WIP/PoC of storing whether we have sent rooms down to clients

1 files changed, 197 insertions, 0 deletions

diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index 3aa139ff1c..1c2ed95660 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -18,6 +18,7 @@
 #
 #
 import logging
+from enum import Enum
 from itertools import chain
 from typing import TYPE_CHECKING, Any, Dict, Final, List, Mapping, Optional, Set, Tuple
 
@@ -38,6 +39,7 @@ from synapse.types import (
     RoomStreamToken,
     SlidingSyncStreamToken,
     StateMap,
+    StrCollection,
     StreamKeyType,
     StreamToken,
     UserID,
@@ -1861,3 +1863,198 @@ class SlidingSyncHandler:
             next_batch=f"{stream_id}",
             events=messages,
         )
+
+
+class HaveSentRoomFlag(Enum):
+    """Flag for whether we have sent the room down a sliding sync connection.
+
+    The valid state changes here are:
+        NEVER -> LIVE
+        LIVE -> PREVIOUSLY
+        PREVIOUSLY -> LIVE
+    """
+
+    # The room has never been sent down (or we have forgotten we have sent it
+    # down).
+    NEVER = 1
+
+    # We have previously sent the room down, but there are updates that we
+    # haven't sent down.
+    PREVIOUSLY = 2
+
+    # We have sent the room down and the client has received all updates.
+    LIVE = 3
+
+
+@attr.s(auto_attribs=True, slots=True, frozen=True)
+class HaveSentRoom:
+    """Whether we have sent the room down a sliding sync connection.
+
+    Attributes:
+        status: Flag of if we have or haven't sent down the room
+        last_token: If the flag is `PREVIOUSLY` then this is non-null and
+            contains the last stream token of the last updates we sent down
+            the room, i.e. we still need to send everything since then to the
+            client.
+    """
+
+    status: HaveSentRoomFlag
+    last_token: Optional[RoomStreamToken]
+
+    @staticmethod
+    def previously(last_token: RoomStreamToken) -> "HaveSentRoom":
+        """Constructor for `PREVIOUSLY` flag."""
+        return HaveSentRoom(HaveSentRoomFlag.PREVIOUSLY, last_token)
+
+
+HAVE_SENT_ROOM_NEVER = HaveSentRoom(HaveSentRoomFlag.NEVER, None)
+HAVE_SENT_ROOM_LIVE = HaveSentRoom(HaveSentRoomFlag.LIVE, None)
+
+
+@attr.s(auto_attribs=True)
+class SlidingSyncConnectionStore:
+    """In-memory store of per-connection state, including what rooms we have
+    previously sent down a sliding sync connection.
+
+    Note: This is NOT safe to run in a worker setup.
+
+    The complication here is that we need to handle requests being resent, i.e.
+    if we sent down a room in a response that the client received, we must
+    consider the room *not* sent when we get the request again.
+
+    This is handled by using an integer "token", which is returned to the client
+    as part of the sync token. For each connection we store a mapping from
+    tokens to the room states, and create a new entry when we send down new
+    rooms.
+
+    Note that for any given sliding sync connection we will only store a maximum
+    of two different tokens: the previous token from the request and a new token
+    sent in the response. When we receive a request with a given token, we then
+    clear out all other entries with a different token.
+
+    Attributes:
+        _connections: Mapping from `(user_id, conn_id)` to mapping of `token`
+            to mapping of room ID to `HaveSentRoom`.
+    """
+
+    # `(user_id, conn_id)` -> `token` -> `room_id` -> `HaveSentRoom`
+    _connections: Dict[Tuple[str, str], Dict[int, Dict[str, HaveSentRoom]]] = (
+        attr.Factory(dict)
+    )
+
+    async def have_sent_room(
+        self, user_id: str, conn_id: str, connection_token: int, room_id: str
+    ) -> HaveSentRoom:
+        """Whether for the given user_id/conn_id/token, return whether we have
+        previously sent the room down
+        """
+
+        sync_statuses = self._connections.setdefault((user_id, conn_id), {})
+        room_status = sync_statuses.get(connection_token, {}).get(
+            room_id, HAVE_SENT_ROOM_NEVER
+        )
+
+        return room_status
+
+    async def record_rooms(
+        self,
+        user_id: str,
+        conn_id: str,
+        from_token: Optional[SlidingSyncStreamToken],
+        *,
+        sent_room_ids: StrCollection,
+        unsent_room_ids: StrCollection,
+    ) -> int:
+        """Record which rooms we have/haven't sent down in a new response
+
+        Attributes:
+            user_id
+            conn_id
+            from_token: The since token from the request, if any
+            sent_room_ids: The set of room IDs that we have sent down as
+                part of this request (only needs to be ones we didn't
+                previously sent down).
+            unsent_room_ids: The set of room IDs that have had updates
+                since the `last_room_token`, but which were not included in
+                this request
+        """
+        prev_connection_token = 0
+        if from_token is not None:
+            prev_connection_token = from_token.connection_token
+
+        # If there are no changes then this is a noop.
+        if not sent_room_ids and not unsent_room_ids:
+            return prev_connection_token
+
+        sync_statuses = self._connections.setdefault((user_id, conn_id), {})
+
+        # Generate a new token, removing any existing entries in that token
+        # (which can happen if requests get resent).
+        new_store_token = prev_connection_token + 1
+        sync_statuses.pop(new_store_token, None)
+
+        # Copy over and update the room mappings.
+        new_room_statuses = dict(sync_statuses.get(prev_connection_token, {}))
+
+        # Whether we have updated the `new_room_statuses`, if we don't by the
+        # end we can treat this as a noop.
+        have_updated = False
+        for room_id in sent_room_ids:
+            new_room_statuses[room_id] = HAVE_SENT_ROOM_LIVE
+            have_updated = True
+
+        # Whether we add/update the entries for unsent rooms depends on the
+        # existing entry:
+        #   - LIVE: We have previously sent down everything up to
+        #     `last_room_token, so we update the entry to be `PREVIOUSLY` with
+        #     `last_room_token`.
+        #   - PREVIOUSLY: We have previously sent down everything up to *a*
+        #     given token, so we don't need to update the entry.
+        #   - NEVER: We have never previously sent down the room, and we haven't
+        #     sent anything down this time either so we leave it as NEVER.
+
+        # Work out the new state for unsent rooms that were `LIVE`.
+        if from_token:
+            new_unsent_state = HaveSentRoom.previously(from_token.stream_token.room_key)
+        else:
+            new_unsent_state = HAVE_SENT_ROOM_NEVER
+
+        for room_id in unsent_room_ids:
+            prev_state = new_room_statuses.get(room_id)
+            if prev_state is not None and prev_state.status == HaveSentRoomFlag.LIVE:
+                new_room_statuses[room_id] = new_unsent_state
+                have_updated = True
+
+        if not have_updated:
+            return prev_connection_token
+
+        sync_statuses[new_store_token] = new_room_statuses
+
+        return new_store_token
+
+    async def mark_token_seen(
+        self,
+        user_id: str,
+        conn_id: str,
+        from_token: Optional[SlidingSyncStreamToken],
+    ) -> None:
+        """We have received a request with the given token, so we can clear out
+        any other tokens associated with the connection.
+
+        If there is no from token then we have started afresh, and so we delete
+        all tokens associated with the device.
+        """
+        # Clear out any tokens for the connection that doesn't match the one
+        # from the request.
+
+        sync_statuses = self._connections.pop((user_id, conn_id), {})
+        if from_token is None:
+            return
+
+        sync_statuses = {
+            i: room_statuses
+            for i, room_statuses in sync_statuses.items()
+            if i == from_token.connection_token
+        }
+        if sync_statuses:
+            self._connections[(user_id, conn_id)] = sync_statuses