fixup: add compat layer on setting.py

Author: pedro-psb

pulpcore/app/settings.py

@@ -56,14 +56,6 @@
 STATIC_URL = "/assets/"
 STATIC_ROOT = DEPLOY_ROOT / STATIC_URL.strip("/")
 
-STORAGES = {
-    "default": {"BACKEND": "pulpcore.app.models.storage.FileSystem"},
-    "staticfiles": {
-        # This is django's default, but when customizing STORAGES we need to add explicitly
-        # https://docs.djangoproject.com/en/4.2/ref/settings/#storages
-        "BACKEND": "django.contrib.staticfiles.storage.StaticFilesStorage",
-    },
-}
 
 REDIRECT_TO_OBJECT_STORAGE = True
 
@@ -371,7 +363,7 @@
 
 # HERE STARTS DYNACONF EXTENSION LOAD (Keep at the very bottom of settings.py)
 # Read more at https://www.dynaconf.com/django/
-from dynaconf import DjangoDynaconf, Validator  # noqa
+from dynaconf import DjangoDynaconf, Validator, get_history  # noqa
 
 # Validators
 storage_validator = (
@@ -481,13 +473,35 @@ def otel_middleware_hook(settings):
         api_root_validator,
         cache_validator,
         sha256_validator,
-        storage_validator,
         unknown_algs_validator,
         json_header_auth_validator,
         authentication_json_header_openapi_security_scheme_validator,
     ],
     post_hooks=otel_middleware_hook,
 )
+
+# begin Compatiblity Layer for DEFAULT_FILE_STORAGE deprecation (remove in 3.85):
+#
+# 1. Removed DEFAULT_FILE_STORAGE from toplevel setting, because STORAGES and DEFAULT_FILE_STORAGE
+#    are mutually exclusive. This enables users to migrate to STORAGES before true removal of the
+#    legacy setting.
+# 2. After dropping this compat-layer, put the default STORAGES in the toplevel module, as usual.
+#    Then, DEFAULT_FILE_STORAGE would not be allowed anymore.
+
+# Dynamically update default storages backend to use Pulp's special class,
+# but only in the case the user has not provided an explicit setting for it.
+dfstorage_history = get_history(settings, key="DEFAULT_FILE_STORAGE")
+django_default_used = (
+    len(dfstorage_history) == 1 and dfstorage_history[0]["identifier"] == "undefined"
+)
+if django_default_used:
+    settings.set("STORAGES.default.BACKEND", "pulpcore.app.models.storage.FileSystem")
+
+settings.validators.register(storage_validator)
+settings.validators.validate(only=["STORAGES.default.BACKEND", "REDIRECT_TO_OBJECT_STORAGE"])
+
+# end Compatibility Layer
+
 # HERE ENDS DYNACONF EXTENSION LOAD (No more code below this line)
 
 _logger = getLogger(__name__)