Support MSC4140: Delayed events (Futures)

changelog.d/17326.feature

@@ -0,0 +1,2 @@
+Add initial implementation of delayed events as proposed by [MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140).
+

docs/usage/configuration/config_documentation.md

@@ -761,6 +761,19 @@ email:
     password_reset: "[%(server_name)s] Password reset"
     email_validation: "[%(server_name)s] Validate your email"
 ```
+---
+### `max_event_delay_duration`
+
+The maximum allowed duration by which sent events can be delayed, as per
+[MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140).
+Must be a positive value if set.
+
+Defaults to no duration (`null`), which disallows sending delayed events.
+
+Example configuration:
+```yaml
+max_event_delay_duration: 24h
+```
 
 ## Homeserver blocking
 Useful options for Synapse admins.

docs/workers.md

@@ -290,6 +290,7 @@ information.
 Additionally, the following REST endpoints can be handled for GET requests:
 
     ^/_matrix/client/(api/v1|r0|v3|unstable)/pushrules/
+    ^/_matrix/client/unstable/org.matrix.msc4140/delayed_events
 
 Pagination requests can also be handled, but all requests for a given
 room must be routed to the same instance. Additionally, care must be taken to

synapse/app/generic_worker.py

@@ -65,6 +65,7 @@
 )
 from synapse.storage.databases.main.censor_events import CensorEventsStore
 from synapse.storage.databases.main.client_ips import ClientIpWorkerStore
+from synapse.storage.databases.main.delayed_events import DelayedEventsStore
 from synapse.storage.databases.main.deviceinbox import DeviceInboxWorkerStore
 from synapse.storage.databases.main.devices import DeviceWorkerStore
 from synapse.storage.databases.main.directory import DirectoryWorkerStore
@@ -161,6 +162,7 @@ class GenericWorkerStore(
     TaskSchedulerWorkerStore,
     ExperimentalFeaturesStore,
     SlidingSyncStore,
+    DelayedEventsStore,
 ):
     # Properties that multiple storage classes define. Tell mypy what the
     # expected type is.

synapse/config/server.py

@@ -2,7 +2,7 @@
 # This file is licensed under the Affero General Public License (AGPL) version 3.
 #
 # Copyright 2014-2021 The Matrix.org Foundation C.I.C.
-# Copyright (C) 2023 New Vector, Ltd
+# Copyright (C) 2023-2024 New Vector, 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
@@ -780,6 +780,17 @@ def read_config(self, config: JsonDict, **kwargs: Any) -> None:
         else:
             self.delete_stale_devices_after = None
 
+        # The maximum allowed delay duration for delayed events (MSC4140).
+        max_event_delay_duration = config.get("max_event_delay_duration")
+        if max_event_delay_duration is not None:
+            self.max_event_delay_ms: Optional[int] = self.parse_duration(
+                max_event_delay_duration
+            )
+            if self.max_event_delay_ms <= 0:
+                raise ConfigError("max_event_delay_duration must be a positive value")
+        else:
+            self.max_event_delay_ms = None
+
     def has_tls_listener(self) -> bool:
         return any(listener.is_tls() for listener in self.listeners)