Don't say MSC4140 is supported when it's disabled (#17780)

changelog.d/17780.bugfix

@@ -0,0 +1 @@
+In the response to `GET /_matrix/client/versions`, set the `unstable_features` flag for MSC4140 to `false` when server configuration disables support for delayed events.

synapse/rest/client/versions.py

@@ -172,7 +172,7 @@ class VersionsRestServlet(RestServlet):
                         )
                     ),
                     # MSC4140: Delayed events
-                    "org.matrix.msc4140": True,
+                    "org.matrix.msc4140": bool(self.config.server.max_event_delay_ms),
                     # MSC4151: Report room API (Client-Server API)
                     "org.matrix.msc4151": self.config.experimental.msc4151_enabled,
                     # Simplified sliding sync

tests/rest/client/test_delayed_events.py

@@ -8,11 +8,12 @@ from parameterized import parameterized
 from twisted.test.proto_helpers import MemoryReactor
 
 from synapse.api.errors import Codes
-from synapse.rest.client import delayed_events, room
+from synapse.rest.client import delayed_events, room, versions
 from synapse.server import HomeServer
 from synapse.types import JsonDict
 from synapse.util import Clock
 
+from tests import unittest
 from tests.unittest import HomeserverTestCase
 
 PATH_PREFIX = "/_matrix/client/unstable/org.matrix.msc4140/delayed_events"
@@ -21,6 +22,21 @@ _HS_NAME = "red"
 _EVENT_TYPE = "com.example.test"
 
 
+class DelayedEventsUnstableSupportTestCase(HomeserverTestCase):
+    servlets = [versions.register_servlets]
+
+    def test_false_by_default(self) -> None:
+        channel = self.make_request("GET", "/_matrix/client/versions")
+        self.assertEqual(channel.code, 200, channel.result)
+        self.assertFalse(channel.json_body["unstable_features"]["org.matrix.msc4140"])
+
+    @unittest.override_config({"max_event_delay_duration": "24h"})
+    def test_true_if_enabled(self) -> None:
+        channel = self.make_request("GET", "/_matrix/client/versions")
+        self.assertEqual(channel.code, 200, channel.result)
+        self.assertTrue(channel.json_body["unstable_features"]["org.matrix.msc4140"])
+
+
 class DelayedEventsTestCase(HomeserverTestCase):
     """Tests getting and managing delayed events."""