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>

1 files changed, 2 insertions, 2 deletions

diff --git a/synapse/app/phone_stats_home.py b/synapse/app/phone_stats_home.py

index 53db1e85b3..897dd3edac 100644
--- a/synapse/app/phone_stats_home.py
+++ b/synapse/app/phone_stats_home.py
@@ -15,7 +15,7 @@ import logging
 import math
 import resource
 import sys
-from typing import TYPE_CHECKING, List, Sized, Tuple
+from typing import TYPE_CHECKING, List, Mapping, Sized, Tuple
 
 from prometheus_client import Gauge
 
@@ -194,7 +194,7 @@ def start_phone_stats_home(hs: "HomeServer") -> None:
     @wrap_as_background_process("generate_monthly_active_users")
     async def generate_monthly_active_users() -> None:
         current_mau_count = 0
-        current_mau_count_by_service = {}
+        current_mau_count_by_service: Mapping[str, int] = {}
         reserved_users: Sized = ()
         store = hs.get_datastores().main
         if hs.config.server.limit_usage_by_mau or hs.config.server.mau_stats_only: