Add room subscriptions to Sliding Sync `/sync` (#17432)

Add room subscriptions to Sliding Sync `/sync`

Based on
[MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575):
Sliding Sync

Currently, you can only subscribe to rooms you have had *any* membership
in before.

In the future, we will allow `world_readable` rooms to be subscribed to
without joining.

5 files changed, 1489 insertions, 358 deletions

diff --git a/changelog.d/17432.feature b/changelog.d/17432.feature
new file mode 100644
index 0000000000..c86f04c118
--- /dev/null
+++ b/changelog.d/17432.feature
@@ -0,0 +1 @@
+Add room subscriptions to experimental [MSC3575](https://github.com/matrix-org/matrix-spec-proposals/pull/3575) Sliding Sync `/sync` endpoint.
diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index 904787ced3..be98b379eb 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -62,32 +62,79 @@ DEFAULT_BUMP_EVENT_TYPES = {
 }
 
 
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class _RoomMembershipForUser:
+    """
+    Attributes:
+        room_id: The room ID of the membership event
+        event_id: The event ID of the membership event
+        event_pos: The stream position of the membership event
+        membership: The membership state of the user in the room
+        sender: The person who sent the membership event
+        newly_joined: Whether the user newly joined the room during the given token
+            range and is still joined to the room at the end of this range.
+        newly_left: Whether the user newly left (or kicked) the room during the given
+            token range and is still "leave" at the end of this range.
+        is_dm: Whether this user considers this room as a direct-message (DM) room
+    """
+
+    room_id: str
+    # Optional because state resets can affect room membership without a corresponding event.
+    event_id: Optional[str]
+    # Even during a state reset which removes the user from the room, we expect this to
+    # be set because `current_state_delta_stream` will note the position that the reset
+    # happened.
+    event_pos: PersistedEventPosition
+    # Even during a state reset which removes the user from the room, we expect this to
+    # be set to `LEAVE` because we can make that assumption based on the situaton (see
+    # `get_current_state_delta_membership_changes_for_user(...)`)
+    membership: str
+    # Optional because state resets can affect room membership without a corresponding event.
+    sender: Optional[str]
+    newly_joined: bool
+    newly_left: bool
+    is_dm: bool
+
+    def copy_and_replace(self, **kwds: Any) -> "_RoomMembershipForUser":
+        return attr.evolve(self, **kwds)
+
+
 def filter_membership_for_sync(
-    *, membership: str, user_id: str, sender: Optional[str]
+    *, user_id: str, room_membership_for_user: _RoomMembershipForUser
 ) -> bool:
     """
     Returns True if the membership event should be included in the sync response,
     otherwise False.
 
     Attributes:
-        membership: The membership state of the user in the room.
         user_id: The user ID that the membership applies to
-        sender: The person who sent the membership event
+        room_membership_for_user: Membership information for the user in the room
     """
 
-    # Everything except `Membership.LEAVE` because we want everything that's *still*
-    # relevant to the user. There are few more things to include in the sync response
-    # (newly_left) but those are handled separately.
+    membership = room_membership_for_user.membership
+    sender = room_membership_for_user.sender
+    newly_left = room_membership_for_user.newly_left
+
+    # We want to allow everything except rooms the user has left unless `newly_left`
+    # because we want everything that's *still* relevant to the user. We include
+    # `newly_left` rooms because the last event that the user should see is their own
+    # leave event.
     #
-    # This logic includes kicks (leave events where the sender is not the same user) and
-    # can be read as "anything that isn't a leave or a leave with a different sender".
+    # A leave != kick. This logic includes kicks (leave events where the sender is not
+    # the same user).
     #
-    # When `sender=None` and `membership=Membership.LEAVE`, it means that a state reset
-    # happened that removed the user from the room, or the user was the last person
-    # locally to leave the room which caused the server to leave the room. In both
-    # cases, we can just remove the rooms since they are no longer relevant to the user.
-    # They could still be added back later if they are `newly_left`.
-    return membership != Membership.LEAVE or sender not in (user_id, None)
+    # When `sender=None`, it means that a state reset happened that removed the user
+    # from the room without a corresponding leave event. We can just remove the rooms
+    # since they are no longer relevant to the user but will still appear if they are
+    # `newly_left`.
+    return (
+        # Anything except leave events
+        membership != Membership.LEAVE
+        # Unless...
+        or newly_left
+        # Allow kicks
+        or (membership == Membership.LEAVE and sender not in (user_id, None))
+    )
 
 
 # We can't freeze this class because we want to update it in place with the
@@ -281,31 +328,6 @@ class StateValues:
     LAZY: Final = "$LAZY"
 
 
-@attr.s(slots=True, frozen=True, auto_attribs=True)
-class _RoomMembershipForUser:
-    """
-    Attributes:
-        event_id: The event ID of the membership event
-        event_pos: The stream position of the membership event
-        membership: The membership state of the user in the room
-        sender: The person who sent the membership event
-        newly_joined: Whether the user newly joined the room during the given token
-            range
-        is_dm: Whether this user considers this room as a direct-message (DM) room
-    """
-
-    room_id: str
-    event_id: Optional[str]
-    event_pos: PersistedEventPosition
-    membership: str
-    sender: Optional[str]
-    newly_joined: bool
-    is_dm: bool
-
-    def copy_and_replace(self, **kwds: Any) -> "_RoomMembershipForUser":
-        return attr.evolve(self, **kwds)
-
-
 class SlidingSyncHandler:
     def __init__(self, hs: "HomeServer"):
         self.clock = hs.get_clock()
@@ -424,18 +446,31 @@ class SlidingSyncHandler:
             # See https://github.com/matrix-org/matrix-doc/issues/1144
             raise NotImplementedError()
 
+        # Get all of the room IDs that the user should be able to see in the sync
+        # response
+        has_lists = sync_config.lists is not None and len(sync_config.lists) > 0
+        has_room_subscriptions = (
+            sync_config.room_subscriptions is not None
+            and len(sync_config.room_subscriptions) > 0
+        )
+        if has_lists or has_room_subscriptions:
+            room_membership_for_user_map = (
+                await self.get_room_membership_for_user_at_to_token(
+                    user=sync_config.user,
+                    to_token=to_token,
+                    from_token=from_token,
+                )
+            )
+
         # Assemble sliding window lists
         lists: Dict[str, SlidingSyncResult.SlidingWindowList] = {}
         # Keep track of the rooms that we're going to display and need to fetch more
         # info about
         relevant_room_map: Dict[str, RoomSyncConfig] = {}
-        if sync_config.lists:
-            # Get all of the room IDs that the user should be able to see in the sync
-            # response
-            sync_room_map = await self.get_sync_room_ids_for_user(
-                sync_config.user,
-                from_token=from_token,
-                to_token=to_token,
+        if has_lists and sync_config.lists is not None:
+            sync_room_map = await self.filter_rooms_relevant_for_sync(
+                user=sync_config.user,
+                room_membership_for_user_map=room_membership_for_user_map,
             )
 
             for list_key, list_config in sync_config.lists.items():
@@ -524,7 +559,35 @@ class SlidingSyncHandler:
                     ops=ops,
                 )
 
-        # TODO: if (sync_config.room_subscriptions):
+        # Handle room subscriptions
+        if has_room_subscriptions and sync_config.room_subscriptions is not None:
+            for room_id, room_subscription in sync_config.room_subscriptions.items():
+                room_membership_for_user_at_to_token = (
+                    await self.check_room_subscription_allowed_for_user(
+                        room_id=room_id,
+                        room_membership_for_user_map=room_membership_for_user_map,
+                        to_token=to_token,
+                    )
+                )
+
+                # Skip this room if the user isn't allowed to see it
+                if not room_membership_for_user_at_to_token:
+                    continue
+
+                room_membership_for_user_map[room_id] = (
+                    room_membership_for_user_at_to_token
+                )
+
+                # Take the superset of the `RoomSyncConfig` for each room.
+                #
+                # Update our `relevant_room_map` with the room we're going to display
+                # and need to fetch more info about.
+                room_sync_config = RoomSyncConfig.from_room_config(room_subscription)
+                existing_room_sync_config = relevant_room_map.get(room_id)
+                if existing_room_sync_config is not None:
+                    existing_room_sync_config.combine_room_sync_config(room_sync_config)
+                else:
+                    relevant_room_map[room_id] = room_sync_config
 
         # Fetch room data
         rooms: Dict[str, SlidingSyncResult.RoomResult] = {}
@@ -533,7 +596,9 @@ class SlidingSyncHandler:
                 user=sync_config.user,
                 room_id=room_id,
                 room_sync_config=room_sync_config,
-                room_membership_for_user_at_to_token=sync_room_map[room_id],
+                room_membership_for_user_at_to_token=room_membership_for_user_map[
+                    room_id
+                ],
                 from_token=from_token,
                 to_token=to_token,
             )
@@ -551,28 +616,23 @@ class SlidingSyncHandler:
             extensions=extensions,
         )
 
-    async def get_sync_room_ids_for_user(
+    async def get_room_membership_for_user_at_to_token(
         self,
         user: UserID,
         to_token: StreamToken,
-        from_token: Optional[StreamToken] = None,
+        from_token: Optional[StreamToken],
     ) -> Dict[str, _RoomMembershipForUser]:
         """
-        Fetch room IDs that should be listed for this user in the sync response (the
-        full room list that will be filtered, sorted, and sliced).
+        Fetch room IDs that the user has had membership in (the full room list including
+        long-lost left rooms that will be filtered, sorted, and sliced).
 
-        We're looking for rooms where the user has the following state in the token
-        range (> `from_token` and <= `to_token`):
+        We're looking for rooms where the user has had any sort of membership in the
+        token range (> `from_token` and <= `to_token`)
 
-        - `invite`, `join`, `knock`, `ban` membership events
-        - Kicks (`leave` membership events where `sender` is different from the
-          `user_id`/`state_key`)
-        - `newly_left` (rooms that were left during the given token range)
-        - In order for bans/kicks to not show up in sync, you need to `/forget` those
-          rooms. This doesn't modify the event itself though and only adds the
-          `forgotten` flag to the `room_memberships` table in Synapse. There isn't a way
-          to tell when a room was forgotten at the moment so we can't factor it into the
-          from/to range.
+        In order for bans/kicks to not show up, you need to `/forget` those rooms. This
+        doesn't modify the event itself though and only adds the `forgotten` flag to the
+        `room_memberships` table in Synapse. There isn't a way to tell when a room was
+        forgotten at the moment so we can't factor it into the token range.
 
         Args:
             user: User to fetch rooms for
@@ -580,8 +640,8 @@ class SlidingSyncHandler:
             from_token: The point in the stream to sync from.
 
         Returns:
-            A dictionary of room IDs that should be listed in the sync response along
-            with membership information in that room at the time of `to_token`.
+            A dictionary of room IDs that the user has had membership in along with
+            membership information in that room at the time of `to_token`.
         """
         user_id = user.to_string()
 
@@ -592,9 +652,6 @@ class SlidingSyncHandler:
             # We want to fetch any kind of membership (joined and left rooms) in order
             # to get the `event_pos` of the latest room membership event for the
             # user.
-            #
-            # We will filter out the rooms that don't belong below (see
-            # `filter_membership_for_sync`)
             membership_list=Membership.LIST,
             excluded_rooms=self.rooms_to_exclude_globally,
         )
@@ -614,7 +671,9 @@ class SlidingSyncHandler:
                 event_pos=room_for_user.event_pos,
                 membership=room_for_user.membership,
                 sender=room_for_user.sender,
+                # We will update these fields below to be accurate
                 newly_joined=False,
+                newly_left=False,
                 is_dm=False,
             )
             for room_for_user in room_for_user_list
@@ -653,12 +712,10 @@ class SlidingSyncHandler:
         # - 1a) Remove rooms that the user joined after the `to_token`
         # - 1b) Add back rooms that the user left after the `to_token`
         # - 1c) Update room membership events to the point in time of the `to_token`
-        # - 2) Add back newly_left rooms (> `from_token` and <= `to_token`)
-        # - 3) Figure out which rooms are `newly_joined`
+        # - 2) Figure out which rooms are `newly_left` rooms (> `from_token` and <= `to_token`)
+        # - 3) Figure out which rooms are `newly_joined` (> `from_token` and <= `to_token`)
         # - 4) Figure out which rooms are DM's
 
-        # 1) -----------------------------------------------------
-
         # 1) Fetch membership changes that fall in the range from `to_token` up to
         # `membership_snapshot_token`
         #
@@ -717,7 +774,9 @@ class SlidingSyncHandler:
                         event_pos=first_membership_change_after_to_token.prev_event_pos,
                         membership=first_membership_change_after_to_token.prev_membership,
                         sender=first_membership_change_after_to_token.prev_sender,
+                        # We will update these fields below to be accurate
                         newly_joined=False,
+                        newly_left=False,
                         is_dm=False,
                     )
                 else:
@@ -726,22 +785,6 @@ class SlidingSyncHandler:
                     # exact membership state and shouldn't rely on the current snapshot.
                     sync_room_id_set.pop(room_id, None)
 
-        # Filter the rooms that that we have updated room membership events to the point
-        # in time of the `to_token` (from the "1)" fixups)
-        filtered_sync_room_id_set = {
-            room_id: room_membership_for_user
-            for room_id, room_membership_for_user in sync_room_id_set.items()
-            if filter_membership_for_sync(
-                membership=room_membership_for_user.membership,
-                user_id=user_id,
-                sender=room_membership_for_user.sender,
-            )
-        }
-
-        # 2) -----------------------------------------------------
-        # We fix-up newly_left rooms after the first fixup because it may have removed
-        # some left rooms that we can figure out are newly_left in the following code
-
         # 2) Fetch membership changes that fall in the range from `from_token` up to `to_token`
         current_state_delta_membership_changes_in_from_to_range = []
         if from_token:
@@ -803,19 +846,40 @@ class SlidingSyncHandler:
             if last_membership_change_in_from_to_range.membership == Membership.JOIN:
                 possibly_newly_joined_room_ids.add(room_id)
 
-            # 2) Add back newly_left rooms (> `from_token` and <= `to_token`). We
-            # include newly_left rooms because the last event that the user should see
-            # is their own leave event
+            # 2) Figure out newly_left rooms (> `from_token` and <= `to_token`).
             if last_membership_change_in_from_to_range.membership == Membership.LEAVE:
-                filtered_sync_room_id_set[room_id] = _RoomMembershipForUser(
-                    room_id=room_id,
-                    event_id=last_membership_change_in_from_to_range.event_id,
-                    event_pos=last_membership_change_in_from_to_range.event_pos,
-                    membership=last_membership_change_in_from_to_range.membership,
-                    sender=last_membership_change_in_from_to_range.sender,
-                    newly_joined=False,
-                    is_dm=False,
-                )
+                # 2) Mark this room as `newly_left`
+
+                # If we're seeing a membership change here, we should expect to already
+                # have it in our snapshot but if a state reset happens, it wouldn't have
+                # shown up in our snapshot but appear as a change here.
+                existing_sync_entry = sync_room_id_set.get(room_id)
+                if existing_sync_entry is not None:
+                    # Normal expected case
+                    sync_room_id_set[room_id] = existing_sync_entry.copy_and_replace(
+                        newly_left=True
+                    )
+                else:
+                    # State reset!
+                    logger.warn(
+                        "State reset detected for room_id %s with %s who is no longer in the room",
+                        room_id,
+                        user_id,
+                    )
+                    # Even though a state reset happened which removed the person from
+                    # the room, we still add it the list so the user knows they left the
+                    # room. Downstream code can check for a state reset by looking for
+                    # `event_id=None and membership is not None`.
+                    sync_room_id_set[room_id] = _RoomMembershipForUser(
+                        room_id=room_id,
+                        event_id=last_membership_change_in_from_to_range.event_id,
+                        event_pos=last_membership_change_in_from_to_range.event_pos,
+                        membership=last_membership_change_in_from_to_range.membership,
+                        sender=last_membership_change_in_from_to_range.sender,
+                        newly_joined=False,
+                        newly_left=True,
+                        is_dm=False,
+                    )
 
         # 3) Figure out `newly_joined`
         for room_id in possibly_newly_joined_room_ids:
@@ -826,9 +890,9 @@ class SlidingSyncHandler:
             # also some non-join in the range, we know they `newly_joined`.
             if has_non_join_in_from_to_range:
                 # We found a `newly_joined` room (we left and joined within the token range)
-                filtered_sync_room_id_set[room_id] = filtered_sync_room_id_set[
-                    room_id
-                ].copy_and_replace(newly_joined=True)
+                sync_room_id_set[room_id] = sync_room_id_set[room_id].copy_and_replace(
+                    newly_joined=True
+                )
             else:
                 prev_event_id = first_membership_change_by_room_id_in_from_to_range[
                     room_id
@@ -840,7 +904,7 @@ class SlidingSyncHandler:
                 if prev_event_id is None:
                     # We found a `newly_joined` room (we are joining the room for the
                     # first time within the token range)
-                    filtered_sync_room_id_set[room_id] = filtered_sync_room_id_set[
+                    sync_room_id_set[room_id] = sync_room_id_set[
                         room_id
                     ].copy_and_replace(newly_joined=True)
                 # Last resort, we need to step back to the previous membership event
@@ -848,7 +912,7 @@ class SlidingSyncHandler:
                 elif prev_membership != Membership.JOIN:
                     # We found a `newly_joined` room (we left before the token range
                     # and joined within the token range)
-                    filtered_sync_room_id_set[room_id] = filtered_sync_room_id_set[
+                    sync_room_id_set[room_id] = sync_room_id_set[
                         room_id
                     ].copy_and_replace(newly_joined=True)
 
@@ -876,12 +940,122 @@ class SlidingSyncHandler:
                             dm_room_id_set.add(room_id)
 
         # 4) Fixup
-        for room_id in filtered_sync_room_id_set:
-            filtered_sync_room_id_set[room_id] = filtered_sync_room_id_set[
-                room_id
-            ].copy_and_replace(is_dm=room_id in dm_room_id_set)
+        for room_id in sync_room_id_set:
+            sync_room_id_set[room_id] = sync_room_id_set[room_id].copy_and_replace(
+                is_dm=room_id in dm_room_id_set
+            )
+
+        return sync_room_id_set
+
+    async def filter_rooms_relevant_for_sync(
+        self,
+        user: UserID,
+        room_membership_for_user_map: Dict[str, _RoomMembershipForUser],
+    ) -> Dict[str, _RoomMembershipForUser]:
+        """
+        Filter room IDs that should/can be listed for this user in the sync response (the
+        full room list that will be further filtered, sorted, and sliced).
+
+        We're looking for rooms where the user has the following state in the token
+        range (> `from_token` and <= `to_token`):
+
+        - `invite`, `join`, `knock`, `ban` membership events
+        - Kicks (`leave` membership events where `sender` is different from the
+          `user_id`/`state_key`)
+        - `newly_left` (rooms that were left during the given token range)
+        - In order for bans/kicks to not show up in sync, you need to `/forget` those
+          rooms. This doesn't modify the event itself though and only adds the
+          `forgotten` flag to the `room_memberships` table in Synapse. There isn't a way
+          to tell when a room was forgotten at the moment so we can't factor it into the
+          from/to range.
+
+        Args:
+            user: User that is syncing
+            room_membership_for_user_map: Room membership for the user
+
+        Returns:
+            A dictionary of room IDs that should be listed in the sync response along
+            with membership information in that room at the time of `to_token`.
+        """
+        user_id = user.to_string()
+
+        # Filter rooms to only what we're interested to sync with
+        filtered_sync_room_map = {
+            room_id: room_membership_for_user
+            for room_id, room_membership_for_user in room_membership_for_user_map.items()
+            if filter_membership_for_sync(
+                user_id=user_id,
+                room_membership_for_user=room_membership_for_user,
+            )
+        }
+
+        return filtered_sync_room_map
+
+    async def check_room_subscription_allowed_for_user(
+        self,
+        room_id: str,
+        room_membership_for_user_map: Dict[str, _RoomMembershipForUser],
+        to_token: StreamToken,
+    ) -> Optional[_RoomMembershipForUser]:
+        """
+        Check whether the user is allowed to see the room based on whether they have
+        ever had membership in the room or if the room is `world_readable`.
 
-        return filtered_sync_room_id_set
+        Similar to `check_user_in_room_or_world_readable(...)`
+
+        Args:
+            room_id: Room to check
+            room_membership_for_user_map: Room membership for the user at the time of
+                the `to_token` (<= `to_token`).
+            to_token: The token to fetch rooms up to.
+
+        Returns:
+            The room membership for the user if they are allowed to subscribe to the
+            room else `None`.
+        """
+
+        # We can first check if they are already allowed to see the room based
+        # on our previous work to assemble the `room_membership_for_user_map`.
+        #
+        # If they have had any membership in the room over time (up to the `to_token`),
+        # let them subscribe and see what they can.
+        existing_membership_for_user = room_membership_for_user_map.get(room_id)
+        if existing_membership_for_user is not None:
+            return existing_membership_for_user
+
+        # TODO: Handle `world_readable` rooms
+        return None
+
+        # If the room is `world_readable`, it doesn't matter whether they can join,
+        # everyone can see the room.
+        # not_in_room_membership_for_user = _RoomMembershipForUser(
+        #     room_id=room_id,
+        #     event_id=None,
+        #     event_pos=None,
+        #     membership=None,
+        #     sender=None,
+        #     newly_joined=False,
+        #     newly_left=False,
+        #     is_dm=False,
+        # )
+        # room_state = await self.get_current_state_at(
+        #     room_id=room_id,
+        #     room_membership_for_user_at_to_token=not_in_room_membership_for_user,
+        #     state_filter=StateFilter.from_types(
+        #         [(EventTypes.RoomHistoryVisibility, "")]
+        #     ),
+        #     to_token=to_token,
+        # )
+
+        # visibility_event = room_state.get((EventTypes.RoomHistoryVisibility, ""))
+        # if (
+        #     visibility_event is not None
+        #     and visibility_event.content.get("history_visibility")
+        #     == HistoryVisibility.WORLD_READABLE
+        # ):
+        #     return not_in_room_membership_for_user
+
+        # return None
 
     async def filter_rooms(
         self,
@@ -1081,7 +1255,6 @@ class SlidingSyncHandler:
                 in the room at the time of `to_token`.
             to_token: The point in the stream to sync up to.
         """
-
         room_state_ids: StateMap[str]
         # People shouldn't see past their leave/ban event
         if room_membership_for_user_at_to_token.membership in (
@@ -1349,10 +1522,10 @@ class SlidingSyncHandler:
             stripped_state.append(strip_event(invite_or_knock_event))
 
         # TODO: Handle state resets. For example, if we see
-        # `room_membership_for_user_at_to_token.membership = Membership.LEAVE` but
-        # `required_state` doesn't include it, we should indicate to the client that a
-        # state reset happened. Perhaps we should indicate this by setting `initial:
-        # True` and empty `required_state`.
+        # `room_membership_for_user_at_to_token.event_id=None and
+        # room_membership_for_user_at_to_token.membership is not None`, we should
+        # indicate to the client that a state reset happened. Perhaps we should indicate
+        # this by setting `initial: True` and empty `required_state`.
 
         # TODO: Since we can't determine whether we've already sent a room down this
         # Sliding Sync connection before (we plan to add this optimization in the
diff --git a/tests/handlers/test_sliding_sync.py b/tests/handlers/test_sliding_sync.py
index eb4b0a05c7..a7aa9bb8af 100644
--- a/tests/handlers/test_sliding_sync.py
+++ b/tests/handlers/test_sliding_sync.py
@@ -19,7 +19,7 @@
 #
 import logging
 from copy import deepcopy
-from typing import Optional
+from typing import Dict, Optional
 from unittest.mock import patch
 
 from parameterized import parameterized
@@ -37,12 +37,16 @@ from synapse.api.constants import (
 from synapse.api.room_versions import RoomVersions
 from synapse.events import make_event_from_dict
 from synapse.events.snapshot import EventContext
-from synapse.handlers.sliding_sync import RoomSyncConfig, StateValues
+from synapse.handlers.sliding_sync import (
+    RoomSyncConfig,
+    StateValues,
+    _RoomMembershipForUser,
+)
 from synapse.rest import admin
 from synapse.rest.client import knock, login, room
 from synapse.server import HomeServer
 from synapse.storage.util.id_generators import MultiWriterIdGenerator
-from synapse.types import JsonDict, UserID
+from synapse.types import JsonDict, StreamToken, UserID
 from synapse.types.handlers import SlidingSyncConfig
 from synapse.util import Clock
 
@@ -581,9 +585,9 @@ class RoomSyncConfigTestCase(TestCase):
         self._assert_room_config_equal(room_sync_config_b, expected, "A into B")
 
 
-class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
+class GetRoomMembershipForUserAtToTokenTestCase(HomeserverTestCase):
     """
-    Tests Sliding Sync handler `get_sync_room_ids_for_user()` to make sure it returns
+    Tests Sliding Sync handler `get_room_membership_for_user_at_to_token()` to make sure it returns
     the correct list of rooms IDs.
     """
 
@@ -616,7 +620,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         now_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=now_token,
                 to_token=now_token,
@@ -643,7 +647,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_room_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room_token,
                 to_token=after_room_token,
@@ -657,9 +661,11 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[room_id].event_id,
             join_response["event_id"],
         )
+        self.assertEqual(room_id_results[room_id].membership, Membership.JOIN)
         # We should be considered `newly_joined` because we joined during the token
         # range
         self.assertEqual(room_id_results[room_id].newly_joined, True)
+        self.assertEqual(room_id_results[room_id].newly_left, False)
 
     def test_get_already_joined_room(self) -> None:
         """
@@ -676,7 +682,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_room_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room_token,
                 to_token=after_room_token,
@@ -690,8 +696,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[room_id].event_id,
             join_response["event_id"],
         )
+        self.assertEqual(room_id_results[room_id].membership, Membership.JOIN)
         # We should *NOT* be `newly_joined` because we joined before the token range
         self.assertEqual(room_id_results[room_id].newly_joined, False)
+        self.assertEqual(room_id_results[room_id].newly_left, False)
 
     def test_get_invited_banned_knocked_room(self) -> None:
         """
@@ -748,7 +756,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_room_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room_token,
                 to_token=after_room_token,
@@ -770,19 +778,25 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[invited_room_id].event_id,
             invite_response["event_id"],
         )
+        self.assertEqual(room_id_results[invited_room_id].membership, Membership.INVITE)
+        self.assertEqual(room_id_results[invited_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[invited_room_id].newly_left, False)
+
         self.assertEqual(
             room_id_results[ban_room_id].event_id,
             ban_response["event_id"],
         )
+        self.assertEqual(room_id_results[ban_room_id].membership, Membership.BAN)
+        self.assertEqual(room_id_results[ban_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[ban_room_id].newly_left, False)
+
         self.assertEqual(
             room_id_results[knock_room_id].event_id,
             knock_room_membership_state_event.event_id,
         )
-        # We should *NOT* be `newly_joined` because we were not joined at the the time
-        # of the `to_token`.
-        self.assertEqual(room_id_results[invited_room_id].newly_joined, False)
-        self.assertEqual(room_id_results[ban_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[knock_room_id].membership, Membership.KNOCK)
         self.assertEqual(room_id_results[knock_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[knock_room_id].newly_left, False)
 
     def test_get_kicked_room(self) -> None:
         """
@@ -814,7 +828,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_kick_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_kick_token,
                 to_token=after_kick_token,
@@ -828,9 +842,12 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[kick_room_id].event_id,
             kick_response["event_id"],
         )
+        self.assertEqual(room_id_results[kick_room_id].membership, Membership.LEAVE)
+        self.assertNotEqual(room_id_results[kick_room_id].sender, user1_id)
         # We should *NOT* be `newly_joined` because we were not joined at the the time
         # of the `to_token`.
         self.assertEqual(room_id_results[kick_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[kick_room_id].newly_left, False)
 
     def test_forgotten_rooms(self) -> None:
         """
@@ -904,7 +921,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.assertEqual(channel.code, 200, channel.result)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room_forgets,
                 to_token=before_room_forgets,
@@ -914,52 +931,58 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         # We shouldn't see the room because it was forgotten
         self.assertEqual(room_id_results.keys(), set())
 
-    def test_only_newly_left_rooms_show_up(self) -> None:
+    def test_newly_left_rooms(self) -> None:
         """
-        Test that newly_left rooms still show up in the sync response but rooms that
-        were left before the `from_token` don't show up. See condition "2)" comments in
-        the `get_sync_room_ids_for_user` method.
+        Test that newly_left are marked properly
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
 
         # Leave before we calculate the `from_token`
         room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
-        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+        leave_response1 = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         after_room1_token = self.event_sources.get_current_token()
 
         # Leave during the from_token/to_token range (newly_left)
         room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
-        _leave_response2 = self.helper.leave(room_id2, user1_id, tok=user1_tok)
+        leave_response2 = self.helper.leave(room_id2, user1_id, tok=user1_tok)
 
         after_room2_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room2_token,
             )
         )
 
-        # Only the newly_left room should show up
-        self.assertEqual(room_id_results.keys(), {room_id2})
-        # It should be pointing to the latest membership event in the from/to range but
-        # the `event_id` is `None` because we left the room causing the server to leave
-        # the room because no other local users are in it (quirk of the
-        # `current_state_delta_stream` table that we source things from)
+        self.assertEqual(room_id_results.keys(), {room_id1, room_id2})
+
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            leave_response1["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined` or `newly_left` because that happened before
+        # the from/to range
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
+
         self.assertEqual(
             room_id_results[room_id2].event_id,
-            None,  # _leave_response2["event_id"],
+            leave_response2["event_id"],
         )
+        self.assertEqual(room_id_results[room_id2].membership, Membership.LEAVE)
         # We should *NOT* be `newly_joined` because we are instead `newly_left`
         self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, True)
 
     def test_no_joins_after_to_token(self) -> None:
         """
         Rooms we join after the `to_token` should *not* show up. See condition "1b)"
-        comments in the `get_sync_room_ids_for_user()` method.
+        comments in the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -978,7 +1001,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.join(room_id2, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -991,14 +1014,16 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[room_id1].event_id,
             join_response1["event_id"],
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_join_during_range_and_left_room_after_to_token(self) -> None:
         """
         Room still shows up if we left the room but were joined during the
         from_token/to_token. See condition "1a)" comments in the
-        `get_sync_room_ids_for_user()` method.
+        `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1016,7 +1041,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1038,14 +1063,16 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_join_before_range_and_left_room_after_to_token(self) -> None:
         """
         Room still shows up if we left the room but were joined before the `from_token`
         so it should show up. See condition "1a)" comments in the
-        `get_sync_room_ids_for_user()` method.
+        `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1061,7 +1088,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
@@ -1082,14 +1109,16 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should *NOT* be `newly_joined` because we joined before the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_kicked_before_range_and_left_after_to_token(self) -> None:
         """
         Room still shows up if we left the room but were kicked before the `from_token`
         so it should show up. See condition "1a)" comments in the
-        `get_sync_room_ids_for_user()` method.
+        `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1123,7 +1152,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response = self.helper.leave(kick_room_id, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_kick_token,
                 to_token=after_kick_token,
@@ -1146,14 +1175,17 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[kick_room_id].membership, Membership.LEAVE)
+        self.assertNotEqual(room_id_results[kick_room_id].sender, user1_id)
         # We should *NOT* be `newly_joined` because we were kicked
         self.assertEqual(room_id_results[kick_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[kick_room_id].newly_left, False)
 
     def test_newly_left_during_range_and_join_leave_after_to_token(self) -> None:
         """
         Newly left room should show up. But we're also testing that joining and leaving
         after the `to_token` doesn't mess with the results. See condition "2)" and "1a)"
-        comments in the `get_sync_room_ids_for_user()` method.
+        comments in the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1176,7 +1208,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response2 = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1199,14 +1231,17 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
-        # We should *NOT* be `newly_joined` because we left during the token range
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined` because we are actually `newly_left` during
+        # the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, True)
 
     def test_newly_left_during_range_and_join_after_to_token(self) -> None:
         """
         Newly left room should show up. But we're also testing that joining after the
         `to_token` doesn't mess with the results. See condition "2)" and "1b)" comments
-        in the `get_sync_room_ids_for_user()` method.
+        in the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1228,7 +1263,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         join_response2 = self.helper.join(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1250,16 +1285,19 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
-        # We should *NOT* be `newly_joined` because we left during the token range
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined` because we are actually `newly_left` during
+        # the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, True)
 
     def test_no_from_token(self) -> None:
         """
-        Test that if we don't provide a `from_token`, we get all the rooms that we we're
-        joined up to the `to_token`.
+        Test that if we don't provide a `from_token`, we get all the rooms that we had
+        membership in up to the `to_token`.
 
-        Providing `from_token` only really has the effect that it adds `newly_left`
-        rooms to the response.
+        Providing `from_token` only really has the effect that it marks rooms as
+        `newly_left` in the response.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1276,7 +1314,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
 
         # Join and leave the room2 before the `to_token`
         self.helper.join(room_id2, user1_id, tok=user1_tok)
-        self.helper.leave(room_id2, user1_id, tok=user1_tok)
+        leave_response2 = self.helper.leave(room_id2, user1_id, tok=user1_tok)
 
         after_room1_token = self.event_sources.get_current_token()
 
@@ -1284,7 +1322,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.join(room_id2, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=None,
                 to_token=after_room1_token,
@@ -1292,15 +1330,31 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         )
 
         # Only rooms we were joined to before the `to_token` should show up
-        self.assertEqual(room_id_results.keys(), {room_id1})
+        self.assertEqual(room_id_results.keys(), {room_id1, room_id2})
+
+        # Room1
         # It should be pointing to the latest membership event in the from/to range
         self.assertEqual(
             room_id_results[room_id1].event_id,
             join_response1["event_id"],
         )
-        # We should *NOT* be `newly_joined` because there is no `from_token` to
-        # define a "live" range to compare against
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
+        # We should *NOT* be `newly_joined`/`newly_left` because there is no
+        # `from_token` to define a "live" range to compare against
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
+
+        # Room2
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id2].event_id,
+            leave_response2["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id2].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined`/`newly_left` because there is no
+        # `from_token` to define a "live" range to compare against
+        self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, False)
 
     def test_from_token_ahead_of_to_token(self) -> None:
         """
@@ -1319,28 +1373,28 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         room_id3 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
         room_id4 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
 
-        # Join room1 before `before_room_token`
-        join_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
+        # Join room1 before `to_token`
+        join_room1_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
 
-        # Join and leave the room2 before `before_room_token`
-        self.helper.join(room_id2, user1_id, tok=user1_tok)
-        self.helper.leave(room_id2, user1_id, tok=user1_tok)
+        # Join and leave the room2 before `to_token`
+        _join_room2_response1 = self.helper.join(room_id2, user1_id, tok=user1_tok)
+        leave_room2_response1 = self.helper.leave(room_id2, user1_id, tok=user1_tok)
 
         # Note: These are purposely swapped. The `from_token` should come after
         # the `to_token` in this test
         to_token = self.event_sources.get_current_token()
 
-        # Join room2 after `before_room_token`
-        self.helper.join(room_id2, user1_id, tok=user1_tok)
+        # Join room2 after `to_token`
+        _join_room2_response2 = self.helper.join(room_id2, user1_id, tok=user1_tok)
 
         # --------
 
-        # Join room3 after `before_room_token`
-        self.helper.join(room_id3, user1_id, tok=user1_tok)
+        # Join room3 after `to_token`
+        _join_room3_response1 = self.helper.join(room_id3, user1_id, tok=user1_tok)
 
-        # Join and leave the room4 after `before_room_token`
-        self.helper.join(room_id4, user1_id, tok=user1_tok)
-        self.helper.leave(room_id4, user1_id, tok=user1_tok)
+        # Join and leave the room4 after `to_token`
+        _join_room4_response1 = self.helper.join(room_id4, user1_id, tok=user1_tok)
+        _leave_room4_response1 = self.helper.leave(room_id4, user1_id, tok=user1_tok)
 
         # Note: These are purposely swapped. The `from_token` should come after the
         # `to_token` in this test
@@ -1350,31 +1404,59 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.join(room_id4, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=from_token,
                 to_token=to_token,
             )
         )
 
-        # Only rooms we were joined to before the `to_token` should show up
-        #
-        # There won't be any newly_left rooms because the `from_token` is ahead of the
-        # `to_token` and that range will give no membership changes to check.
-        self.assertEqual(room_id_results.keys(), {room_id1})
+        # In the "current" state snapshot, we're joined to all of the rooms but in the
+        # from/to token range...
+        self.assertIncludes(
+            room_id_results.keys(),
+            {
+                # Included because we were joined before both tokens
+                room_id1,
+                # Included because we had membership before the to_token
+                room_id2,
+                # Excluded because we joined after the `to_token`
+                # room_id3,
+                # Excluded because we joined after the `to_token`
+                # room_id4,
+            },
+            exact=True,
+        )
+
+        # Room1
         # It should be pointing to the latest membership event in the from/to range
         self.assertEqual(
             room_id_results[room_id1].event_id,
-            join_response1["event_id"],
+            join_room1_response1["event_id"],
         )
-        # We should *NOT* be `newly_joined` because we joined `room1` before either of the tokens
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
+        # We should *NOT* be `newly_joined`/`newly_left` because we joined `room1`
+        # before either of the tokens
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
+
+        # Room2
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id2].event_id,
+            leave_room2_response1["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id2].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined`/`newly_left` because we joined and left
+        # `room1` before either of the tokens
+        self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, False)
 
     def test_leave_before_range_and_join_leave_after_to_token(self) -> None:
         """
-        Old left room shouldn't show up. But we're also testing that joining and leaving
-        after the `to_token` doesn't mess with the results. See condition "1a)" comments
-        in the `get_sync_room_ids_for_user()` method.
+        Test old left rooms. But we're also testing that joining and leaving after the
+        `to_token` doesn't mess with the results. See condition "1a)" comments in the
+        `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1386,7 +1468,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
         # Join and leave the room before the from/to range
         self.helper.join(room_id1, user1_id, tok=user1_tok)
-        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+        leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         after_room1_token = self.event_sources.get_current_token()
 
@@ -1395,21 +1477,30 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
             )
         )
 
-        # Room shouldn't show up because it was left before the `from_token`
-        self.assertEqual(room_id_results.keys(), set())
+        self.assertEqual(room_id_results.keys(), {room_id1})
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            leave_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined`/`newly_left` because we joined and left
+        # `room1` before either of the tokens
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_leave_before_range_and_join_after_to_token(self) -> None:
         """
-        Old left room shouldn't show up. But we're also testing that joining after the
-        `to_token` doesn't mess with the results. See condition "1b)" comments in the
-        `get_sync_room_ids_for_user()` method.
+        Test old left room. But we're also testing that joining after the `to_token`
+        doesn't mess with the results. See condition "1b)" comments in the
+        `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1421,7 +1512,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
         # Join and leave the room before the from/to range
         self.helper.join(room_id1, user1_id, tok=user1_tok)
-        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+        leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         after_room1_token = self.event_sources.get_current_token()
 
@@ -1429,24 +1520,32 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.join(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
             )
         )
 
-        # Room shouldn't show up because it was left before the `from_token`
-        self.assertEqual(room_id_results.keys(), set())
+        self.assertEqual(room_id_results.keys(), {room_id1})
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            leave_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined`/`newly_left` because we joined and left
+        # `room1` before either of the tokens
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_join_leave_multiple_times_during_range_and_after_to_token(
         self,
     ) -> None:
         """
         Join and leave multiple times shouldn't affect rooms from showing up. It just
-        matters that we were joined or newly_left in the from/to range. But we're also
-        testing that joining and leaving after the `to_token` doesn't mess with the
-        results.
+        matters that we had membership in the from/to range. But we're also testing that
+        joining and leaving after the `to_token` doesn't mess with the results.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1458,7 +1557,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         # We create the room with user2 so the room isn't left with no members when we
         # leave and can still re-join.
         room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
-        # Join, leave, join back to the room before the from/to range
+        # Join, leave, join back to the room during the from/to range
         join_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
         leave_response1 = self.helper.leave(room_id1, user1_id, tok=user1_tok)
         join_response2 = self.helper.join(room_id1, user1_id, tok=user1_tok)
@@ -1471,7 +1570,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response3 = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1496,15 +1595,19 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        # We should *NOT* be `newly_left` because we joined during the token range and
+        # was still joined at the end of the range
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_join_leave_multiple_times_before_range_and_after_to_token(
         self,
     ) -> None:
         """
         Join and leave multiple times before the from/to range shouldn't affect rooms
-        from showing up. It just matters that we were joined or newly_left in the
+        from showing up. It just matters that we had membership in the
         from/to range. But we're also testing that joining and leaving after the
         `to_token` doesn't mess with the results.
         """
@@ -1529,7 +1632,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response3 = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
@@ -1554,8 +1657,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should *NOT* be `newly_joined` because we joined before the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_invite_before_range_and_join_leave_after_to_token(
         self,
@@ -1563,7 +1668,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         """
         Make it look like we joined after the token range but we were invited before the
         from/to range so the room should still show up. See condition "1a)" comments in
-        the `get_sync_room_ids_for_user()` method.
+        the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1586,7 +1691,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
@@ -1608,9 +1713,11 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.INVITE)
         # We should *NOT* be `newly_joined` because we were only invited before the
         # token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_join_and_display_name_changes_in_token_range(
         self,
@@ -1658,7 +1765,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         )
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1684,8 +1791,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_display_name_changes_in_token_range(
         self,
@@ -1721,7 +1830,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_change1_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_change1_token,
@@ -1744,8 +1853,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should *NOT* be `newly_joined` because we joined before the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_display_name_changes_before_and_after_token_range(
         self,
@@ -1791,7 +1902,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         )
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_room1_token,
@@ -1817,8 +1928,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should *NOT* be `newly_joined` because we joined before the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_display_name_changes_leave_after_token_range(
         self,
@@ -1828,7 +1941,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         if there are multiple `join` membership events in a row indicating
         `displayname`/`avatar_url` updates and we leave after the `to_token`.
 
-        See condition "1a)" comments in the `get_sync_room_ids_for_user()` method.
+        See condition "1a)" comments in the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1871,7 +1984,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.leave(room_id1, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1897,8 +2010,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_display_name_changes_join_after_token_range(
         self,
@@ -1908,7 +2023,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         indicating `displayname`/`avatar_url` updates doesn't affect the results (we
         joined after the token range so it shouldn't show up)
 
-        See condition "1b)" comments in the `get_sync_room_ids_for_user()` method.
+        See condition "1b)" comments in the `get_room_membership_for_user_at_to_token()` method.
         """
         user1_id = self.register_user("user1", "pass")
         user1_tok = self.login(user1_id, "pass")
@@ -1937,7 +2052,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         )
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -1973,7 +2088,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_more_changes_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=after_room1_token,
                 to_token=after_more_changes_token,
@@ -1987,9 +2102,11 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
             room_id_results[room_id1].event_id,
             join_response2["event_id"],
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be considered `newly_joined` because there is some non-join event in
         # between our latest join event.
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_newly_joined_only_joins_during_token_range(
         self,
@@ -2036,7 +2153,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         after_room1_token = self.event_sources.get_current_token()
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room1_token,
                 to_token=after_room1_token,
@@ -2062,8 +2179,10 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
                 }
             ),
         )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
         # We should be `newly_joined` because we first joined during the token range
         self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
 
     def test_multiple_rooms_are_not_confused(
         self,
@@ -2086,16 +2205,18 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
 
         # Invited and left the room before the token
         self.helper.invite(room_id1, src=user2_id, targ=user1_id, tok=user2_tok)
-        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+        leave_room1_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
         # Invited to room2
-        self.helper.invite(room_id2, src=user2_id, targ=user1_id, tok=user2_tok)
+        invite_room2_response = self.helper.invite(
+            room_id2, src=user2_id, targ=user1_id, tok=user2_tok
+        )
 
         before_room3_token = self.event_sources.get_current_token()
 
         # Invited and left room3 during the from/to range
         room_id3 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=True)
         self.helper.invite(room_id3, src=user2_id, targ=user1_id, tok=user2_tok)
-        self.helper.leave(room_id3, user1_id, tok=user1_tok)
+        leave_room3_response = self.helper.leave(room_id3, user1_id, tok=user1_tok)
 
         after_room3_token = self.event_sources.get_current_token()
 
@@ -2108,7 +2229,7 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.helper.leave(room_id3, user1_id, tok=user1_tok)
 
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_room3_token,
                 to_token=after_room3_token,
@@ -2118,19 +2239,158 @@ class GetSyncRoomIdsForUserTestCase(HomeserverTestCase):
         self.assertEqual(
             room_id_results.keys(),
             {
-                # `room_id1` shouldn't show up because we left before the from/to range
-                #
-                # Room should show up because we were invited before the from/to range
+                # Left before the from/to range
+                room_id1,
+                # Invited before the from/to range
                 room_id2,
-                # Room should show up because it was newly_left during the from/to range
+                # `newly_left` during the from/to range
                 room_id3,
             },
         )
 
+        # Room1
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            leave_room1_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined`/`newly_left` because we were invited and left
+        # before the token range
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
+
+        # Room2
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id2].event_id,
+            invite_room2_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id2].membership, Membership.INVITE)
+        # We should *NOT* be `newly_joined`/`newly_left` because we were invited before
+        # the token range
+        self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, False)
+
+        # Room3
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id3].event_id,
+            leave_room3_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id3].membership, Membership.LEAVE)
+        # We should be `newly_left` because we were invited and left during
+        # the token range
+        self.assertEqual(room_id_results[room_id3].newly_joined, False)
+        self.assertEqual(room_id_results[room_id3].newly_left, True)
+
+    def test_state_reset(self) -> None:
+        """
+        Test a state reset scenario where the user gets removed from the room (when
+        there is no corresponding leave event)
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        # The room where the state reset will happen
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        join_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
+
+        # Join another room so we don't hit the short-circuit and return early if they
+        # have no room membership
+        room_id2 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        self.helper.join(room_id2, user1_id, tok=user1_tok)
+
+        before_reset_token = self.event_sources.get_current_token()
+
+        # Send another state event to make a position for the state reset to happen at
+        dummy_state_response = self.helper.send_state(
+            room_id1,
+            event_type="foobarbaz",
+            state_key="",
+            body={"foo": "bar"},
+            tok=user2_tok,
+        )
+        dummy_state_pos = self.get_success(
+            self.store.get_position_for_event(dummy_state_response["event_id"])
+        )
+
+        # Mock a state reset removing the membership for user1 in the current state
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="current_state_events",
+                keyvalues={
+                    "room_id": room_id1,
+                    "type": EventTypes.Member,
+                    "state_key": user1_id,
+                },
+                desc="state reset user in current_state_events",
+            )
+        )
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="local_current_membership",
+                keyvalues={
+                    "room_id": room_id1,
+                    "user_id": user1_id,
+                },
+                desc="state reset user in local_current_membership",
+            )
+        )
+        self.get_success(
+            self.store.db_pool.simple_insert(
+                table="current_state_delta_stream",
+                values={
+                    "stream_id": dummy_state_pos.stream,
+                    "room_id": room_id1,
+                    "type": EventTypes.Member,
+                    "state_key": user1_id,
+                    "event_id": None,
+                    "prev_event_id": join_response1["event_id"],
+                    "instance_name": dummy_state_pos.instance_name,
+                },
+                desc="state reset user in current_state_delta_stream",
+            )
+        )
+
+        # Manually bust the cache since we we're just manually messing with the database
+        # and not causing an actual state reset.
+        self.store._membership_stream_cache.entity_has_changed(
+            user1_id, dummy_state_pos.stream
+        )
+
+        after_reset_token = self.event_sources.get_current_token()
+
+        # The function under test
+        room_id_results = self.get_success(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
+                UserID.from_string(user1_id),
+                from_token=before_reset_token,
+                to_token=after_reset_token,
+            )
+        )
 
-class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
+        # Room1 should show up because it was `newly_left` via state reset during the from/to range
+        self.assertEqual(room_id_results.keys(), {room_id1, room_id2})
+        # It should be pointing to no event because we were removed from the room
+        # without a corresponding leave event
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            None,
+        )
+        # State reset caused us to leave the room and there is no corresponding leave event
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined` because we joined before the token range
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        # We should be `newly_left` because we were removed via state reset during the from/to range
+        self.assertEqual(room_id_results[room_id1].newly_left, True)
+
+
+class GetRoomMembershipForUserAtToTokenShardTestCase(BaseMultiWorkerStreamTestCase):
     """
-    Tests Sliding Sync handler `get_sync_room_ids_for_user()` to make sure it works with
+    Tests Sliding Sync handler `get_room_membership_for_user_at_to_token()` to make sure it works with
     sharded event stream_writers enabled
     """
 
@@ -2189,7 +2449,7 @@ class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
 
         We then send some events to advance the stream positions of worker1 and worker3
         but worker2 is lagging behind because it's stuck. We are specifically testing
-        that `get_sync_room_ids_for_user(from_token=xxx, to_token=xxx)` should work
+        that `get_room_membership_for_user_at_to_token(from_token=xxx, to_token=xxx)` should work
         correctly in these adverse conditions.
         """
         user1_id = self.register_user("user1", "pass")
@@ -2228,7 +2488,7 @@ class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
         join_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
         join_response2 = self.helper.join(room_id2, user1_id, tok=user1_tok)
         # Leave room2
-        self.helper.leave(room_id2, user1_id, tok=user1_tok)
+        leave_room2_response = self.helper.leave(room_id2, user1_id, tok=user1_tok)
         join_response3 = self.helper.join(room_id3, user1_id, tok=user1_tok)
         # Leave room3
         self.helper.leave(room_id3, user1_id, tok=user1_tok)
@@ -2265,7 +2525,7 @@ class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
         # For room_id1/worker1: leave and join the room to advance the stream position
         # and generate membership changes.
         self.helper.leave(room_id1, user1_id, tok=user1_tok)
-        self.helper.join(room_id1, user1_id, tok=user1_tok)
+        join_room1_response = self.helper.join(room_id1, user1_id, tok=user1_tok)
         # For room_id2/worker2: which is currently stuck, join the room.
         join_on_worker2_response = self.helper.join(room_id2, user1_id, tok=user1_tok)
         # For room_id3/worker3: leave and join the room to advance the stream position
@@ -2319,7 +2579,7 @@ class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
 
         # The function under test
         room_id_results = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
                 UserID.from_string(user1_id),
                 from_token=before_stuck_activity_token,
                 to_token=stuck_activity_token,
@@ -2330,18 +2590,411 @@ class GetSyncRoomIdsForUserEventShardTestCase(BaseMultiWorkerStreamTestCase):
             room_id_results.keys(),
             {
                 room_id1,
-                # room_id2 shouldn't show up because we left before the from/to range
-                # and the join event during the range happened while worker2 was stuck.
-                # This means that from the perspective of the master, where the
-                # `stuck_activity_token` is generated, the stream position for worker2
-                # wasn't advanced to the join yet. Looking at the `instance_map`, the
-                # join technically comes after `stuck_activity_token``.
-                #
-                # room_id2,
+                room_id2,
                 room_id3,
             },
         )
 
+        # Room1
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            join_room1_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id1].membership, Membership.JOIN)
+        # We should be `newly_joined` because we joined during the token range
+        self.assertEqual(room_id_results[room_id1].newly_joined, True)
+        self.assertEqual(room_id_results[room_id1].newly_left, False)
+
+        # Room2
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id2].event_id,
+            leave_room2_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id2].membership, Membership.LEAVE)
+        # room_id2 should *NOT* be considered `newly_left` because we left before the
+        # from/to range and the join event during the range happened while worker2 was
+        # stuck. This means that from the perspective of the master, where the
+        # `stuck_activity_token` is generated, the stream position for worker2 wasn't
+        # advanced to the join yet. Looking at the `instance_map`, the join technically
+        # comes after `stuck_activity_token`.
+        self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, False)
+
+        # Room3
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[room_id3].event_id,
+            join_on_worker3_response["event_id"],
+        )
+        self.assertEqual(room_id_results[room_id3].membership, Membership.JOIN)
+        # We should be `newly_joined` because we joined during the token range
+        self.assertEqual(room_id_results[room_id3].newly_joined, True)
+        self.assertEqual(room_id_results[room_id3].newly_left, False)
+
+
+class FilterRoomsRelevantForSyncTestCase(HomeserverTestCase):
+    """
+    Tests Sliding Sync handler `filter_rooms_relevant_for_sync()` to make sure it returns
+    the correct list of rooms IDs.
+    """
+
+    servlets = [
+        admin.register_servlets,
+        knock.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+    ]
+
+    def default_config(self) -> JsonDict:
+        config = super().default_config()
+        # Enable sliding sync
+        config["experimental_features"] = {"msc3575_enabled": True}
+        return config
+
+    def prepare(self, reactor: MemoryReactor, clock: Clock, hs: HomeServer) -> None:
+        self.sliding_sync_handler = self.hs.get_sliding_sync_handler()
+        self.store = self.hs.get_datastores().main
+        self.event_sources = hs.get_event_sources()
+        self.storage_controllers = hs.get_storage_controllers()
+
+    def _get_sync_room_ids_for_user(
+        self,
+        user: UserID,
+        to_token: StreamToken,
+        from_token: Optional[StreamToken],
+    ) -> Dict[str, _RoomMembershipForUser]:
+        """
+        Get the rooms the user should be syncing with
+        """
+        room_membership_for_user_map = self.get_success(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
+                user=user,
+                from_token=from_token,
+                to_token=to_token,
+            )
+        )
+        filtered_sync_room_map = self.get_success(
+            self.sliding_sync_handler.filter_rooms_relevant_for_sync(
+                user=user,
+                room_membership_for_user_map=room_membership_for_user_map,
+            )
+        )
+
+        return filtered_sync_room_map
+
+    def test_no_rooms(self) -> None:
+        """
+        Test when the user has never joined any rooms before
+        """
+        user1_id = self.register_user("user1", "pass")
+        # user1_tok = self.login(user1_id, "pass")
+
+        now_token = self.event_sources.get_current_token()
+
+        room_id_results = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=now_token,
+            to_token=now_token,
+        )
+
+        self.assertEqual(room_id_results.keys(), set())
+
+    def test_basic_rooms(self) -> None:
+        """
+        Test that rooms that the user is joined to, invited to, banned from, and knocked
+        on show up.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        before_room_token = self.event_sources.get_current_token()
+
+        join_room_id = self.helper.create_room_as(user2_id, tok=user2_tok)
+        join_response = self.helper.join(join_room_id, user1_id, tok=user1_tok)
+
+        # Setup the invited room (user2 invites user1 to the room)
+        invited_room_id = self.helper.create_room_as(user2_id, tok=user2_tok)
+        invite_response = self.helper.invite(
+            invited_room_id, targ=user1_id, tok=user2_tok
+        )
+
+        # Setup the ban room (user2 bans user1 from the room)
+        ban_room_id = self.helper.create_room_as(
+            user2_id, tok=user2_tok, is_public=True
+        )
+        self.helper.join(ban_room_id, user1_id, tok=user1_tok)
+        ban_response = self.helper.ban(
+            ban_room_id, src=user2_id, targ=user1_id, tok=user2_tok
+        )
+
+        # Setup the knock room (user1 knocks on the room)
+        knock_room_id = self.helper.create_room_as(
+            user2_id, tok=user2_tok, room_version=RoomVersions.V7.identifier
+        )
+        self.helper.send_state(
+            knock_room_id,
+            EventTypes.JoinRules,
+            {"join_rule": JoinRules.KNOCK},
+            tok=user2_tok,
+        )
+        # User1 knocks on the room
+        knock_channel = self.make_request(
+            "POST",
+            "/_matrix/client/r0/knock/%s" % (knock_room_id,),
+            b"{}",
+            user1_tok,
+        )
+        self.assertEqual(knock_channel.code, 200, knock_channel.result)
+        knock_room_membership_state_event = self.get_success(
+            self.storage_controllers.state.get_current_state_event(
+                knock_room_id, EventTypes.Member, user1_id
+            )
+        )
+        assert knock_room_membership_state_event is not None
+
+        after_room_token = self.event_sources.get_current_token()
+
+        room_id_results = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=before_room_token,
+            to_token=after_room_token,
+        )
+
+        # Ensure that the invited, ban, and knock rooms show up
+        self.assertEqual(
+            room_id_results.keys(),
+            {
+                join_room_id,
+                invited_room_id,
+                ban_room_id,
+                knock_room_id,
+            },
+        )
+        # It should be pointing to the the respective membership event (latest
+        # membership event in the from/to range)
+        self.assertEqual(
+            room_id_results[join_room_id].event_id,
+            join_response["event_id"],
+        )
+        self.assertEqual(room_id_results[join_room_id].membership, Membership.JOIN)
+        self.assertEqual(room_id_results[join_room_id].newly_joined, True)
+        self.assertEqual(room_id_results[join_room_id].newly_left, False)
+
+        self.assertEqual(
+            room_id_results[invited_room_id].event_id,
+            invite_response["event_id"],
+        )
+        self.assertEqual(room_id_results[invited_room_id].membership, Membership.INVITE)
+        self.assertEqual(room_id_results[invited_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[invited_room_id].newly_left, False)
+
+        self.assertEqual(
+            room_id_results[ban_room_id].event_id,
+            ban_response["event_id"],
+        )
+        self.assertEqual(room_id_results[ban_room_id].membership, Membership.BAN)
+        self.assertEqual(room_id_results[ban_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[ban_room_id].newly_left, False)
+
+        self.assertEqual(
+            room_id_results[knock_room_id].event_id,
+            knock_room_membership_state_event.event_id,
+        )
+        self.assertEqual(room_id_results[knock_room_id].membership, Membership.KNOCK)
+        self.assertEqual(room_id_results[knock_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[knock_room_id].newly_left, False)
+
+    def test_only_newly_left_rooms_show_up(self) -> None:
+        """
+        Test that `newly_left` rooms still show up in the sync response but rooms that
+        were left before the `from_token` don't show up. See condition "2)" comments in
+        the `get_room_membership_for_user_at_to_token()` method.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+
+        # Leave before we calculate the `from_token`
+        room_id1 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        self.helper.leave(room_id1, user1_id, tok=user1_tok)
+
+        after_room1_token = self.event_sources.get_current_token()
+
+        # Leave during the from_token/to_token range (newly_left)
+        room_id2 = self.helper.create_room_as(user1_id, tok=user1_tok)
+        _leave_response2 = self.helper.leave(room_id2, user1_id, tok=user1_tok)
+
+        after_room2_token = self.event_sources.get_current_token()
+
+        room_id_results = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=after_room1_token,
+            to_token=after_room2_token,
+        )
+
+        # Only the `newly_left` room should show up
+        self.assertEqual(room_id_results.keys(), {room_id2})
+        self.assertEqual(
+            room_id_results[room_id2].event_id,
+            _leave_response2["event_id"],
+        )
+        # We should *NOT* be `newly_joined` because we are instead `newly_left`
+        self.assertEqual(room_id_results[room_id2].newly_joined, False)
+        self.assertEqual(room_id_results[room_id2].newly_left, True)
+
+    def test_get_kicked_room(self) -> None:
+        """
+        Test that a room that the user was kicked from still shows up. When the user
+        comes back to their client, they should see that they were kicked.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        # Setup the kick room (user2 kicks user1 from the room)
+        kick_room_id = self.helper.create_room_as(
+            user2_id, tok=user2_tok, is_public=True
+        )
+        self.helper.join(kick_room_id, user1_id, tok=user1_tok)
+        # Kick user1 from the room
+        kick_response = self.helper.change_membership(
+            room=kick_room_id,
+            src=user2_id,
+            targ=user1_id,
+            tok=user2_tok,
+            membership=Membership.LEAVE,
+            extra_data={
+                "reason": "Bad manners",
+            },
+        )
+
+        after_kick_token = self.event_sources.get_current_token()
+
+        room_id_results = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=after_kick_token,
+            to_token=after_kick_token,
+        )
+
+        # The kicked room should show up
+        self.assertEqual(room_id_results.keys(), {kick_room_id})
+        # It should be pointing to the latest membership event in the from/to range
+        self.assertEqual(
+            room_id_results[kick_room_id].event_id,
+            kick_response["event_id"],
+        )
+        self.assertEqual(room_id_results[kick_room_id].membership, Membership.LEAVE)
+        self.assertNotEqual(room_id_results[kick_room_id].sender, user1_id)
+        # We should *NOT* be `newly_joined` because we were not joined at the the time
+        # of the `to_token`.
+        self.assertEqual(room_id_results[kick_room_id].newly_joined, False)
+        self.assertEqual(room_id_results[kick_room_id].newly_left, False)
+
+    def test_state_reset(self) -> None:
+        """
+        Test a state reset scenario where the user gets removed from the room (when
+        there is no corresponding leave event)
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        # The room where the state reset will happen
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        join_response1 = self.helper.join(room_id1, user1_id, tok=user1_tok)
+
+        # Join another room so we don't hit the short-circuit and return early if they
+        # have no room membership
+        room_id2 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        self.helper.join(room_id2, user1_id, tok=user1_tok)
+
+        before_reset_token = self.event_sources.get_current_token()
+
+        # Send another state event to make a position for the state reset to happen at
+        dummy_state_response = self.helper.send_state(
+            room_id1,
+            event_type="foobarbaz",
+            state_key="",
+            body={"foo": "bar"},
+            tok=user2_tok,
+        )
+        dummy_state_pos = self.get_success(
+            self.store.get_position_for_event(dummy_state_response["event_id"])
+        )
+
+        # Mock a state reset removing the membership for user1 in the current state
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="current_state_events",
+                keyvalues={
+                    "room_id": room_id1,
+                    "type": EventTypes.Member,
+                    "state_key": user1_id,
+                },
+                desc="state reset user in current_state_events",
+            )
+        )
+        self.get_success(
+            self.store.db_pool.simple_delete(
+                table="local_current_membership",
+                keyvalues={
+                    "room_id": room_id1,
+                    "user_id": user1_id,
+                },
+                desc="state reset user in local_current_membership",
+            )
+        )
+        self.get_success(
+            self.store.db_pool.simple_insert(
+                table="current_state_delta_stream",
+                values={
+                    "stream_id": dummy_state_pos.stream,
+                    "room_id": room_id1,
+                    "type": EventTypes.Member,
+                    "state_key": user1_id,
+                    "event_id": None,
+                    "prev_event_id": join_response1["event_id"],
+                    "instance_name": dummy_state_pos.instance_name,
+                },
+                desc="state reset user in current_state_delta_stream",
+            )
+        )
+
+        # Manually bust the cache since we we're just manually messing with the database
+        # and not causing an actual state reset.
+        self.store._membership_stream_cache.entity_has_changed(
+            user1_id, dummy_state_pos.stream
+        )
+
+        after_reset_token = self.event_sources.get_current_token()
+
+        # The function under test
+        room_id_results = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=before_reset_token,
+            to_token=after_reset_token,
+        )
+
+        # Room1 should show up because it was `newly_left` via state reset during the from/to range
+        self.assertEqual(room_id_results.keys(), {room_id1, room_id2})
+        # It should be pointing to no event because we were removed from the room
+        # without a corresponding leave event
+        self.assertEqual(
+            room_id_results[room_id1].event_id,
+            None,
+        )
+        # State reset caused us to leave the room and there is no corresponding leave event
+        self.assertEqual(room_id_results[room_id1].membership, Membership.LEAVE)
+        # We should *NOT* be `newly_joined` because we joined before the token range
+        self.assertEqual(room_id_results[room_id1].newly_joined, False)
+        # We should be `newly_left` because we were removed via state reset during the from/to range
+        self.assertEqual(room_id_results[room_id1].newly_left, True)
+
 
 class FilterRoomsTestCase(HomeserverTestCase):
     """
@@ -2367,6 +3020,31 @@ class FilterRoomsTestCase(HomeserverTestCase):
         self.store = self.hs.get_datastores().main
         self.event_sources = hs.get_event_sources()
 
+    def _get_sync_room_ids_for_user(
+        self,
+        user: UserID,
+        to_token: StreamToken,
+        from_token: Optional[StreamToken],
+    ) -> Dict[str, _RoomMembershipForUser]:
+        """
+        Get the rooms the user should be syncing with
+        """
+        room_membership_for_user_map = self.get_success(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
+                user=user,
+                from_token=from_token,
+                to_token=to_token,
+            )
+        )
+        filtered_sync_room_map = self.get_success(
+            self.sliding_sync_handler.filter_rooms_relevant_for_sync(
+                user=user,
+                room_membership_for_user_map=room_membership_for_user_map,
+            )
+        )
+
+        return filtered_sync_room_map
+
     def _create_dm_room(
         self,
         inviter_user_id: str,
@@ -2438,12 +3116,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Try with `is_dm=True`
@@ -2496,12 +3172,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Try with `is_encrypted=True`
@@ -2552,12 +3226,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Try with `is_invite=True`
@@ -2621,12 +3293,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Try finding only normal rooms
@@ -2714,12 +3384,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Try finding *NOT* normal rooms
@@ -2838,12 +3506,10 @@ class FilterRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         filtered_room_map = self.get_success(
@@ -2884,6 +3550,31 @@ class SortRoomsTestCase(HomeserverTestCase):
         self.store = self.hs.get_datastores().main
         self.event_sources = hs.get_event_sources()
 
+    def _get_sync_room_ids_for_user(
+        self,
+        user: UserID,
+        to_token: StreamToken,
+        from_token: Optional[StreamToken],
+    ) -> Dict[str, _RoomMembershipForUser]:
+        """
+        Get the rooms the user should be syncing with
+        """
+        room_membership_for_user_map = self.get_success(
+            self.sliding_sync_handler.get_room_membership_for_user_at_to_token(
+                user=user,
+                from_token=from_token,
+                to_token=to_token,
+            )
+        )
+        filtered_sync_room_map = self.get_success(
+            self.sliding_sync_handler.filter_rooms_relevant_for_sync(
+                user=user,
+                room_membership_for_user_map=room_membership_for_user_map,
+            )
+        )
+
+        return filtered_sync_room_map
+
     def test_sort_activity_basic(self) -> None:
         """
         Rooms with newer activity are sorted first.
@@ -2903,12 +3594,10 @@ class SortRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Sort the rooms (what we're testing)
@@ -2986,12 +3675,10 @@ class SortRoomsTestCase(HomeserverTestCase):
         self.helper.send(room_id3, "activity in room3", tok=user2_tok)
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=before_rooms_token,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=before_rooms_token,
+            to_token=after_rooms_token,
         )
 
         # Sort the rooms (what we're testing)
@@ -3052,12 +3739,10 @@ class SortRoomsTestCase(HomeserverTestCase):
         after_rooms_token = self.event_sources.get_current_token()
 
         # Get the rooms the user should be syncing with
-        sync_room_map = self.get_success(
-            self.sliding_sync_handler.get_sync_room_ids_for_user(
-                UserID.from_string(user1_id),
-                from_token=None,
-                to_token=after_rooms_token,
-            )
+        sync_room_map = self._get_sync_room_ids_for_user(
+            UserID.from_string(user1_id),
+            from_token=None,
+            to_token=after_rooms_token,
         )
 
         # Sort the rooms (what we're testing)
diff --git a/tests/rest/client/test_sync.py b/tests/rest/client/test_sync.py
index 4236812db5..f5d57e689c 100644
--- a/tests/rest/client/test_sync.py
+++ b/tests/rest/client/test_sync.py
@@ -20,7 +20,8 @@
 #
 import json
 import logging
-from typing import AbstractSet, Any, Dict, Iterable, List, Optional
+from http import HTTPStatus
+from typing import Any, Dict, Iterable, List
 
 from parameterized import parameterized, parameterized_class
 
@@ -1259,7 +1260,7 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
         exact: bool = False,
     ) -> None:
         """
-        Wrapper around `_assertIncludes` to give slightly better looking diff error
+        Wrapper around `assertIncludes` to give slightly better looking diff error
         messages that include some context "$event_id (type, state_key)".
 
         Args:
@@ -1275,7 +1276,7 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
         for event in actual_required_state:
             assert isinstance(event, dict)
 
-        self._assertIncludes(
+        self.assertIncludes(
             {
                 f'{event["event_id"]} ("{event["type"]}", "{event["state_key"]}")'
                 for event in actual_required_state
@@ -1289,56 +1290,6 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
             message=str(actual_required_state),
         )
 
-    def _assertIncludes(
-        self,
-        actual_items: AbstractSet[str],
-        expected_items: AbstractSet[str],
-        exact: bool = False,
-        message: Optional[str] = None,
-    ) -> None:
-        """
-        Assert that all of the `expected_items` are included in the `actual_items`.
-
-        This assert could also be called `assertContains`, `assertItemsInSet`
-
-        Args:
-            actual_items: The container
-            expected_items: The items to check for in the container
-            exact: Whether the actual state should be exactly equal to the expected
-                state (no extras).
-            message: Optional message to include in the failure message.
-        """
-        # Check that each set has the same items
-        if exact and actual_items == expected_items:
-            return
-        # Check for a superset
-        elif not exact and actual_items >= expected_items:
-            return
-
-        expected_lines: List[str] = []
-        for expected_item in expected_items:
-            is_expected_in_actual = expected_item in actual_items
-            expected_lines.append(
-                "{}  {}".format(" " if is_expected_in_actual else "?", expected_item)
-            )
-
-        actual_lines: List[str] = []
-        for actual_item in actual_items:
-            is_actual_in_expected = actual_item in expected_items
-            actual_lines.append(
-                "{}  {}".format("+" if is_actual_in_expected else " ", actual_item)
-            )
-
-        newline = "\n"
-        expected_string = f"Expected items to be in actual ('?' = missing expected items):\n {{\n{newline.join(expected_lines)}\n }}"
-        actual_string = f"Actual ('+' = found expected items):\n {{\n{newline.join(actual_lines)}\n }}"
-        first_message = (
-            "Items must match exactly" if exact else "Some expected items are missing."
-        )
-        diff_message = f"{first_message}\n{expected_string}\n{actual_string}"
-
-        self.fail(f"{diff_message}\n{message}")
-
     def _add_new_dm_to_global_account_data(
         self, source_user_id: str, target_user_id: str, target_room_id: str
     ) -> None:
@@ -3868,6 +3819,13 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
             body={"foo": "bar"},
             tok=user2_tok,
         )
+        self.helper.send_state(
+            room_id1,
+            event_type="org.matrix.bar_state",
+            state_key="",
+            body={"bar": "qux"},
+            tok=user2_tok,
+        )
 
         # Make the Sliding Sync request with wildcards for the `state_key`
         channel = self.make_request(
@@ -3891,16 +3849,13 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
                         ],
                         "timeline_limit": 0,
                     },
-                }
-                # TODO: Room subscription should also combine with the `required_state`
-                # "room_subscriptions": {
-                #     room_id1: {
-                #         "required_state": [
-                #             ["org.matrix.bar_state", ""]
-                #         ],
-                #         "timeline_limit": 0,
-                #     }
-                # }
+                },
+                "room_subscriptions": {
+                    room_id1: {
+                        "required_state": [["org.matrix.bar_state", ""]],
+                        "timeline_limit": 0,
+                    }
+                },
             },
             access_token=user1_tok,
         )
@@ -3917,6 +3872,7 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
                 state_map[(EventTypes.Member, user1_id)],
                 state_map[(EventTypes.Member, user2_id)],
                 state_map[("org.matrix.foo_state", "")],
+                state_map[("org.matrix.bar_state", "")],
             },
             exact=True,
         )
@@ -4009,6 +3965,271 @@ class SlidingSyncTestCase(unittest.HomeserverTestCase):
             channel.json_body["lists"]["foo-list"],
         )
 
+    def test_room_subscriptions_with_join_membership(self) -> None:
+        """
+        Test `room_subscriptions` with a joined room should give us timeline and current
+        state events.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        join_response = self.helper.join(room_id1, user1_id, tok=user1_tok)
+
+        # Make the Sliding Sync request with just the room subscription
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint,
+            {
+                "room_subscriptions": {
+                    room_id1: {
+                        "required_state": [
+                            [EventTypes.Create, ""],
+                        ],
+                        "timeline_limit": 1,
+                    }
+                },
+            },
+            access_token=user1_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        state_map = self.get_success(
+            self.storage_controllers.state.get_current_state(room_id1)
+        )
+
+        # We should see some state
+        self._assertRequiredStateIncludes(
+            channel.json_body["rooms"][room_id1]["required_state"],
+            {
+                state_map[(EventTypes.Create, "")],
+            },
+            exact=True,
+        )
+        self.assertIsNone(channel.json_body["rooms"][room_id1].get("invite_state"))
+
+        # We should see some events
+        self.assertEqual(
+            [
+                event["event_id"]
+                for event in channel.json_body["rooms"][room_id1]["timeline"]
+            ],
+            [
+                join_response["event_id"],
+            ],
+            channel.json_body["rooms"][room_id1]["timeline"],
+        )
+        # No "live" events in an initial sync (no `from_token` to define the "live"
+        # range)
+        self.assertEqual(
+            channel.json_body["rooms"][room_id1]["num_live"],
+            0,
+            channel.json_body["rooms"][room_id1],
+        )
+        # There are more events to paginate to
+        self.assertEqual(
+            channel.json_body["rooms"][room_id1]["limited"],
+            True,
+            channel.json_body["rooms"][room_id1],
+        )
+
+    def test_room_subscriptions_with_leave_membership(self) -> None:
+        """
+        Test `room_subscriptions` with a leave room should give us timeline and state
+        events up to the leave event.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok)
+        self.helper.send_state(
+            room_id1,
+            event_type="org.matrix.foo_state",
+            state_key="",
+            body={"foo": "bar"},
+            tok=user2_tok,
+        )
+
+        join_response = self.helper.join(room_id1, user1_id, tok=user1_tok)
+        leave_response = self.helper.leave(room_id1, user1_id, tok=user1_tok)
+
+        state_map = self.get_success(
+            self.storage_controllers.state.get_current_state(room_id1)
+        )
+
+        # Send some events after user1 leaves
+        self.helper.send(room_id1, "activity after leave", tok=user2_tok)
+        # Update state after user1 leaves
+        self.helper.send_state(
+            room_id1,
+            event_type="org.matrix.foo_state",
+            state_key="",
+            body={"foo": "qux"},
+            tok=user2_tok,
+        )
+
+        # Make the Sliding Sync request with just the room subscription
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint,
+            {
+                "room_subscriptions": {
+                    room_id1: {
+                        "required_state": [
+                            ["org.matrix.foo_state", ""],
+                        ],
+                        "timeline_limit": 2,
+                    }
+                },
+            },
+            access_token=user1_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # We should see the state at the time of the leave
+        self._assertRequiredStateIncludes(
+            channel.json_body["rooms"][room_id1]["required_state"],
+            {
+                state_map[("org.matrix.foo_state", "")],
+            },
+            exact=True,
+        )
+        self.assertIsNone(channel.json_body["rooms"][room_id1].get("invite_state"))
+
+        # We should see some before we left (nothing after)
+        self.assertEqual(
+            [
+                event["event_id"]
+                for event in channel.json_body["rooms"][room_id1]["timeline"]
+            ],
+            [
+                join_response["event_id"],
+                leave_response["event_id"],
+            ],
+            channel.json_body["rooms"][room_id1]["timeline"],
+        )
+        # No "live" events in an initial sync (no `from_token` to define the "live"
+        # range)
+        self.assertEqual(
+            channel.json_body["rooms"][room_id1]["num_live"],
+            0,
+            channel.json_body["rooms"][room_id1],
+        )
+        # There are more events to paginate to
+        self.assertEqual(
+            channel.json_body["rooms"][room_id1]["limited"],
+            True,
+            channel.json_body["rooms"][room_id1],
+        )
+
+    def test_room_subscriptions_no_leak_private_room(self) -> None:
+        """
+        Test `room_subscriptions` with a private room we have never been in should not
+        leak any data to the user.
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        room_id1 = self.helper.create_room_as(user2_id, tok=user2_tok, is_public=False)
+
+        # We should not be able to join the private room
+        self.helper.join(
+            room_id1, user1_id, tok=user1_tok, expect_code=HTTPStatus.FORBIDDEN
+        )
+
+        # Make the Sliding Sync request with just the room subscription
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint,
+            {
+                "room_subscriptions": {
+                    room_id1: {
+                        "required_state": [
+                            [EventTypes.Create, ""],
+                        ],
+                        "timeline_limit": 1,
+                    }
+                },
+            },
+            access_token=user1_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # We should not see the room at all (we're not in it)
+        self.assertIsNone(
+            channel.json_body["rooms"].get(room_id1), channel.json_body["rooms"]
+        )
+
+    def test_room_subscriptions_world_readable(self) -> None:
+        """
+        Test `room_subscriptions` with a room that has `world_readable` history visibility
+
+        FIXME: We should be able to see the room timeline and state
+        """
+        user1_id = self.register_user("user1", "pass")
+        user1_tok = self.login(user1_id, "pass")
+        user2_id = self.register_user("user2", "pass")
+        user2_tok = self.login(user2_id, "pass")
+
+        # Create a room with `world_readable` history visibility
+        room_id1 = self.helper.create_room_as(
+            user2_id,
+            tok=user2_tok,
+            extra_content={
+                "preset": "public_chat",
+                "initial_state": [
+                    {
+                        "content": {
+                            "history_visibility": HistoryVisibility.WORLD_READABLE
+                        },
+                        "state_key": "",
+                        "type": EventTypes.RoomHistoryVisibility,
+                    }
+                ],
+            },
+        )
+        # Ensure we're testing with a room with `world_readable` history visibility
+        # which means events are visible to anyone even without membership.
+        history_visibility_response = self.helper.get_state(
+            room_id1, EventTypes.RoomHistoryVisibility, tok=user2_tok
+        )
+        self.assertEqual(
+            history_visibility_response.get("history_visibility"),
+            HistoryVisibility.WORLD_READABLE,
+        )
+
+        # Note: We never join the room
+
+        # Make the Sliding Sync request with just the room subscription
+        channel = self.make_request(
+            "POST",
+            self.sync_endpoint,
+            {
+                "room_subscriptions": {
+                    room_id1: {
+                        "required_state": [
+                            [EventTypes.Create, ""],
+                        ],
+                        "timeline_limit": 1,
+                    }
+                },
+            },
+            access_token=user1_tok,
+        )
+        self.assertEqual(channel.code, 200, channel.json_body)
+
+        # FIXME: In the future, we should be able to see the room because it's
+        # `world_readable` but currently we don't support this.
+        self.assertIsNone(
+            channel.json_body["rooms"].get(room_id1), channel.json_body["rooms"]
+        )
+
 
 class SlidingSyncToDeviceExtensionTestCase(unittest.HomeserverTestCase):
     """Tests for the to-device sliding sync extension"""
diff --git a/tests/unittest.py b/tests/unittest.py
index a7c20556a0..4aa7f56106 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -28,6 +28,7 @@ import logging
 import secrets
 import time
 from typing import (
+    AbstractSet,
     Any,
     Awaitable,
     Callable,
@@ -269,6 +270,56 @@ class TestCase(unittest.TestCase):
                 required[key], actual[key], msg="%s mismatch. %s" % (key, actual)
             )
 
+    def assertIncludes(
+        self,
+        actual_items: AbstractSet[str],
+        expected_items: AbstractSet[str],
+        exact: bool = False,
+        message: Optional[str] = None,
+    ) -> None:
+        """
+        Assert that all of the `expected_items` are included in the `actual_items`.
+
+        This assert could also be called `assertContains`, `assertItemsInSet`
+
+        Args:
+            actual_items: The container
+            expected_items: The items to check for in the container
+            exact: Whether the actual state should be exactly equal to the expected
+                state (no extras).
+            message: Optional message to include in the failure message.
+        """
+        # Check that each set has the same items
+        if exact and actual_items == expected_items:
+            return
+        # Check for a superset
+        elif not exact and actual_items >= expected_items:
+            return
+
+        expected_lines: List[str] = []
+        for expected_item in expected_items:
+            is_expected_in_actual = expected_item in actual_items
+            expected_lines.append(
+                "{}  {}".format(" " if is_expected_in_actual else "?", expected_item)
+            )
+
+        actual_lines: List[str] = []
+        for actual_item in actual_items:
+            is_actual_in_expected = actual_item in expected_items
+            actual_lines.append(
+                "{}  {}".format("+" if is_actual_in_expected else " ", actual_item)
+            )
+
+        newline = "\n"
+        expected_string = f"Expected items to be in actual ('?' = missing expected items):\n {{\n{newline.join(expected_lines)}\n }}"
+        actual_string = f"Actual ('+' = found expected items):\n {{\n{newline.join(actual_lines)}\n }}"
+        first_message = (
+            "Items must match exactly" if exact else "Some expected items are missing."
+        )
+        diff_message = f"{first_message}\n{expected_string}\n{actual_string}"
+
+        self.fail(f"{diff_message}\n{message}")
+
 
 def DEBUG(target: TV) -> TV:
     """A decorator to set the .loglevel attribute to logging.DEBUG.