Add some handy db methods for knocking

This just adds some database methods that we'll need to use when
implementing rescinding of knocks of clients. They are equivalent to
their invite-related counterparts.

synapse/storage/databases/main/roommember.py

@@ -272,6 +272,21 @@ class RoomMemberWorkerStore(EventsWorkerStore):
             user_id, [Membership.INVITE]
         )
 
+    @cached()
+    async def get_knocked_rooms_for_local_user(self, user_id: str) -> RoomsForUser:
+        """Get all the rooms the *local* user currently has the membership of knock for.
+
+        Args:
+            user_id: The user ID.
+
+        Returns:
+            A list of RoomsForUser.
+        """
+
+        return await self.get_rooms_for_local_user_where_membership_is(
+            user_id, [Membership.KNOCK]
+        )
+
     async def get_invite_for_local_user_in_room(
         self, user_id: str, room_id: str
     ) -> Optional[RoomsForUser]:
@@ -290,6 +305,24 @@ class RoomMemberWorkerStore(EventsWorkerStore):
                 return invite
         return None
 
+    async def get_knock_for_local_user_in_room(
+        self, user_id: str, room_id: str
+    ) -> Optional[RoomsForUser]:
+        """Gets the knock for the given *local* user and room.
+
+        Args:
+            user_id: The user ID to find the knock of.
+            room_id: The room to user knocked on.
+
+        Returns:
+            Either a RoomsForUser or None if no knock was found.
+        """
+        knocks = await self.get_knocked_rooms_for_local_user(user_id)
+        for knock in knocks:
+            if knock.room_id == room_id:
+                return knock
+        return None
+
     async def get_rooms_for_local_user_where_membership_is(
         self, user_id: str, membership_list: Collection[str]
     ) -> List[RoomsForUser]: