Add companion endpoint for MSC4360: Sliding Sync Threads Extension

changelog.d/19041.feature

@@ -0,0 +1 @@
+Add companion endpoint for MSC4360: Sliding Sync Threads Extension.

synapse/rest/client/relations.py

@@ -20,17 +20,22 @@
 
 import logging
 import re
-from typing import TYPE_CHECKING, Optional, Tuple
+from typing import TYPE_CHECKING, Dict, Optional, Sequence, Tuple
 
 from synapse.api.constants import Direction
-from synapse.handlers.relations import ThreadsListInclude
+from synapse.api.errors import SynapseError
+from synapse.events.utils import SerializeEventConfig
+from synapse.handlers.relations import BundledAggregations, ThreadsListInclude
 from synapse.http.server import HttpServer
 from synapse.http.servlet import RestServlet, parse_boolean, parse_integer, parse_string
 from synapse.http.site import SynapseRequest
 from synapse.rest.client._base import client_patterns
-from synapse.storage.databases.main.relations import ThreadsNextBatch
-from synapse.streams.config import PaginationConfig
-from synapse.types import JsonDict
+from synapse.storage.databases.main.relations import ThreadsNextBatch, ThreadUpdateInfo
+from synapse.streams.config import (
+    PaginationConfig,
+    extract_stream_token_from_pagination_token,
+)
+from synapse.types import JsonDict, RoomStreamToken, StreamToken
 
 if TYPE_CHECKING:
     from synapse.server import HomeServer
@@ -133,6 +138,187 @@ async def on_GET(
         return 200, result
 
 
+class ThreadUpdatesServlet(RestServlet):
+    """
+    Companion endpoint to the Sliding Sync threads extension (MSC4360).
+    Allows clients to bulk fetch thread updates across all joined rooms.
+    """
+
+    PATTERNS = client_patterns(
+        "/io.element.msc4360/thread_updates$",
+        unstable=True,
+        releases=(),
+    )
+    CATEGORY = "Client API requests"
+
+    def __init__(self, hs: "HomeServer"):
+        super().__init__()
+        self.clock = hs.get_clock()
+        self.auth = hs.get_auth()
+        self.store = hs.get_datastores().main
+        self.relations_handler = hs.get_relations_handler()
+        self.event_serializer = hs.get_event_client_serializer()
+
+    async def _serialize_thread_updates(
+        self,
+        thread_updates: Sequence[ThreadUpdateInfo],
+        bundled_aggregations: Dict[str, BundledAggregations],
+        time_now: int,
+        serialize_options: SerializeEventConfig,
+    ) -> Dict[str, Dict[str, JsonDict]]:
+        """
+        Serialize thread updates into the response format.
+
+        Args:
+            thread_updates: List of thread update info from storage
+            bundled_aggregations: Map of event_id to bundled aggregations
+            time_now: Current time in milliseconds
+            serialize_options: Serialization configuration
+
+        Returns:
+            Nested dict mapping room_id -> thread_root_id -> thread update dict
+        """
+        chunk: Dict[str, Dict[str, JsonDict]] = {}
+
+        for update in thread_updates:
+            room_id = update.room_id
+            thread_id = update.thread_id
+
+            if room_id not in chunk:
+                chunk[room_id] = {}
+
+            update_dict: JsonDict = {}
+
+            # Serialize thread root if present
+            if update.thread_root_event is not None:
+                bundle_aggs_map = (
+                    {thread_id: bundled_aggregations[thread_id]}
+                    if thread_id in bundled_aggregations
+                    else None
+                )
+                serialized_events = await self.event_serializer.serialize_events(
+                    [update.thread_root_event],
+                    time_now,
+                    config=serialize_options,
+                    bundle_aggregations=bundle_aggs_map,
+                )
+                if serialized_events:
+                    update_dict["thread_root"] = serialized_events[0]
+
+            # Add per-thread prev_batch if present
+            if update.prev_batch is not None:
+                update_dict["prev_batch"] = await update.prev_batch.to_string(
+                    self.store
+                )
+
+            chunk[room_id][thread_id] = update_dict
+
+        return chunk
+
+    async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
+        requester = await self.auth.get_user_by_req(request)
+
+        # Parse parameters
+        dir_str = parse_string(request, "dir", default="b")
+        if dir_str != "b":
+            raise SynapseError(
+                400,
+                "The 'dir' parameter must be 'b' (backward). Forward pagination is not supported.",
+            )
+
+        limit = parse_integer(request, "limit", default=100)
+        if limit <= 0:
+            raise SynapseError(400, "The 'limit' parameter must be positive.")
+
+        from_token_str = parse_string(request, "from")
+        to_token_str = parse_string(request, "to")
+
+        # Parse pagination tokens
+        from_token: Optional[RoomStreamToken] = None
+        to_token: Optional[RoomStreamToken] = None
+
+        if from_token_str:
+            try:
+                stream_token_str = extract_stream_token_from_pagination_token(
+                    from_token_str
+                )
+                stream_token = await StreamToken.from_string(
+                    self.store, stream_token_str
+                )
+                from_token = stream_token.room_key
+            except Exception:
+                raise SynapseError(400, "'from' parameter is invalid")
+
+        if to_token_str:
+            try:
+                stream_token_str = extract_stream_token_from_pagination_token(
+                    to_token_str
+                )
+                stream_token = await StreamToken.from_string(
+                    self.store, stream_token_str
+                )
+                to_token = stream_token.room_key
+            except Exception:
+                raise SynapseError(400, "'to' parameter is invalid")
+
+        # Fetch thread updates from storage
+        # For backward pagination:
+        # - 'from' (upper bound, exclusive) maps to 'to_token' (inclusive with <=)
+        #   Since next_batch is (last_returned - 1), <= excludes the last returned item
+        # - 'to' (lower bound, exclusive) maps to 'from_token' (exclusive with >)
+        thread_updates, next_token = await self.store.get_thread_updates_for_user(
+            user_id=requester.user.to_string(),
+            from_token=to_token,
+            to_token=from_token,
+            limit=limit,
+            include_thread_roots=True,
+        )
+
+        # Serialize response
+        chunk: Dict[str, Dict[str, JsonDict]] = {}
+
+        if thread_updates:
+            # Get bundled aggregations for all thread roots
+            thread_root_events = [
+                update.thread_root_event
+                for update in thread_updates
+                if update.thread_root_event is not None
+            ]
+
+            bundled_aggregations = {}
+            if thread_root_events:
+                bundled_aggregations = (
+                    await self.relations_handler.get_bundled_aggregations(
+                        thread_root_events, requester.user.to_string()
+                    )
+                )
+
+            # Set up serialization
+            time_now = self.clock.time_msec()
+            serialize_options = SerializeEventConfig(
+                requester=requester,
+            )
+
+            # Serialize all thread updates
+            chunk = await self._serialize_thread_updates(
+                thread_updates=thread_updates,
+                bundled_aggregations=bundled_aggregations,
+                time_now=time_now,
+                serialize_options=serialize_options,
+            )
+
+        # Build response
+        response: JsonDict = {"chunk": chunk}
+
+        # Add next_batch token for pagination
+        if next_token is not None:
+            response["next_batch"] = await next_token.to_string(self.store)
+
+        return 200, response
+
+
 def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     RelationPaginationServlet(hs).register(http_server)
     ThreadsServlet(hs).register(http_server)
+    if hs.config.experimental.msc4360_enabled:
+        ThreadUpdatesServlet(hs).register(http_server)

synapse/streams/config.py

@@ -36,6 +36,32 @@
 MAX_LIMIT = 1000
 
 
+def extract_stream_token_from_pagination_token(token_str: str) -> str:
+    """
+    Extract the StreamToken portion from a pagination 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.
+
+    Args:
+        token_str: The token string to parse
+
+    Returns:
+        The StreamToken portion of the token
+    """
+    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
+
+
 @attr.s(slots=True, auto_attribs=True)
 class PaginationConfig:
     """A configuration object which stores pagination parameters."""
@@ -58,40 +84,24 @@ async def from_request(
         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:
-                stream_token_str = extract_stream_token(from_tok_str)
+                stream_token_str = extract_stream_token_from_pagination_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:
-                stream_token_str = extract_stream_token(to_tok_str)
+                stream_token_str = extract_stream_token_from_pagination_token(
+                    to_tok_str
+                )
                 to_tok = await StreamToken.from_string(store, stream_token_str)
         except Exception:
             raise SynapseError(400, "'to' parameter is invalid")