi_headers)
     register(no_translate_debug_logs)
@@ -526,3 +535,4 @@ def factory(register):
     register(no_test_log)
     register(validate_assertIsNone)
     register(validate_assertTrue)
+    register(no_assert_called_once)

cinder/tests/unit/test_hacking.py

@@ -448,3 +448,9 @@ class HackingTestCase(test.TestCase):
     def test_no_test_log(self, first, second, third, fourth):
         self.assertEqual(first, len(list(checks.no_test_log(
             "%s('arg')" % second, third, fourth))))
+
+    def test_no_assert_called_once(self):
+        self.assertEqual(1, len(list(checks.no_assert_called_once(
+            "masker.assert_called_once(", "cinder/tests/unit/fake.py"))))
+        self.assertEqual(0, len(list(checks.no_assert_called_once(
+            "masker.assert_called_once_with(", "cinder/tests/unit/fake.py"))))