Implement MSC-4169: backwards-compatible send of redactions for rooms < v11 using the /send endpoint

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

@@ -552,6 +552,9 @@ def read_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

@@ -1013,6 +1013,15 @@ async def create_and_send_nonmember_event(
 
             if room_version.updated_redaction_rules:
                 redacts = event_dict["content"].get("redacts")
+            elif self.hs.config.experimental.msc4169_enabled:
+                # Legacy room versions need the "redacts" field outside of the event's
+                # content. However clients may still send it within the content, so copy
+                # the field if necessary for compatibility.
+                redacts = event_dict.get("redacts") or event_dict["content"].pop(
+                    "redacts", None
+                )
+                if redacts and "redacts" not in event_dict:
+                    event_dict["redacts"] = redacts
             else:
                 redacts = event_dict.get("redacts")
 

synapse/rest/client/versions.py

@@ -177,6 +177,8 @@ async def on_GET(self, request: SynapseRequest) -> Tuple[int, JsonDict]:
                     "uk.tcpip.msc4133": self.config.experimental.msc4133_enabled,
                     # MSC4155: Invite filtering
                     "org.matrix.msc4155": self.config.experimental.msc4155_enabled,
+                    # MSC4169: Backwards-compatible redaction sending useing `/send`
+                    "com.beeper.msc4169": self.config.experimental.msc4169_enabled,
                 },
             },
         )