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.

3 files changed, 43 insertions, 37 deletions

diff --git a/changelog.d/9416.bugfix b/changelog.d/9416.bugfix
new file mode 100644
index 0000000000..4d79cb2228
--- /dev/null
+++ b/changelog.d/9416.bugfix
@@ -0,0 +1 @@
+Fix a bug that caused multiple calls to the experimental `shared_rooms` endpoint to return stale results.
\ No newline at end of file
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]:
diff --git a/tests/rest/client/v2_alpha/test_shared_rooms.py b/tests/rest/client/v2_alpha/test_shared_rooms.py
index 116ace1812..dd83a1f8ff 100644
--- a/tests/rest/client/v2_alpha/test_shared_rooms.py
+++ b/tests/rest/client/v2_alpha/test_shared_rooms.py
@@ -54,61 +54,62 @@ class UserSharedRoomsTest(unittest.HomeserverTestCase):
         A room should show up in the shared list of rooms between two users
         if it is public.
         """
-        u1 = self.register_user("user1", "pass")
-        u1_token = self.login(u1, "pass")
-        u2 = self.register_user("user2", "pass")
-        u2_token = self.login(u2, "pass")
-
-        room = self.helper.create_room_as(u1, is_public=True, tok=u1_token)
-        self.helper.invite(room, src=u1, targ=u2, tok=u1_token)
-        self.helper.join(room, user=u2, tok=u2_token)
-
-        channel = self._get_shared_rooms(u1_token, u2)
-        self.assertEquals(200, channel.code, channel.result)
-        self.assertEquals(len(channel.json_body["joined"]), 1)
-        self.assertEquals(channel.json_body["joined"][0], room)
+        self._check_shared_rooms_with(room_one_is_public=True, room_two_is_public=True)
 
     def test_shared_room_list_private(self):
         """
         A room should show up in the shared list of rooms between two users
         if it is private.
         """
-        u1 = self.register_user("user1", "pass")
-        u1_token = self.login(u1, "pass")
-        u2 = self.register_user("user2", "pass")
-        u2_token = self.login(u2, "pass")
-
-        room = self.helper.create_room_as(u1, is_public=False, tok=u1_token)
-        self.helper.invite(room, src=u1, targ=u2, tok=u1_token)
-        self.helper.join(room, user=u2, tok=u2_token)
-
-        channel = self._get_shared_rooms(u1_token, u2)
-        self.assertEquals(200, channel.code, channel.result)
-        self.assertEquals(len(channel.json_body["joined"]), 1)
-        self.assertEquals(channel.json_body["joined"][0], room)
+        self._check_shared_rooms_with(
+            room_one_is_public=False, room_two_is_public=False
+        )
 
     def test_shared_room_list_mixed(self):
         """
         The shared room list between two users should contain both public and private
         rooms.
         """
+        self._check_shared_rooms_with(room_one_is_public=True, room_two_is_public=False)
+
+    def _check_shared_rooms_with(
+        self, room_one_is_public: bool, room_two_is_public: bool
+    ):
+        """Checks that shared public or private rooms between two users appear in
+        their shared room lists
+        """
         u1 = self.register_user("user1", "pass")
         u1_token = self.login(u1, "pass")
         u2 = self.register_user("user2", "pass")
         u2_token = self.login(u2, "pass")
 
-        room_public = self.helper.create_room_as(u1, is_public=True, tok=u1_token)
-        room_private = self.helper.create_room_as(u2, is_public=False, tok=u2_token)
-        self.helper.invite(room_public, src=u1, targ=u2, tok=u1_token)
-        self.helper.invite(room_private, src=u2, targ=u1, tok=u2_token)
-        self.helper.join(room_public, user=u2, tok=u2_token)
-        self.helper.join(room_private, user=u1, tok=u1_token)
+        # Create a room. user1 invites user2, who joins
+        room_id_one = self.helper.create_room_as(
+            u1, is_public=room_one_is_public, tok=u1_token
+        )
+        self.helper.invite(room_id_one, src=u1, targ=u2, tok=u1_token)
+        self.helper.join(room_id_one, user=u2, tok=u2_token)
 
+        # Check shared rooms from user1's perspective.
+        # We should see the one room in common
+        channel = self._get_shared_rooms(u1_token, u2)
+        self.assertEquals(200, channel.code, channel.result)
+        self.assertEquals(len(channel.json_body["joined"]), 1)
+        self.assertEquals(channel.json_body["joined"][0], room_id_one)
+
+        # Create another room and invite user2 to it
+        room_id_two = self.helper.create_room_as(
+            u1, is_public=room_two_is_public, tok=u1_token
+        )
+        self.helper.invite(room_id_two, src=u1, targ=u2, tok=u1_token)
+        self.helper.join(room_id_two, user=u2, tok=u2_token)
+
+        # Check shared rooms again. We should now see both rooms.
         channel = self._get_shared_rooms(u1_token, u2)
         self.assertEquals(200, channel.code, channel.result)
         self.assertEquals(len(channel.json_body["joined"]), 2)
-        self.assertTrue(room_public in channel.json_body["joined"])
-        self.assertTrue(room_private in channel.json_body["joined"])
+        for room_id_id in channel.json_body["joined"]:
+            self.assertIn(room_id_id, [room_id_one, room_id_two])
 
     def test_shared_room_list_after_leave(self):
         """
@@ -132,6 +133,12 @@ class UserSharedRoomsTest(unittest.HomeserverTestCase):
 
         self.helper.leave(room, user=u1, tok=u1_token)
 
+        # Check user1's view of shared rooms with user2
+        channel = self._get_shared_rooms(u1_token, u2)
+        self.assertEquals(200, channel.code, channel.result)
+        self.assertEquals(len(channel.json_body["joined"]), 0)
+
+        # Check user2's view of shared rooms with user1
         channel = self._get_shared_rooms(u2_token, u1)
         self.assertEquals(200, channel.code, channel.result)
         self.assertEquals(len(channel.json_body["joined"]), 0)