Return read-only collections from @cached methods (#13755)

It's important that collections returned from `@cached` methods are not
modified, otherwise future retrievals from the cache will return the
modified collection.

This applies to the return values from `@cached` methods and the values
inside the dictionaries returned by `@cachedList` methods. It's not
necessary for the dictionaries returned by `@cachedList` methods
themselves to be read-only.

Signed-off-by: Sean Quah <seanq@matrix.org>
Co-authored-by: David Robertson <davidr@element.io>

synapse/storage/databases/main/registration.py

@@ -16,7 +16,7 @@
 import logging
 import random
 import re
-from typing import TYPE_CHECKING, Any, Dict, List, Optional, Tuple, Union, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Mapping, Optional, Tuple, Union, cast
 
 import attr
 
@@ -192,7 +192,7 @@ class RegistrationWorkerStore(CacheInvalidationWorkerStore):
             )
 
     @cached()
-    async def get_user_by_id(self, user_id: str) -> Optional[Dict[str, Any]]:
+    async def get_user_by_id(self, user_id: str) -> Optional[Mapping[str, Any]]:
         """Deprecated: use get_userinfo_by_id instead"""
 
         def get_user_by_id_txn(txn: LoggingTransaction) -> Optional[Dict[str, Any]]: