Better already migrated detection

.gitignore

@@ -10,3 +10,4 @@ test.db
 coverage.xml
 docs/_build
 .idea
+constance/_version.py

constance/migrations/0003_drop_pickle.py

@@ -1,3 +1,4 @@
+import json
 import logging
 import pickle
 from base64 import b64decode
@@ -11,6 +12,19 @@
 logger = logging.getLogger(__name__)
 
 
+def is_already_migrated(value):
+    migrated_shape = {'__type__', '__value__'}
+    try:
+        data = json.loads(value)
+        if (isinstance(data, list) and all(set(item.keys()) == migrated_shape for item in data)) or (
+            isinstance(data, dict) and set(data.keys()) == migrated_shape
+        ):
+            return True
+    except (json.JSONDecodeError, TypeError):
+        return False
+    return False
+
+
 def import_module_attr(path):
     package, module = path.rsplit('.', 1)
     return getattr(import_module(package), module)
@@ -39,15 +53,8 @@ def migrate_pickled_data(apps, schema_editor) -> None:  # pragma: no cover
         for key in settings.CONFIG:
             prefixed_key = f'{_prefix}{key}'
             value = _rd.get(prefixed_key)
-            if value is not None:
-                try:
-                    redis_migrated_data[prefixed_key] = dumps(pickle.loads(value))  # noqa: S301
-                except pickle.UnpicklingError as e:
-                    if value.startswith(b'{"__'):
-                        # Seems like we're facing already migrated data
-                        # Might be related to defaults and when config was accessed while django inits for migration
-                        continue
-                    raise e
+            if value is not None and not is_already_migrated(value):
+                redis_migrated_data[prefixed_key] = dumps(pickle.loads(value))  # noqa: S301
         for prefixed_key, value in redis_migrated_data.items():
             _rd.set(prefixed_key, value)