Initial implementation of MSC3981: recursive relations API (#15315)

Adds an optional keyword argument to the /relations API which
will recurse a limited number of event relationships.

This will cause the API to return not just the events related to the
parent event, but also events related to those related to the parent
event, etc.

This is disabled by default behind an experimental configuration
flag and is currently implemented using prefixed parameters.

1 files changed, 9 insertions, 1 deletions

diff --git a/synapse/rest/client/relations.py b/synapse/rest/client/relations.py
index b8b296bc0c..785dfa08d8 100644
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@@ -19,7 +19,7 @@ from typing import TYPE_CHECKING, Optional, Tuple
 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 @@ class RelationPaginationServlet(RestServlet):
         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 @@ class RelationPaginationServlet(RestServlet):
         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 @@ class RelationPaginationServlet(RestServlet):
             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,