Comment cleanup

changelog.d/19005.feature

@@ -0,0 +1 @@
+Add experimental support for MSC4360: Sliding Sync Threads Extension.

synapse/api/constants.py

@@ -272,6 +272,9 @@ class EventContentFields:
     M_TOPIC: Final = "m.topic"
     M_TEXT: Final = "m.text"
 
+    # Event relations
+    RELATIONS: Final = "m.relates_to"
+
 
 class EventUnsignedContentFields:
     """Fields found inside the 'unsigned' data on events"""
@@ -360,3 +363,10 @@ class Direction(enum.Enum):
 class ProfileFields:
     DISPLAYNAME: Final = "displayname"
     AVATAR_URL: Final = "avatar_url"
+
+
+class MRelatesToFields:
+    """Fields found inside m.relates_to content blocks."""
+
+    EVENT_ID: Final = "event_id"
+    REL_TYPE: Final = "rel_type"

synapse/config/experimental.py

@@ -593,3 +593,6 @@ class ExperimentalConfig(Config):
         # MSC4306: Thread Subscriptions
         # (and MSC4308: Thread Subscriptions extension to Sliding Sync)
         self.msc4306_enabled: bool = experimental.get("msc4306_enabled", False)
+
+        # MSC4360: Threads Extension to Sliding Sync
+        self.msc4360_enabled: bool = experimental.get("msc4360_enabled", False)

synapse/handlers/relations.py

@@ -105,8 +105,6 @@ class RelationsHandler:
     ) -> JsonDict:
         """Get related events of a event, ordered by topological ordering.
 
-        TODO Accept a PaginationConfig instead of individual pagination parameters.
-
         Args:
             requester: The user requesting the relations.
             event_id: Fetch events that relate to this event ID.

synapse/handlers/sliding_sync/__init__.py

@@ -305,6 +305,7 @@ class SlidingSyncHandler:
             # account data, read receipts, typing indicators, to-device messages, etc).
             actual_room_ids=set(relevant_room_map.keys()),
             actual_room_response_map=rooms,
+            room_membership_for_user_at_to_token_map=room_membership_for_user_map,
             from_token=from_token,
             to_token=to_token,
         )

synapse/handlers/sliding_sync/extensions.py

@@ -14,6 +14,7 @@
 
 import itertools
 import logging
+from collections import defaultdict
 from typing import (
     TYPE_CHECKING,
     AbstractSet,
@@ -26,16 +27,28 @@ from typing import (
 
 from typing_extensions import TypeAlias, assert_never
 
-from synapse.api.constants import AccountDataTypes, EduTypes
+from synapse.api.constants import (
+    AccountDataTypes,
+    EduTypes,
+    EventContentFields,
+    Membership,
+    MRelatesToFields,
+    RelationTypes,
+)
+from synapse.events import EventBase
 from synapse.handlers.receipts import ReceiptEventSource
+from synapse.handlers.sliding_sync.room_lists import RoomsForUserType
 from synapse.logging.opentracing import trace
 from synapse.storage.databases.main.receipts import ReceiptInRoom
+from synapse.storage.databases.main.relations import ThreadUpdateInfo
 from synapse.types import (
     DeviceListUpdates,
     JsonMapping,
     MultiWriterStreamToken,
+    RoomStreamToken,
     SlidingSyncStreamToken,
     StrCollection,
+    StreamKeyType,
     StreamToken,
     ThreadSubscriptionsToken,
 )
@@ -51,6 +64,7 @@ from synapse.util.async_helpers import (
     concurrently_execute,
     gather_optional_coroutines,
 )
+from synapse.visibility import filter_events_for_client
 
 _ThreadSubscription: TypeAlias = (
     SlidingSyncResult.Extensions.ThreadSubscriptionsExtension.ThreadSubscription
@@ -58,6 +72,7 @@ _ThreadSubscription: TypeAlias = (
 _ThreadUnsubscription: TypeAlias = (
     SlidingSyncResult.Extensions.ThreadSubscriptionsExtension.ThreadUnsubscription
 )
+_ThreadUpdate: TypeAlias = SlidingSyncResult.Extensions.ThreadsExtension.ThreadUpdate
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -73,7 +88,10 @@ class SlidingSyncExtensionHandler:
         self.event_sources = hs.get_event_sources()
         self.device_handler = hs.get_device_handler()
         self.push_rules_handler = hs.get_push_rules_handler()
+        self.relations_handler = hs.get_relations_handler()
+        self._storage_controllers = hs.get_storage_controllers()
         self._enable_thread_subscriptions = hs.config.experimental.msc4306_enabled
+        self._enable_threads_ext = hs.config.experimental.msc4360_enabled
 
     @trace
     async def get_extensions_response(
@@ -84,6 +102,7 @@ class SlidingSyncExtensionHandler:
         actual_lists: Mapping[str, SlidingSyncResult.SlidingWindowList],
         actual_room_ids: set[str],
         actual_room_response_map: Mapping[str, SlidingSyncResult.RoomResult],
+        room_membership_for_user_at_to_token_map: Mapping[str, RoomsForUserType],
         to_token: StreamToken,
         from_token: SlidingSyncStreamToken | None,
     ) -> SlidingSyncResult.Extensions:
@@ -99,6 +118,8 @@ class SlidingSyncExtensionHandler:
             actual_room_ids: The actual room IDs in the the Sliding Sync response.
             actual_room_response_map: A map of room ID to room results in the the
                 Sliding Sync response.
+            room_membership_for_user_at_to_token_map: A map of room ID to the membership
+                information for the user in the room at the time of `to_token`.
             to_token: The latest point in the stream to sync up to.
             from_token: The point in the stream to sync from.
         """
@@ -174,6 +195,18 @@ class SlidingSyncExtensionHandler:
                 from_token=from_token,
             )
 
+        threads_coro = None
+        if sync_config.extensions.threads is not None and self._enable_threads_ext:
+            threads_coro = self.get_threads_extension_response(
+                sync_config=sync_config,
+                threads_request=sync_config.extensions.threads,
+                actual_room_ids=actual_room_ids,
+                actual_room_response_map=actual_room_response_map,
+                room_membership_for_user_at_to_token_map=room_membership_for_user_at_to_token_map,
+                to_token=to_token,
+                from_token=from_token,
+            )
+
         (
             to_device_response,
             e2ee_response,
@@ -181,6 +214,7 @@ class SlidingSyncExtensionHandler:
             receipts_response,
             typing_response,
             thread_subs_response,
+            threads_response,
         ) = await gather_optional_coroutines(
             to_device_coro,
             e2ee_coro,
@@ -188,6 +222,7 @@ class SlidingSyncExtensionHandler:
             receipts_coro,
             typing_coro,
             thread_subs_coro,
+            threads_coro,
         )
 
         return SlidingSyncResult.Extensions(
@@ -197,6 +232,7 @@ class SlidingSyncExtensionHandler:
             receipts=receipts_response,
             typing=typing_response,
             thread_subscriptions=thread_subs_response,
+            threads=threads_response,
         )
 
     def find_relevant_room_ids_for_extension(
@@ -967,3 +1003,273 @@ class SlidingSyncExtensionHandler:
             unsubscribed=unsubscribed_threads,
             prev_batch=prev_batch,
         )
+
+    def _extract_thread_id_from_event(self, event: EventBase) -> str | None:
+        """Extract thread ID from event if it's a thread reply.
+
+        Args:
+            event: The event to check.
+
+        Returns:
+            The thread ID if the event is a thread reply, None otherwise.
+        """
+        relates_to = event.content.get(EventContentFields.RELATIONS)
+        if isinstance(relates_to, dict):
+            if relates_to.get(MRelatesToFields.REL_TYPE) == RelationTypes.THREAD:
+                return relates_to.get(MRelatesToFields.EVENT_ID)
+        return None
+
+    def _find_threads_in_timeline(
+        self,
+        actual_room_response_map: Mapping[str, SlidingSyncResult.RoomResult],
+    ) -> set[str]:
+        """Find all thread IDs that have events in room timelines.
+
+        Args:
+            actual_room_response_map: A map of room ID to room results.
+
+        Returns:
+            A set of thread IDs (thread root event IDs) that appear in the timeline.
+        """
+        threads_in_timeline: set[str] = set()
+        for room_result in actual_room_response_map.values():
+            if room_result.timeline_events:
+                for event in room_result.timeline_events:
+                    thread_id = self._extract_thread_id_from_event(event)
+                    if thread_id:
+                        threads_in_timeline.add(thread_id)
+        return threads_in_timeline
+
+    def _merge_prev_batch_token(
+        self,
+        current_token: StreamToken | None,
+        new_token: StreamToken | None,
+    ) -> StreamToken | None:
+        """Merge two prev_batch tokens, taking the maximum (latest) for backwards pagination.
+
+        Args:
+            current_token: The current prev_batch token (may be None)
+            new_token: The new prev_batch token to merge (may be None)
+
+        Returns:
+            The merged token (maximum of the two, or None if both are None)
+        """
+        if new_token is None:
+            return current_token
+        if current_token is None:
+            return new_token
+        if new_token.room_key.stream > current_token.room_key.stream:
+            return new_token
+        return current_token
+
+    def _merge_thread_updates(
+        self,
+        target: dict[str, list[ThreadUpdateInfo]],
+        source: dict[str, list[ThreadUpdateInfo]],
+    ) -> None:
+        """Merge thread updates from source into target.
+
+        Args:
+            target: The target dict to merge into (modified in place)
+            source: The source dict to merge from
+        """
+        for thread_id, updates in source.items():
+            target.setdefault(thread_id, []).extend(updates)
+
+    async def get_threads_extension_response(
+        self,
+        sync_config: SlidingSyncConfig,
+        threads_request: SlidingSyncConfig.Extensions.ThreadsExtension,
+        actual_room_ids: set[str],
+        actual_room_response_map: Mapping[str, SlidingSyncResult.RoomResult],
+        room_membership_for_user_at_to_token_map: Mapping[str, RoomsForUserType],
+        to_token: StreamToken,
+        from_token: SlidingSyncStreamToken | None,
+    ) -> SlidingSyncResult.Extensions.ThreadsExtension | None:
+        """Handle Threads extension (MSC4360)
+
+        Args:
+            sync_config: Sync configuration.
+            threads_request: The threads extension from the request.
+            actual_room_ids: The actual room IDs in the the Sliding Sync response.
+            actual_room_response_map: A map of room ID to room results in the
+                sliding sync response. Used to determine which threads already have
+                events in the room timeline.
+            room_membership_for_user_at_to_token_map: A map of room ID to the membership
+                information for the user in the room at the time of `to_token`.
+            to_token: The point in the stream to sync up to.
+            from_token: The point in the stream to sync from.
+
+        Returns:
+            the response (None if empty or threads extension is disabled)
+        """
+        if not threads_request.enabled:
+            return None
+
+        # Identify which threads already have events in the room timelines.
+        # If include_roots=False, we'll exclude these threads from the DB query
+        # since the client already sees the thread activity in the timeline.
+        # If include_roots=True, we fetch all threads regardless, because the client
+        # wants the thread root events.
+        threads_to_exclude: set[str] | None = None
+        if not threads_request.include_roots:
+            threads_to_exclude = self._find_threads_in_timeline(
+                actual_room_response_map
+            )
+
+        # Separate rooms into groups based on membership status.
+        # For LEAVE/BAN rooms, we need to bound the to_token to prevent leaking events
+        # that occurred after the user left/was banned.
+        leave_ban_rooms: set[str] = set()
+        other_rooms: set[str] = set()
+
+        for room_id in actual_room_ids:
+            membership_info = room_membership_for_user_at_to_token_map.get(room_id)
+            if membership_info and membership_info.membership in (
+                Membership.LEAVE,
+                Membership.BAN,
+            ):
+                leave_ban_rooms.add(room_id)
+            else:
+                other_rooms.add(room_id)
+
+        # Fetch thread updates, handling LEAVE/BAN rooms separately to avoid data leaks.
+        all_thread_updates: dict[str, list[ThreadUpdateInfo]] = {}
+        prev_batch_token: StreamToken | None = None
+        remaining_limit = threads_request.limit
+
+        # Query for rooms where the user has left or been banned, using their leave/ban
+        # event position as the upper bound to prevent seeing events after they left.
+        if leave_ban_rooms:
+            for room_id in leave_ban_rooms:
+                if remaining_limit <= 0:
+                    # We've already fetched enough updates, but we still need to set
+                    # prev_batch to indicate there are more results.
+                    prev_batch_token = to_token
+                    break
+
+                membership_info = room_membership_for_user_at_to_token_map[room_id]
+                bounded_to_token = membership_info.event_pos.to_room_stream_token()
+
+                (
+                    room_thread_updates,
+                    room_prev_batch,
+                ) = await self.store.get_thread_updates_for_rooms(
+                    room_ids={room_id},
+                    from_token=from_token.stream_token.room_key if from_token else None,
+                    to_token=bounded_to_token,
+                    limit=remaining_limit,
+                    exclude_thread_ids=threads_to_exclude,
+                )
+
+                # Count how many updates we fetched and reduce the remaining limit
+                num_updates = sum(
+                    len(updates) for updates in room_thread_updates.values()
+                )
+                remaining_limit -= num_updates
+
+                self._merge_thread_updates(all_thread_updates, room_thread_updates)
+                prev_batch_token = self._merge_prev_batch_token(
+                    prev_batch_token, room_prev_batch
+                )
+
+        # Query for rooms where the user is joined, invited, or knocking, using the
+        # normal to_token as the upper bound.
+        if other_rooms and remaining_limit > 0:
+            (
+                other_thread_updates,
+                other_prev_batch,
+            ) = await self.store.get_thread_updates_for_rooms(
+                room_ids=other_rooms,
+                from_token=from_token.stream_token.room_key if from_token else None,
+                to_token=to_token.room_key,
+                limit=remaining_limit,
+                exclude_thread_ids=threads_to_exclude,
+            )
+
+            self._merge_thread_updates(all_thread_updates, other_thread_updates)
+            prev_batch_token = self._merge_prev_batch_token(
+                prev_batch_token, other_prev_batch
+            )
+
+        if len(all_thread_updates) == 0:
+            return None
+
+        # Build a mapping of event_id -> (thread_id, update) for efficient lookup
+        # during visibility filtering.
+        event_to_thread_map: dict[str, tuple[str, ThreadUpdateInfo]] = {}
+        for thread_id, updates in all_thread_updates.items():
+            for update in updates:
+                event_to_thread_map[update.event_id] = (thread_id, update)
+
+        # Fetch and filter events for visibility
+        all_events = await self.store.get_events_as_list(event_to_thread_map.keys())
+        filtered_events = await filter_events_for_client(
+            self._storage_controllers, sync_config.user.to_string(), all_events
+        )
+
+        # Rebuild thread updates from filtered events
+        filtered_updates: dict[str, list[ThreadUpdateInfo]] = defaultdict(list)
+        for event in filtered_events:
+            if event.event_id in event_to_thread_map:
+                thread_id, update = event_to_thread_map[event.event_id]
+                filtered_updates[thread_id].append(update)
+
+        if not filtered_updates:
+            return None
+
+        # Note: Updates are already sorted by stream_ordering DESC from the database query,
+        # and filter_events_for_client preserves order, so updates[0] is guaranteed to be
+        # the latest event for each thread.
+
+        # Optionally fetch thread root events and their bundled aggregations
+        thread_root_event_map = {}
+        aggregations_map = {}
+        if threads_request.include_roots:
+            thread_root_events = await self.store.get_events_as_list(
+                filtered_updates.keys()
+            )
+            thread_root_event_map = {e.event_id: e for e in thread_root_events}
+
+            if thread_root_event_map:
+                aggregations_map = (
+                    await self.relations_handler.get_bundled_aggregations(
+                        thread_root_event_map.values(),
+                        sync_config.user.to_string(),
+                    )
+                )
+
+        thread_updates: dict[str, dict[str, _ThreadUpdate]] = {}
+        for thread_root, updates in filtered_updates.items():
+            # We only care about the latest update for the thread.
+            # After sorting above, updates[0] is guaranteed to be the latest (highest stream_ordering).
+            latest_update = updates[0]
+
+            # Generate per-thread prev_batch token if this thread has multiple visible updates.
+            # When we hit the global limit, we generate prev_batch tokens for all threads, even if
+            # we only saw 1 update for them. This is to cover the case where we only saw
+            # a single update for a given thread, but the global limit prevents us from
+            # obtaining other updates which would have otherwise been included in the
+            # range.
+            per_thread_prev_batch = None
+            if len(updates) > 1 or prev_batch_token is not None:
+                # Create a token pointing to one position before the latest event's stream position.
+                # This makes it exclusive - /relations with dir=b won't return the latest event again.
+                # Use StreamToken.START as base (all other streams at 0) since only room position matters.
+                per_thread_prev_batch = StreamToken.START.copy_and_replace(
+                    StreamKeyType.ROOM,
+                    RoomStreamToken(stream=latest_update.stream_ordering - 1),
+                )
+
+            thread_updates.setdefault(latest_update.room_id, {})[thread_root] = (
+                _ThreadUpdate(
+                    thread_root=thread_root_event_map.get(thread_root),
+                    prev_batch=per_thread_prev_batch,
+                    bundled_aggregations=aggregations_map.get(thread_root),
+                )
+            )
+
+        return SlidingSyncResult.Extensions.ThreadsExtension(
+            updates=thread_updates,
+            prev_batch=prev_batch_token,
+        )

synapse/rest/client/sync.py

@@ -31,6 +31,7 @@ from synapse.api.filtering import FilterCollection
 from synapse.api.presence import UserPresenceState
 from synapse.api.ratelimiting import Ratelimiter
 from synapse.events.utils import (
+    EventClientSerializer,
     SerializeEventConfig,
     format_event_for_client_v2_without_room_id,
     format_event_raw,
@@ -56,6 +57,7 @@ from synapse.http.servlet import (
 from synapse.http.site import SynapseRequest
 from synapse.logging.opentracing import log_kv, set_tag, trace_with_opname
 from synapse.rest.admin.experimental_features import ExperimentalFeature
+from synapse.storage.databases.main import DataStore
 from synapse.types import JsonDict, Requester, SlidingSyncStreamToken, StreamToken
 from synapse.types.rest.client import SlidingSyncBody
 from synapse.util.caches.lrucache import LruCache
@@ -646,6 +648,7 @@ class SlidingSyncRestServlet(RestServlet):
         - receipts (MSC3960)
         - account data (MSC3959)
         - thread subscriptions (MSC4308)
+        - threads (MSC4360)
 
     Request query parameters:
         timeout: How long to wait for new events in milliseconds.
@@ -849,7 +852,10 @@ class SlidingSyncRestServlet(RestServlet):
             logger.info("Client has disconnected; not serializing response.")
             return 200, {}
 
-        response_content = await self.encode_response(requester, sliding_sync_results)
+        time_now = self.clock.time_msec()
+        response_content = await self.encode_response(
+            requester, sliding_sync_results, time_now
+        )
 
         return 200, response_content
 
@@ -858,6 +864,7 @@ class SlidingSyncRestServlet(RestServlet):
         self,
         requester: Requester,
         sliding_sync_result: SlidingSyncResult,
+        time_now: int,
     ) -> JsonDict:
         response: JsonDict = defaultdict(dict)
 
@@ -866,10 +873,10 @@ class SlidingSyncRestServlet(RestServlet):
         if serialized_lists:
             response["lists"] = serialized_lists
         response["rooms"] = await self.encode_rooms(
-            requester, sliding_sync_result.rooms
+            requester, sliding_sync_result.rooms, time_now
         )
         response["extensions"] = await self.encode_extensions(
-            requester, sliding_sync_result.extensions
+            requester, sliding_sync_result.extensions, time_now
         )
 
         return response
@@ -901,9 +908,8 @@ class SlidingSyncRestServlet(RestServlet):
         self,
         requester: Requester,
         rooms: dict[str, SlidingSyncResult.RoomResult],
+        time_now: int,
     ) -> JsonDict:
-        time_now = self.clock.time_msec()
-
         serialize_options = SerializeEventConfig(
             event_format=format_event_for_client_v2_without_room_id,
             requester=requester,
@@ -1019,7 +1025,10 @@ class SlidingSyncRestServlet(RestServlet):
 
     @trace_with_opname("sliding_sync.encode_extensions")
     async def encode_extensions(
-        self, requester: Requester, extensions: SlidingSyncResult.Extensions
+        self,
+        requester: Requester,
+        extensions: SlidingSyncResult.Extensions,
+        time_now: int,
     ) -> JsonDict:
         serialized_extensions: JsonDict = {}
 
@@ -1089,6 +1098,17 @@ class SlidingSyncRestServlet(RestServlet):
                 _serialise_thread_subscriptions(extensions.thread_subscriptions)
             )
 
+        # excludes both None and falsy `threads`
+        if extensions.threads:
+            serialized_extensions[
+                "io.element.msc4360.threads"
+            ] = await _serialise_threads(
+                self.event_serializer,
+                time_now,
+                extensions.threads,
+                self.store,
+            )
+
         return serialized_extensions
 
 
@@ -1125,6 +1145,72 @@ def _serialise_thread_subscriptions(
     return out
 
 
+async def _serialise_threads(
+    event_serializer: EventClientSerializer,
+    time_now: int,
+    threads: SlidingSyncResult.Extensions.ThreadsExtension,
+    store: "DataStore",
+) -> JsonDict:
+    """
+    Serialize the threads extension response for sliding sync.
+
+    Args:
+        event_serializer: The event serializer to use for serializing thread root events.
+        time_now: The current time in milliseconds, used for event serialization.
+        threads: The threads extension data containing thread updates and pagination tokens.
+        store: The datastore, needed for serializing stream tokens.
+
+    Returns:
+        A JSON-serializable dict containing:
+        - "updates": A nested dict mapping room_id -> thread_root_id -> thread update.
+          Each thread update may contain:
+          - "thread_root": The serialized thread root event (if include_roots was True),
+            with bundled aggregations including the latest_event in unsigned.m.relations.m.thread.
+          - "prev_batch": A pagination token for fetching older events in the thread.
+        - "prev_batch": A pagination token for fetching older thread updates (if available).
+    """
+    out: JsonDict = {}
+
+    if threads.updates:
+        updates_dict: JsonDict = {}
+        for room_id, thread_updates in threads.updates.items():
+            room_updates: JsonDict = {}
+            for thread_root_id, update in thread_updates.items():
+                # Serialize the update
+                update_dict: JsonDict = {}
+
+                # Serialize the thread_root event if present
+                if update.thread_root is not None:
+                    # Create a mapping of event_id to bundled_aggregations
+                    bundle_aggs_map = (
+                        {thread_root_id: update.bundled_aggregations}
+                        if update.bundled_aggregations
+                        else None
+                    )
+                    serialized_events = await event_serializer.serialize_events(
+                        [update.thread_root],
+                        time_now,
+                        bundle_aggregations=bundle_aggs_map,
+                    )
+                    if serialized_events:
+                        update_dict["thread_root"] = serialized_events[0]
+
+                # Add prev_batch if present
+                if update.prev_batch is not None:
+                    update_dict["prev_batch"] = await update.prev_batch.to_string(store)
+
+                room_updates[thread_root_id] = update_dict
+
+            updates_dict[room_id] = room_updates
+
+        out["updates"] = updates_dict
+
+    if threads.prev_batch:
+        out["prev_batch"] = await threads.prev_batch.to_string(store)
+
+    return out
+
+
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     SyncRestServlet(hs).register(http_server)
 

synapse/storage/databases/main/relations.py

@@ -19,6 +19,7 @@
 #
 
 import logging
+from collections import defaultdict
 from typing import (
     TYPE_CHECKING,
     Collection,
@@ -40,13 +41,19 @@ from synapse.storage.database import (
     LoggingTransaction,
     make_in_list_sql_clause,
 )
+from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.stream import (
     generate_next_token,
     generate_pagination_bounds,
     generate_pagination_where_clause,
 )
 from synapse.storage.engines import PostgresEngine
-from synapse.types import JsonDict, StreamKeyType, StreamToken
+from synapse.types import (
+    JsonDict,
+    RoomStreamToken,
+    StreamKeyType,
+    StreamToken,
+)
 from synapse.util.caches.descriptors import cached, cachedList
 
 if TYPE_CHECKING:
@@ -88,7 +95,23 @@ class _RelatedEvent:
     sender: str
 
 
-class RelationsWorkerStore(SQLBaseStore):
+@attr.s(slots=True, frozen=True, auto_attribs=True)
+class ThreadUpdateInfo:
+    """
+    Information about a thread update for the sliding sync threads extension.
+
+    Attributes:
+        event_id: The event ID of the event in the thread.
+        room_id: The room ID where this thread exists.
+        stream_ordering: The stream ordering of this event.
+    """
+
+    event_id: str
+    room_id: str
+    stream_ordering: int
+
+
+class RelationsWorkerStore(EventsWorkerStore, SQLBaseStore):
     def __init__(
         self,
         database: DatabasePool,
@@ -584,14 +607,18 @@ class RelationsWorkerStore(SQLBaseStore):
             "get_applicable_edits", _get_applicable_edits_txn
         )
 
-        edits = await self.get_events(edit_ids.values())  # type: ignore[attr-defined]
+        edits = await self.get_events(edit_ids.values())
 
         # Map to the original event IDs to the edit events.
         #
         # There might not be an edit event due to there being no edits or
         # due to the event not being known, either case is treated the same.
         return {
-            original_event_id: edits.get(edit_ids.get(original_event_id))
+            original_event_id: (
+                edits.get(edit_id)
+                if (edit_id := edit_ids.get(original_event_id))
+                else None
+            )
             for original_event_id in event_ids
         }
 
@@ -699,7 +726,7 @@ class RelationsWorkerStore(SQLBaseStore):
             "get_thread_summaries", _get_thread_summaries_txn
         )
 
-        latest_events = await self.get_events(latest_event_ids.values())  # type: ignore[attr-defined]
+        latest_events = await self.get_events(latest_event_ids.values())
 
         # Map to the event IDs to the thread summary.
         #
@@ -1111,6 +1138,148 @@ class RelationsWorkerStore(SQLBaseStore):
             "get_related_thread_id", _get_related_thread_id
         )
 
+    async def get_thread_updates_for_rooms(
+        self,
+        *,
+        room_ids: Collection[str],
+        from_token: RoomStreamToken | None = None,
+        to_token: RoomStreamToken | None = None,
+        limit: int = 5,
+        exclude_thread_ids: Collection[str] | None = None,
+    ) -> tuple[dict[str, list[ThreadUpdateInfo]], StreamToken | None]:
+        """Get a list of updated threads, ordered by stream ordering of their
+        latest reply, filtered to only include threads in rooms where the user
+        is currently joined.
+
+        Args:
+            room_ids: The room IDs to fetch thread updates for.
+            from_token: The lower bound (exclusive) for thread updates. If None,
+                fetch from the start of the room timeline.
+            to_token: The upper bound (inclusive) for thread updates. If None,
+                fetch up to the current position in the room timeline.
+            limit: Maximum number of thread updates to return.
+            exclude_thread_ids: Optional collection of thread root event IDs to exclude
+                from the results. Useful for filtering out threads already visible
+                in the room timeline.
+
+        Returns:
+            A tuple of:
+                A dict mapping thread_id to list of ThreadUpdateInfo objects,
+                ordered by stream_ordering descending (most recent first).
+                A prev_batch StreamToken (exclusive) if there are more results available,
+                None otherwise.
+        """
+        # Ensure bad limits aren't being passed in.
+        assert limit > 0
+
+        if len(room_ids) == 0:
+            return ({}), None
+
+        def _get_thread_updates_for_user_txn(
+            txn: LoggingTransaction,
+        ) -> tuple[list[tuple[str, str, str, int]], int | None]:
+            room_clause, room_id_values = make_in_list_sql_clause(
+                txn.database_engine, "e.room_id", room_ids
+            )
+
+            # Generate the pagination clause, if necessary.
+            pagination_clause = ""
+            pagination_args: list[str] = []
+            if from_token:
+                from_bound = from_token.stream
+                pagination_clause += " AND stream_ordering > ?"
+                pagination_args.append(str(from_bound))
+
+            if to_token:
+                to_bound = to_token.stream
+                pagination_clause += " AND stream_ordering <= ?"
+                pagination_args.append(str(to_bound))
+
+            # Generate the exclusion clause for thread IDs, if necessary.
+            exclusion_clause = ""
+            exclusion_args: list[str] = []
+            if exclude_thread_ids:
+                exclusion_clause, exclusion_args = make_in_list_sql_clause(
+                    txn.database_engine,
+                    "er.relates_to_id",
+                    exclude_thread_ids,
+                    negative=True,
+                )
+                exclusion_clause = f" AND {exclusion_clause}"
+
+            # TODO: improve the fact that multiple hits for the same thread means we
+            # won't get as many overall updates for the sss response
+
+            # Find any thread events between the stream ordering bounds.
+            sql = f"""
+                SELECT e.event_id, er.relates_to_id, e.room_id, e.stream_ordering
+                  FROM event_relations AS er
+                  INNER JOIN events AS e ON er.event_id = e.event_id
+                  WHERE er.relation_type = '{RelationTypes.THREAD}'
+                  AND {room_clause}
+                  {exclusion_clause}
+                  {pagination_clause}
+                  ORDER BY stream_ordering DESC
+                  LIMIT ?
+            """
+
+            # Fetch `limit + 1` rows as a way to detect if there are more results beyond
+            # what we're returning. If we get exactly `limit + 1` rows back, we know there
+            # are more results available and we can set `next_token`. We only return the
+            # first `limit` rows to the caller. This avoids needing a separate COUNT query.
+            txn.execute(
+                sql,
+                (
+                    *room_id_values,
+                    *exclusion_args,
+                    *pagination_args,
+                    limit + 1,
+                ),
+            )
+
+            # SQL returns: event_id, thread_id, room_id, stream_ordering
+            rows = cast(list[tuple[str, str, str, int]], txn.fetchall())
+
+            # If there are more events, generate the next pagination key from the
+            # last thread which will be returned.
+            next_token = None
+            if len(rows) > limit:
+                # Set the next_token to be the second last row in the result set since
+                # that will be the last row we return from this function.
+                # This works as an exclusive bound that can be backpaginated from.
+                # Use the stream_ordering field (index 2 in original rows)
+                next_token = rows[-2][3]
+
+            return rows[:limit], next_token
+
+        thread_infos, next_token_int = await self.db_pool.runInteraction(
+            "get_thread_updates_for_user", _get_thread_updates_for_user_txn
+        )
+
+        # Convert the next_token int (stream ordering) to a StreamToken.
+        # Use StreamToken.START as base (all other streams at 0) since only room
+        # position matters.
+        # Subtract 1 to make it exclusive - the client can paginate from this point without
+        # receiving the last thread update that was already returned.
+        next_token = None
+        if next_token_int is not None:
+            next_token = StreamToken.START.copy_and_replace(
+                StreamKeyType.ROOM, RoomStreamToken(stream=next_token_int - 1)
+            )
+
+        # Build ThreadUpdateInfo objects.
+        thread_update_infos: dict[str, list[ThreadUpdateInfo]] = defaultdict(list)
+        for event_id, thread_id, room_id, stream_ordering in thread_infos:
+            thread_update_infos[thread_id].append(
+                ThreadUpdateInfo(
+                    event_id=event_id,
+                    room_id=room_id,
+                    stream_ordering=stream_ordering,
+                )
+            )
+
+        return (thread_update_infos, next_token)
+
 
 class RelationsStore(RelationsWorkerStore):
     pass

synapse/storage/schema/main/delta/92/10_thread_updates_indexes.sql

@@ -0,0 +1,33 @@
+--
+-- This file is licensed under the Affero General Public License (AGPL) version 3.
+--
+-- Copyright (C) 2025 New Vector, Ltd
+--
+-- This program is free software: you can redistribute it and/or modify
+-- it under the terms of the GNU Affero General Public License as
+-- published by the Free Software Foundation, either version 3 of the
+-- License, or (at your option) any later version.
+--
+-- See the GNU Affero General Public License for more details:
+-- <https://www.gnu.org/licenses/agpl-3.0.html>.
+
+-- Add indexes to improve performance of the thread_updates endpoint and
+-- sliding sync threads extension (MSC4360).
+
+-- Index for efficiently finding all events that relate to a specific event
+-- (e.g., all replies to a thread root). This is used by the correlated subquery
+-- in get_thread_updates_for_user that counts thread updates.
+-- Also useful for other relation queries (edits, reactions, etc.).
+CREATE INDEX IF NOT EXISTS event_relations_relates_to_id_type
+    ON event_relations(relates_to_id, relation_type);
+
+-- Index for the /thread_updates endpoint's cross-room query.
+-- Allows efficient descending ordering and range filtering of threads
+-- by stream_ordering across all rooms.
+CREATE INDEX IF NOT EXISTS threads_stream_ordering_desc
+    ON threads(stream_ordering DESC);
+
+-- Index for the EXISTS clause that filters threads to only joined rooms.
+-- Allows efficient lookup of a user's current room memberships.
+CREATE INDEX IF NOT EXISTS local_current_membership_user_room
+    ON local_current_membership(user_id, membership, room_id);

synapse/streams/config.py

@@ -57,19 +57,41 @@ class PaginationConfig:
         from_tok_str = parse_string(request, "from")
         to_tok_str = parse_string(request, "to")
 
+        # Helper function to extract StreamToken from either StreamToken or SlidingSyncStreamToken format
+        def extract_stream_token(token_str: str) -> str:
+            """
+            Extract the StreamToken portion from a token string.
+
+            Handles both:
+            - StreamToken format: "s123_456_..."
+            - SlidingSyncStreamToken format: "5/s123_456_..." (extracts part after /)
+
+            This allows clients using sliding sync to use their pos tokens
+            with endpoints like /relations and /messages.
+            """
+            if "/" in token_str:
+                # SlidingSyncStreamToken format: "connection_position/stream_token"
+                # Split and return just the stream_token part
+                parts = token_str.split("/", 1)
+                if len(parts) == 2:
+                    return parts[1]
+            return token_str
+
         try:
             from_tok = None
             if from_tok_str == "END":
                 from_tok = None  # For backwards compat.
             elif from_tok_str:
-                from_tok = await StreamToken.from_string(store, from_tok_str)
+                stream_token_str = extract_stream_token(from_tok_str)
+                from_tok = await StreamToken.from_string(store, stream_token_str)
         except Exception:
             raise SynapseError(400, "'from' parameter is invalid")
 
         try:
             to_tok = None
             if to_tok_str:
-                to_tok = await StreamToken.from_string(store, to_tok_str)
+                stream_token_str = extract_stream_token(to_tok_str)
+                to_tok = await StreamToken.from_string(store, stream_token_str)
         except Exception:
             raise SynapseError(400, "'to' parameter is invalid")
 

synapse/types/handlers/sliding_sync.py

@@ -35,6 +35,9 @@ from pydantic import ConfigDict
 
 from synapse.api.constants import EventTypes
 from synapse.events import EventBase
+
+if TYPE_CHECKING:
+    from synapse.handlers.relations import BundledAggregations
 from synapse.types import (
     DeviceListUpdates,
     JsonDict,
@@ -388,12 +391,60 @@ class SlidingSyncResult:
                     or bool(self.prev_batch)
                 )
 
+        @attr.s(slots=True, frozen=True, auto_attribs=True)
+        class ThreadsExtension:
+            """The Threads extension (MSC4360)
+
+            Provides thread updates for threads that have new activity across all of the
+            user's joined rooms within the sync window.
+
+            Attributes:
+                updates: A nested mapping of room_id -> thread_root_id -> ThreadUpdate.
+                    Each ThreadUpdate contains information about a thread that has new activity,
+                    including the thread root event (if requested) and a pagination token
+                    for fetching older events in that specific thread.
+                prev_batch: A pagination token for fetching more thread updates across all rooms.
+                    If present, indicates there are more thread updates available beyond what
+                    was returned in this response. This token can be used with a future request
+                    to paginate through older thread updates.
+            """
+
+            @attr.s(slots=True, frozen=True, auto_attribs=True)
+            class ThreadUpdate:
+                """Information about a single thread that has new activity.
+
+                Attributes:
+                    thread_root: The thread root event, if requested via include_roots in the
+                        request. This is the event that started the thread.
+                    prev_batch: A pagination token (exclusive) for fetching older events in this
+                        specific thread. Only present if the thread has multiple updates in the
+                        sync window. This token can be used with the /relations endpoint with
+                        dir=b to paginate backwards through the thread's history.
+                    bundled_aggregations: Bundled aggregations for the thread root event,
+                        including the latest_event in the thread (found in
+                        unsigned.m.relations.m.thread). Only present if thread_root is included.
+                """
+
+                thread_root: EventBase | None
+                prev_batch: StreamToken | None
+                bundled_aggregations: "BundledAggregations | None" = None
+
+                def __bool__(self) -> bool:
+                    return bool(self.thread_root) or bool(self.prev_batch)
+
+            updates: Mapping[str, Mapping[str, ThreadUpdate]] | None
+            prev_batch: StreamToken | None
+
+            def __bool__(self) -> bool:
+                return bool(self.updates) or bool(self.prev_batch)
+
         to_device: ToDeviceExtension | None = None
         e2ee: E2eeExtension | None = None
         account_data: AccountDataExtension | None = None
         receipts: ReceiptsExtension | None = None
         typing: TypingExtension | None = None
         thread_subscriptions: ThreadSubscriptionsExtension | None = None
+        threads: ThreadsExtension | None = None
 
         def __bool__(self) -> bool:
             return bool(
@@ -403,6 +454,7 @@ class SlidingSyncResult:
                 or self.receipts
                 or self.typing
                 or self.thread_subscriptions
+                or self.threads
             )
 
     next_pos: SlidingSyncStreamToken
@@ -852,6 +904,7 @@ class PerConnectionState:
     Attributes:
         rooms: The status of each room for the events stream.
         receipts: The status of each room for the receipts stream.
+        account_data: The status of each room for the account data stream.
         room_configs: Map from room_id to the `RoomSyncConfig` of all
             rooms that we have previously sent down.
     """

synapse/types/rest/client/__init__.py

@@ -383,6 +383,19 @@ class SlidingSyncBody(RequestBodyModel):
             enabled: StrictBool | None = False
             limit: StrictInt = 100
 
+        class ThreadsExtension(RequestBodyModel):
+            """The Threads extension (MSC4360)
+
+            Attributes:
+                enabled: Whether the threads extension is enabled.
+                include_roots: whether to include thread root events in the extension response.
+                limit: maximum number of thread updates to return across all joined rooms.
+            """
+
+            enabled: StrictBool | None = False
+            include_roots: StrictBool = False
+            limit: StrictInt = 100
+
         to_device: ToDeviceExtension | None = None
         e2ee: E2eeExtension | None = None
         account_data: AccountDataExtension | None = None
@@ -391,6 +404,9 @@ class SlidingSyncBody(RequestBodyModel):
         thread_subscriptions: ThreadSubscriptionsExtension | None = Field(
             None, alias="io.element.msc4308.thread_subscriptions"
         )
+        threads: ThreadsExtension | None = Field(
+            None, alias="io.element.msc4360.threads"
+        )
 
     conn_id: StrictStr | None = None
     lists: (

synapse/util/async_helpers.py

@@ -340,6 +340,7 @@ T3 = TypeVar("T3")
 T4 = TypeVar("T4")
 T5 = TypeVar("T5")
 T6 = TypeVar("T6")
+T7 = TypeVar("T7")
 
 
 @overload
@@ -469,6 +470,30 @@ async def gather_optional_coroutines(
 ) -> tuple[T1 | None, T2 | None, T3 | None, T4 | None, T5 | None, T6 | None]: ...
 
 
+@overload
+async def gather_optional_coroutines(
+    *coroutines: Unpack[
+        tuple[
+            Coroutine[Any, Any, T1] | None,
+            Coroutine[Any, Any, T2] | None,
+            Coroutine[Any, Any, T3] | None,
+            Coroutine[Any, Any, T4] | None,
+            Coroutine[Any, Any, T5] | None,
+            Coroutine[Any, Any, T6] | None,
+            Coroutine[Any, Any, T7] | None,
+        ]
+    ],
+) -> tuple[
+    T1 | None,
+    T2 | None,
+    T3 | None,
+    T4 | None,
+    T5 | None,
+    T6 | None,
+    T7 | None,
+]: ...
+
+
 async def gather_optional_coroutines(
     *coroutines: Unpack[tuple[Coroutine[Any, Any, T1] | None, ...]],
 ) -> tuple[T1 | None, ...]: