Initial implementation of MSC3981.

changelog.d/15315.feature

@@ -0,0 +1 @@
+Experimental support to recursively provide relations per [MSC3981](https://github.com/matrix-org/matrix-spec-proposals/pull/3981).

synapse/config/experimental.py

@@ -181,3 +181,8 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
 
         # MSC2659: Application service ping endpoint
         self.msc2659_enabled = experimental.get("msc2659_enabled", False)
+
+        # MSC3981: Recurse relations
+        self.msc3981_recurse_relations = experimental.get(
+            "msc3981_recurse_relations", False
+        )

synapse/handlers/relations.py

@@ -85,6 +85,7 @@ async def get_relations(
         event_id: str,
         room_id: str,
         pagin_config: PaginationConfig,
+        recurse: bool,
         include_original_event: bool,
         relation_type: Optional[str] = None,
         event_type: Optional[str] = None,
@@ -98,6 +99,7 @@ async def get_relations(
             event_id: Fetch events that relate to this event ID.
             room_id: The room the event belongs to.
             pagin_config: The pagination config rules to apply, if any.
+            recurse: Whether to recursively find relations.
             include_original_event: Whether to include the parent event.
             relation_type: Only fetch events with this relation type, if given.
             event_type: Only fetch events with this event type, if given.
@@ -132,6 +134,7 @@ async def get_relations(
             direction=pagin_config.direction,
             from_token=pagin_config.from_token,
             to_token=pagin_config.to_token,
+            recurse=recurse,
         )
 
         events = await self._main_store.get_events_as_list(

synapse/rest/client/relations.py

@@ -19,7 +19,7 @@
 from synapse.api.constants import Direction
 from synapse.handlers.relations import ThreadsListInclude
 from synapse.http.server import HttpServer
-from synapse.http.servlet import RestServlet, parse_integer, parse_string
+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
@@ -49,6 +49,7 @@ def __init__(self, hs: "HomeServer"):
         self.auth = hs.get_auth()
         self._store = hs.get_datastores().main
         self._relations_handler = hs.get_relations_handler()
+        self._support_recurse = hs.config.experimental.msc3981_recurse_relations
 
     async def on_GET(
         self,
@@ -63,6 +64,12 @@ async def on_GET(
         pagination_config = await PaginationConfig.from_request(
             self._store, request, default_limit=5, default_dir=Direction.BACKWARDS
         )
+        if self._support_recurse:
+            recurse = parse_boolean(
+                request, "org.matrix.msc3981.recurse", default=False
+            )
+        else:
+            recurse = False
 
         # The unstable version of this API returns an extra field for client
         # compatibility, see https://github.com/matrix-org/synapse/issues/12930.
@@ -75,6 +82,7 @@ async def on_GET(
             event_id=parent_id,
             room_id=room_id,
             pagin_config=pagination_config,
+            recurse=recurse,
             include_original_event=include_original_event,
             relation_type=relation_type,
             event_type=event_type,

synapse/storage/databases/main/relations.py

@@ -172,6 +172,7 @@ async def get_relations_for_event(
         direction: Direction = Direction.BACKWARDS,
         from_token: Optional[StreamToken] = None,
         to_token: Optional[StreamToken] = None,
+        recurse: bool = False,
     ) -> Tuple[Sequence[_RelatedEvent], Optional[StreamToken]]:
         """Get a list of relations for an event, ordered by topological ordering.
 
@@ -186,6 +187,7 @@ async def get_relations_for_event(
                 oldest first (forwards).
             from_token: Fetch rows from the given token, or from the start if None.
             to_token: Fetch rows up to the given token, or up to the end if None.
+            recurse: Whether to recursively find relations.
 
         Returns:
             A tuple of:
@@ -200,7 +202,7 @@ async def get_relations_for_event(
         # Ensure bad limits aren't being passed in.
         assert limit >= 0
 
-        where_clause = ["relates_to_id = ?", "room_id = ?"]
+        where_clause = ["room_id = ?"]
         where_args: List[Union[str, int]] = [event.event_id, room_id]
         is_redacted = event.internal_metadata.is_redacted()
 
@@ -229,18 +231,42 @@ async def get_relations_for_event(
         if pagination_clause:
             where_clause.append(pagination_clause)
 
-        sql = """
-            SELECT event_id, relation_type, sender, topological_ordering, stream_ordering
-            FROM event_relations
-            INNER JOIN events USING (event_id)
-            WHERE %s
-            ORDER BY topological_ordering %s, stream_ordering %s
-            LIMIT ?
-        """ % (
-            " AND ".join(where_clause),
-            order,
-            order,
-        )
+        # TODO This needs to be a recursive query that maybe still matches some of the times above.
+        if recurse:
+            sql = """
+                WITH RECURSIVE related_events AS (
+                    SELECT event_id, relation_type, relates_to_id, 0 depth
+                    FROM event_relations
+                    WHERE relates_to_id = ?
+                    UNION SELECT e.event_id, e.relation_type, e.relates_to_id, depth + 1
+                    FROM event_relations e
+                    INNER JOIN related_events r ON r.event_id = e.relates_to_id
+                    WHERE depth <= 3
+                )
+                SELECT event_id, relation_type, sender, topological_ordering, stream_ordering
+                FROM related_events
+                INNER JOIN events USING (event_id)
+                WHERE %s
+                ORDER BY topological_ordering %s, stream_ordering %s
+                LIMIT ?;
+            """ % (
+                " AND ".join(where_clause),
+                order,
+                order,
+            )
+        else:
+            sql = """
+                SELECT event_id, relation_type, sender, topological_ordering, stream_ordering
+                FROM event_relations
+                INNER JOIN events USING (event_id)
+                WHERE relates_to_id = ? AND %s
+                ORDER BY topological_ordering %s, stream_ordering %s
+                LIMIT ?
+            """ % (
+                " AND ".join(where_clause),
+                order,
+                order,
+            )
 
         def _get_recent_references_for_event_txn(
             txn: LoggingTransaction,