Support the MSC3715 for `/relations`. (#11941)

This adds an unstable org.matrix.msc3715.dir parameter
which acts like dir on /mesages.

changelog.d/11941.feature

@@ -0,0 +1 @@
+Support the `dir` parameter on the `/relations` endpoint, per [MSC3715](https://github.com/matrix-org/matrix-doc/pull/3715).

synapse/rest/client/relations.py

@@ -111,6 +111,9 @@ class RelationPaginationServlet(RestServlet):
             raise SynapseError(404, "Unknown parent event.")
 
         limit = parse_integer(request, "limit", default=5)
+        direction = parse_string(
+            request, "org.matrix.msc3715.dir", default="b", allowed_values=["f", "b"]
+        )
         from_token_str = parse_string(request, "from")
         to_token_str = parse_string(request, "to")
 
@@ -128,6 +131,7 @@ class RelationPaginationServlet(RestServlet):
                 relation_type=relation_type,
                 event_type=event_type,
                 limit=limit,
+                direction=direction,
                 from_token=from_token,
                 to_token=to_token,
             )

tests/rest/client/test_relations.py

@@ -169,24 +169,28 @@ class RelationsTestCase(unittest.HomeserverTestCase):
         """Tests that calling pagination API correctly the latest relations."""
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")
         self.assertEquals(200, channel.code, channel.json_body)
+        first_annotation_id = channel.json_body["event_id"]
 
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "b")
         self.assertEquals(200, channel.code, channel.json_body)
-        annotation_id = channel.json_body["event_id"]
+        second_annotation_id = channel.json_body["event_id"]
 
         channel = self.make_request(
             "GET",
-            "/_matrix/client/unstable/rooms/%s/relations/%s?limit=1"
+            f"/_matrix/client/unstable/rooms/{self.room}/relations/{self.parent_id}?limit=1",
-            % (self.room, self.parent_id),
             access_token=self.user_token,
         )
         self.assertEquals(200, channel.code, channel.json_body)
 
-        # We expect to get back a single pagination result, which is the full
+        # We expect to get back a single pagination result, which is the latest
-        # relation event we sent above.
+        # full relation event we sent above.
         self.assertEquals(len(channel.json_body["chunk"]), 1, channel.json_body)
         self.assert_dict(
-            {"event_id": annotation_id, "sender": self.user_id, "type": "m.reaction"},
+            {
+                "event_id": second_annotation_id,
+                "sender": self.user_id,
+                "type": "m.reaction",
+            },
             channel.json_body["chunk"][0],
         )
 
@@ -201,6 +205,27 @@ class RelationsTestCase(unittest.HomeserverTestCase):
             channel.json_body.get("next_batch"), str, channel.json_body
         )
 
+        # Request the relations again, but with a different direction.
+        channel = self.make_request(
+            "GET",
+            f"/_matrix/client/unstable/rooms/{self.room}/relations"
+            f"/{self.parent_id}?limit=1&org.matrix.msc3715.dir=f",
+            access_token=self.user_token,
+        )
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        # We expect to get back a single pagination result, which is the earliest
+        # full relation event we sent above.
+        self.assertEquals(len(channel.json_body["chunk"]), 1, channel.json_body)
+        self.assert_dict(
+            {
+                "event_id": first_annotation_id,
+                "sender": self.user_id,
+                "type": "m.reaction",
+            },
+            channel.json_body["chunk"][0],
+        )
+
     def _stream_token_to_relation_token(self, token: str) -> str:
         """Convert a StreamToken into a legacy token (RelationPaginationToken)."""
         room_key = self.get_success(StreamToken.from_string(self.store, token)).room_key