Do not return unspecced original_event field when using the stable /relations endpoint. (#14025)

Keep the old behavior (of including the original_event field) for any
requests to the /unstable version of the endpoint, but do not include
the field when the /v1 version is used.

This should avoid new clients from depending on this field, but will
not help with current dependencies.

4 files changed, 28 insertions, 17 deletions

diff --git a/changelog.d/14025.bugfix b/changelog.d/14025.bugfix
new file mode 100644
index 0000000000..391364f44d
--- /dev/null
+++ b/changelog.d/14025.bugfix
@@ -0,0 +1 @@
+Do not return an unspecified `original_event` field when using the stable `/relations` endpoint. Introduced in Synapse v1.57.0.
diff --git a/synapse/handlers/relations.py b/synapse/handlers/relations.py
index 28d7093f08..63bc6a7aa5 100644
--- a/synapse/handlers/relations.py
+++ b/synapse/handlers/relations.py
@@ -78,6 +78,7 @@ class RelationsHandler:
         direction: str = "b",
         from_token: Optional[StreamToken] = None,
         to_token: Optional[StreamToken] = None,
+        include_original_event: bool = False,
     ) -> JsonDict:
         """Get related events of a event, ordered by topological ordering.
 
@@ -94,6 +95,7 @@ class RelationsHandler:
                 oldest first (`"f"`).
             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.
+            include_original_event: Whether to include the parent event.
 
         Returns:
             The pagination chunk.
@@ -138,25 +140,24 @@ class RelationsHandler:
             is_peeking=(member_event_id is None),
         )
 
-        now = self._clock.time_msec()
-        # Do not bundle aggregations when retrieving the original event because
-        # we want the content before relations are applied to it.
-        original_event = self._event_serializer.serialize_event(
-            event, now, bundle_aggregations=None
-        )
         # The relations returned for the requested event do include their
         # bundled aggregations.
         aggregations = await self.get_bundled_aggregations(
             events, requester.user.to_string()
         )
-        serialized_events = self._event_serializer.serialize_events(
-            events, now, bundle_aggregations=aggregations
-        )
 
-        return_value = {
-            "chunk": serialized_events,
-            "original_event": original_event,
+        now = self._clock.time_msec()
+        return_value: JsonDict = {
+            "chunk": self._event_serializer.serialize_events(
+                events, now, bundle_aggregations=aggregations
+            ),
         }
+        if include_original_event:
+            # Do not bundle aggregations when retrieving the original event because
+            # we want the content before relations are applied to it.
+            return_value["original_event"] = self._event_serializer.serialize_event(
+                event, now, bundle_aggregations=None
+            )
 
         if next_token:
             return_value["next_batch"] = await next_token.to_string(self._main_store)
diff --git a/synapse/rest/client/relations.py b/synapse/rest/client/relations.py
index 205c556f64..7a25de5c85 100644
--- a/synapse/rest/client/relations.py
+++ b/synapse/rest/client/relations.py
@@ -82,6 +82,11 @@ class RelationPaginationServlet(RestServlet):
         if to_token_str:
             to_token = await StreamToken.from_string(self.store, to_token_str)
 
+        # The unstable version of this API returns an extra field for client
+        # compatibility, see https://github.com/matrix-org/synapse/issues/12930.
+        assert request.path is not None
+        include_original_event = request.path.startswith(b"/_matrix/client/unstable/")
+
         result = await self._relations_handler.get_relations(
             requester=requester,
             event_id=parent_id,
@@ -92,6 +97,7 @@ class RelationPaginationServlet(RestServlet):
             direction=direction,
             from_token=from_token,
             to_token=to_token,
+            include_original_event=include_original_event,
         )
 
         return 200, result
diff --git a/tests/rest/client/test_relations.py b/tests/rest/client/test_relations.py
index fef3b72d76..988cdb746d 100644
--- a/tests/rest/client/test_relations.py
+++ b/tests/rest/client/test_relations.py
@@ -654,6 +654,14 @@ class RelationsTestCase(BaseRelationsTestCase):
         )
 
         # We also expect to get the original event (the id of which is self.parent_id)
+        # when requesting the unstable endpoint.
+        self.assertNotIn("original_event", channel.json_body)
+        channel = self.make_request(
+            "GET",
+            f"/_matrix/client/unstable/rooms/{self.room}/relations/{self.parent_id}?limit=1",
+            access_token=self.user_token,
+        )
+        self.assertEqual(200, channel.code, channel.json_body)
         self.assertEqual(
             channel.json_body["original_event"]["event_id"], self.parent_id
         )
@@ -755,11 +763,6 @@ class RelationPaginationTestCase(BaseRelationsTestCase):
             channel.json_body["chunk"][0],
         )
 
-        # We also expect to get the original event (the id of which is self.parent_id)
-        self.assertEqual(
-            channel.json_body["original_event"]["event_id"], self.parent_id
-        )
-
         # Make sure next_batch has something in it that looks like it could be a
         # valid token.
         self.assertIsInstance(