Remove cache for get_shared_rooms_for_users (#9416)

This PR remove the cache for the `get_shared_rooms_for_users` storage method (the db method driving the experimental "what rooms do I share with this user?" feature: [MSC2666](https://github.com/matrix-org/matrix-doc/pull/2666)). Currently subsequent requests to the endpoint will return the same result, even if your shared rooms with that user have changed.

The cache was added in https://github.com/matrix-org/synapse/pull/7785, but we forgot to ensure it was invalidated appropriately.

Upon attempting to invalidate it, I found that the cache had to be entirely invalidated whenever a user (remote or local) joined or left a room. This didn't make for a very useful cache, especially for a function that may or may not be called very often. Thus, I've opted to remove it instead of invalidating it.

1 files changed, 1 insertions, 3 deletions

diff --git a/synapse/storage/databases/main/user_directory.py b/synapse/storage/databases/main/user_directory.py
index 02ee15676c..1026f321e5 100644
--- a/synapse/storage/databases/main/user_directory.py
+++ b/synapse/storage/databases/main/user_directory.py
@@ -497,8 +497,7 @@ class UserDirectoryBackgroundUpdateStore(StateDeltasStore):
     async def add_users_in_public_rooms(
         self, room_id: str, user_ids: Iterable[str]
     ) -> None:
-        """Insert entries into the users_who_share_private_rooms table. The first
-        user should be a local user.
+        """Insert entries into the users_in_public_rooms table.
 
         Args:
             room_id
@@ -670,7 +669,6 @@ class UserDirectoryStore(UserDirectoryBackgroundUpdateStore):
         users.update(rows)
         return list(users)
 
-    @cached()
     async def get_shared_rooms_for_users(
         self, user_id: str, other_user_id: str
     ) -> Set[str]: