Only use in-memory cache for database settings, set ttl=5 (#12166)

* Only use in-memory cache for database settings

Make necessary adjustments to monkeypatch
  as it is very vunerable to recursion
  Remove migration exception that is now redundant

Clear cache if a setting is changed

* Use dedicated middleware for setting cache stuff
  Clear cache for each request

* Add tests for in-memory cache

awx/conf/tests/unit/test_settings.py

@@ -8,6 +8,8 @@ import codecs
 from uuid import uuid4
 import time
 
+from unittest import mock
+
 from django.conf import LazySettings
 from django.core.cache.backends.locmem import LocMemCache
 from django.core.exceptions import ImproperlyConfigured
@@ -299,3 +301,33 @@ def test_readonly_sensitive_cache_data_is_encrypted(settings):
     cache.set('AWX_ENCRYPTED', 'SECRET!')
     assert cache.get('AWX_ENCRYPTED') == 'SECRET!'
     assert native_cache.get('AWX_ENCRYPTED') == 'FRPERG!'
+
+
+@pytest.mark.defined_in_file(AWX_VAR='DEFAULT')
+def test_in_memory_cache_only_for_registered_settings(settings):
+    "Test that we only make use of the in-memory TTL cache for registered settings"
+    settings._awx_conf_memoizedcache.clear()
+    settings.MIDDLEWARE
+    assert len(settings._awx_conf_memoizedcache) == 0  # does not cache MIDDLEWARE
+    settings.registry.register('AWX_VAR', field_class=fields.CharField, category=_('System'), category_slug='system')
+    settings._wrapped.__dict__['all_supported_settings'] = ['AWX_VAR']  # because it is cached_property
+    settings._awx_conf_memoizedcache.clear()
+    assert settings.AWX_VAR == 'DEFAULT'
+    assert len(settings._awx_conf_memoizedcache) == 1  # caches registered settings
+
+
+@pytest.mark.defined_in_file(AWX_VAR='DEFAULT')
+def test_in_memory_cache_works(settings):
+    settings._awx_conf_memoizedcache.clear()
+    settings.registry.register('AWX_VAR', field_class=fields.CharField, category=_('System'), category_slug='system')
+    settings._wrapped.__dict__['all_supported_settings'] = ['AWX_VAR']
+
+    settings._awx_conf_memoizedcache.clear()
+
+    with mock.patch('awx.conf.settings.SettingsWrapper._get_local', return_value='DEFAULT') as mock_get:
+        assert settings.AWX_VAR == 'DEFAULT'
+        mock_get.assert_called_once_with('AWX_VAR')
+
+    with mock.patch.object(settings, '_get_local') as mock_get:
+        assert settings.AWX_VAR == 'DEFAULT'
+        mock_get.assert_not_called()