Create MSC4370 endpoint

synapse/config/experimental.py

@@ -3,6 +3,7 @@
 #
 # Copyright 2021 The Matrix.org Foundation C.I.C.
 # Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2025 Element Creations Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -533,6 +534,9 @@ class ExperimentalConfig(Config):
             "msc4108_delegation_endpoint", None
         )
 
+        # MSC4370: Get extremities federation endpoint
+        self.msc4370_enabled = experimental.get("msc4370_enabled", False)
+
         auth_delegated = self.msc3861.enabled or (
             config.get("matrix_authentication_service") or {}
         ).get("enabled", False)

synapse/federation/federation_server.py

@@ -4,6 +4,7 @@
 # Copyright 2019-2021 Matrix.org Federation C.I.C
 # Copyright 2015, 2016 OpenMarket Ltd
 # Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2025 Element Creations Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -683,6 +684,16 @@ class FederationServer(FederationBase):
         resp = await self.registry.on_query(query_type, args)
         return 200, resp
 
+    async def on_get_extremities_request(self, origin: str, room_id: str) -> JsonDict:
+        origin_host, _ = parse_server_name(origin)
+        await self.check_server_matches_acl(origin_host, room_id)
+
+        await self._event_auth_handler.assert_host_in_room(room_id, origin)
+
+        extremities = await self.store.get_forward_extremities_for_room(room_id)
+        prev_event_ids = list(e[0] for e in extremities)
+        return {"prev_events": prev_event_ids}
+
     async def on_make_join_request(
         self, origin: str, room_id: str, user_id: str, supported_versions: list[str]
     ) -> dict[str, Any]:

synapse/federation/transport/server/__init__.py

@@ -4,6 +4,7 @@
 # Copyright 2020 Sorunome
 # Copyright 2014-2021 The Matrix.org Foundation C.I.C.
 # Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2025 Element Creations Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -33,6 +34,7 @@ from synapse.federation.transport.server.federation import (
     FederationMediaDownloadServlet,
     FederationMediaThumbnailServlet,
     FederationUnstableClientKeysClaimServlet,
+    FederationUnstableGetExtremitiesServlet,
 )
 from synapse.http.server import HttpServer, JsonResource
 from synapse.http.servlet import (
@@ -326,6 +328,12 @@ def register_servlets(
                 if not hs.config.media.can_load_media_repo:
                     continue
 
+            if (
+                servletclass == FederationUnstableGetExtremitiesServlet
+                and not hs.config.experimental.msc4370_enabled
+            ):
+                continue
+
             servletclass(
                 hs=hs,
                 authenticator=authenticator,

synapse/federation/transport/server/federation.py

@@ -1,8 +1,9 @@
 #
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
-#  Copyright 2021 The Matrix.org Foundation C.I.C.
+# Copyright 2021 The Matrix.org Foundation C.I.C.
 # Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2025 Element Creations Ltd
 #
 # This program is free software: you can redistribute it and/or modify
 # it under the terms of the GNU Affero General Public License as
@@ -273,6 +274,22 @@ class FederationQueryServlet(BaseFederationServerServlet):
         return await self.handler.on_query_request(query_type, args)
 
 
+class FederationUnstableGetExtremitiesServlet(BaseFederationServerServlet):
+    PREFIX = FEDERATION_UNSTABLE_PREFIX + "/org.matrix.msc4370"
+    PATH = "/extremities/(?P<room_id>[^/]*)"
+    CATEGORY = "Federation requests"
+
+    async def on_GET(
+        self,
+        origin: str,
+        content: Literal[None],
+        query: dict[bytes, list[bytes]],
+        room_id: str,
+    ) -> tuple[int, JsonDict]:
+        result = await self.handler.on_get_extremities_request(origin, room_id)
+        return 200, result
+
+
 class FederationMakeJoinServlet(BaseFederationServerServlet):
     PATH = "/make_join/(?P<room_id>[^/]*)/(?P<user_id>[^/]*)"
     CATEGORY = "Federation requests"
@@ -884,6 +901,7 @@ FEDERATION_SERVLET_CLASSES: tuple[type[BaseFederationServlet], ...] = (
     FederationBackfillServlet,
     FederationTimestampLookupServlet,
     FederationQueryServlet,
+    FederationUnstableGetExtremitiesServlet,
     FederationMakeJoinServlet,
     FederationMakeLeaveServlet,
     FederationEventServlet,