Move the (unstable) `dir` parameter for /relations behind an experimental flag. (#12984)

MSC3715 defines this parameter, but the unstable version of it should be
behind an experimental flag.

changelog.d/12984.misc

@@ -0,0 +1 @@
+Move [MSC3715](https://github.com/matrix-org/matrix-spec-proposals/pull/3715) behind an experimental config flag.

synapse/config/experimental.py

@@ -84,3 +84,6 @@ class ExperimentalConfig(Config):
 
         # MSC3772: A push rule for mutual relations.
         self.msc3772_enabled: bool = experimental.get("msc3772_enabled", False)
+
+        # MSC3715: dir param on /relations.
+        self.msc3715_enabled: bool = experimental.get("msc3715_enabled", False)

synapse/rest/client/relations.py

@@ -43,6 +43,7 @@ class RelationPaginationServlet(RestServlet):
         self.auth = hs.get_auth()
         self.store = hs.get_datastores().main
         self._relations_handler = hs.get_relations_handler()
+        self._msc3715_enabled = hs.config.experimental.msc3715_enabled
 
     async def on_GET(
         self,
@@ -55,9 +56,15 @@ class RelationPaginationServlet(RestServlet):
         requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         limit = parse_integer(request, "limit", default=5)
+        if self._msc3715_enabled:
             direction = parse_string(
-            request, "org.matrix.msc3715.dir", default="b", allowed_values=["f", "b"]
+                request,
+                "org.matrix.msc3715.dir",
+                default="b",
+                allowed_values=["f", "b"],
             )
+        else:
+            direction = "b"
         from_token_str = parse_string(request, "from")
         to_token_str = parse_string(request, "to")
 

tests/rest/client/test_relations.py

@@ -728,6 +728,7 @@ class RelationsTestCase(BaseRelationsTestCase):
 
 
 class RelationPaginationTestCase(BaseRelationsTestCase):
+    @unittest.override_config({"experimental_features": {"msc3715_enabled": True}})
     def test_basic_paginate_relations(self) -> None:
         """Tests that calling pagination API correctly the latest relations."""
         channel = self._send_relation(RelationTypes.ANNOTATION, "m.reaction", "a")