add temp waffle flag default metric (#24284)

The argument flag_undefined_default is soon to be retired
once ARCHBOM-132 is closed. The following will be used to
help ensure the rollout is complete.

- Add a temporary metric if flag_undefined_default is used.
- Add deprecation warning for flag_undefined_default.
- Add minor fix for waffle flag metric when no request found.

ARCHBOM-132

openedx/core/djangoapps/waffle_utils/__init__.py

@@ -56,6 +56,7 @@ can't yet be deleted, for example if there are still course overrides.
 """
 
 import logging
+import warnings
 from abc import ABCMeta
 from contextlib import contextmanager
 
@@ -254,6 +255,13 @@ class WaffleFlagNamespace(six.with_metaclass(ABCMeta, WaffleNamespace)):
         # Import is placed here to avoid model import at project startup.
         from waffle.models import Flag
 
+        if flag_undefined_default:
+            warnings.warn(
+                # NOTE: This will be removed once ARCHBOM-132, currently in-progress, is complete.
+                'flag_undefined_default has been deprecated. For existing uses this is already actively being fixed.',
+                DeprecationWarning
+            )
+
         # validate arguments
         namespaced_flag_name = self._namespaced_name(flag_name)
         value = None
@@ -271,6 +279,10 @@ class WaffleFlagNamespace(six.with_metaclass(ABCMeta, WaffleNamespace)):
                     try:
                         Flag.objects.get(name=namespaced_flag_name)
                     except Flag.DoesNotExist:
+                        if flag_undefined_default:
+                            # This metric will go away once this has been fully retired with ARCHBOM-132.
+                            # Also, even though the value will only track the last flag, that should be enough.
+                            set_custom_metric('temp_flag_default_used', namespaced_flag_name)
                         value = flag_undefined_default
 
                 if value is None:
@@ -279,12 +291,15 @@ class WaffleFlagNamespace(six.with_metaclass(ABCMeta, WaffleNamespace)):
                         value = flag_is_active(request, namespaced_flag_name)
                     else:
                         log.warning(u"%sFlag '%s' accessed without a request", self.log_prefix, namespaced_flag_name)
+                        set_custom_metric('warn_flag_no_request', True)
                         # Return the default value if not in a request context.
                         # Note: this skips the cache as the value might be different
                         # in a normal request context. This case seems to occur when
                         # a page redirects to a 404. In this case, we'll just return
                         # the default value.
-                        return bool(flag_undefined_default)
+                        value = bool(flag_undefined_default)
+                        self._set_waffle_flag_metric(namespaced_flag_name, value)
+                        return value
 
                 self._cached_flags[namespaced_flag_name] = value
 

openedx/core/djangoapps/waffle_utils/tests/test_init.py

@@ -127,7 +127,11 @@ class TestCourseWaffleFlag(TestCase):
                     self.TEST_COURSE_KEY
                 )
 
-        self._assert_waffle_flag_metric(mock_set_custom_metric, expected_flag_value=str(data['result']))
+        self._assert_waffle_flag_metric(
+            mock_set_custom_metric,
+            expected_flag_value=str(data['result']),
+            flag_undefined_default=data['flag_undefined_default'],
+        )
 
     @ddt.data(
         {'flag_undefined_default': None, 'result': False},
@@ -165,11 +169,12 @@ class TestCourseWaffleFlag(TestCase):
 
         self.assertEqual(mock_set_custom_metric.call_count, data['expected_count'])
 
-    def _assert_waffle_flag_metric(self, mock_set_custom_metric, expected_flag_value=None):
+    def _assert_waffle_flag_metric(self, mock_set_custom_metric, expected_flag_value=None, flag_undefined_default=None):
         if expected_flag_value:
             expected_calls = [call(self.NAMESPACED_FLAG_NAME, expected_flag_value)]
             mock_set_custom_metric.assert_has_calls(expected_calls)
-            self.assertEqual(mock_set_custom_metric.call_count, 1)
+            expected_call_count = 2 if flag_undefined_default else 1
+            self.assertEqual(mock_set_custom_metric.call_count, expected_call_count)
         else:
             self.assertEqual(mock_set_custom_metric.call_count, 0)