Implement MSC4169: backwards-compatible redaction sending for rooms < v11 using the /send endpoint (#18898)

Implement
[MSC4169](https://github.com/matrix-org/matrix-spec-proposals/pull/4169)

While there is a dedicated API endpoint for redactions, being able to
send redactions using the normal send endpoint is useful when using
[MSC4140](https://github.com/matrix-org/matrix-spec-proposals/pull/4140)
for sending delayed redactions to replicate expiring messages. Currently
this would only work on rooms >= v11 but fail with an internal server
error on older room versions when setting the `redacts` field in the
content, since older rooms would require that field to be outside of
`content`. We can address this by copying it over if necessary.

Relevant spec at
https://spec.matrix.org/v1.8/rooms/v11/#moving-the-redacts-property-of-mroomredaction-events-to-a-content-property

---------

Co-authored-by: Tulir Asokan <tulir@maunium.net>

changelog.d/18898.feature

@@ -0,0 +1 @@
+Support [MSC4169](https://github.com/matrix-org/matrix-spec-proposals/pull/4169) for backwards-compatible redaction sending using the `/send` endpoint. Contributed by @SpiritCroc @ Beeper.

synapse/config/experimental.py

@@ -556,6 +556,9 @@ class ExperimentalConfig(Config):
         # MSC4133: Custom profile fields
         self.msc4133_enabled: bool = experimental.get("msc4133_enabled", False)
 
+        # MSC4169: Backwards-compatible redaction sending using `/send`
+        self.msc4169_enabled: bool = experimental.get("msc4169_enabled", False)
+
         # MSC4210: Remove legacy mentions
         self.msc4210_enabled: bool = experimental.get("msc4210_enabled", False)
 

synapse/handlers/message.py

@@ -1013,14 +1013,37 @@ class EventCreationHandler:
             await self.clock.sleep(random.randint(1, 10))
             raise ShadowBanError()
 
-        if ratelimit:
+        room_version = None
+
+        if (
+            event_dict["type"] == EventTypes.Redaction
+            and "redacts" in event_dict["content"]
+            and self.hs.config.experimental.msc4169_enabled
+        ):
             room_id = event_dict["room_id"]
             try:
                 room_version = await self.store.get_room_version(room_id)
             except NotFoundError:
-                # The room doesn't exist.
                 raise AuthError(403, f"User {requester.user} not in room {room_id}")
 
+            if not room_version.updated_redaction_rules:
+                # Legacy room versions need the "redacts" field outside of the event's
+                # content. However clients may still send it within the content, so move
+                # the field if necessary for compatibility.
+                redacts = event_dict.get("redacts") or event_dict["content"].pop(
+                    "redacts", None
+                )
+                if redacts is not None and "redacts" not in event_dict:
+                    event_dict["redacts"] = redacts
+
+        if ratelimit:
+            if room_version is None:
+                room_id = event_dict["room_id"]
+                try:
+                    room_version = await self.store.get_room_version(room_id)
+                except NotFoundError:
+                    raise AuthError(403, f"User {requester.user} not in room {room_id}")
+
             if room_version.updated_redaction_rules:
                 redacts = event_dict["content"].get("redacts")
             else:

synapse/rest/client/versions.py

@@ -180,6 +180,8 @@ class VersionsRestServlet(RestServlet):
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
                     # MSC4306: Support for thread subscriptions
                     "org.matrix.msc4306": self.config.experimental.msc4306_enabled,
+                    # MSC4169: Backwards-compatible redaction sending using `/send`
+                    "com.beeper.msc4169": self.config.experimental.msc4169_enabled,
                 },
             },
         )