Add support for MSC4293 - Redact on Kick/Ban

synapse/config/experimental.py

@@ -584,8 +584,7 @@ def read_config(
 
         # MSC4293: Redact on Kick/Ban
         self.msc4293_enabled: bool = experimental.get("msc4293_enabled", False)
-      
+
         # MSC4306: Thread Subscriptions
         # (and MSC4308: sliding sync extension for thread subscriptions)
         self.msc4306_enabled: bool = experimental.get("msc4306_enabled", False)
-

synapse/storage/databases/main/events.py

@@ -422,6 +422,10 @@ async def _persist_events_and_state_updates(
 
                 # check that sender can redact
                 redact_allowed = await self._can_sender_redact(event)
+
+                # Signal that this user's past events in this room
+                # should be redacted by adding an entry to
+                # `room_ban_redactions`.
                 if redact_allowed:
                     await self.db_pool.simple_upsert(
                         "room_ban_redactions",
@@ -451,31 +455,45 @@ async def _persist_events_and_state_updates(
                 )
 
     async def _can_sender_redact(self, event: EventBase) -> bool:
-        state_filter = StateFilter.from_types([(EventTypes.PowerLevels, "")])
+        state_filter = StateFilter.from_types(
+            [(EventTypes.PowerLevels, ""), (EventTypes.Create, "")]
+        )
         state = await self.store.get_partial_filtered_current_state_ids(
             event.room_id, state_filter
         )
         pl_id = state[(EventTypes.PowerLevels, "")]
-        pl_event = await self.store.get_event(pl_id)
-        if pl_event:
-            sender_level = pl_event.content.get("users", {}).get(event.sender)
-            if sender_level is None:
-                sender_level = pl_event.content.get("users_default", 0)
-
-            redact_level = pl_event.content.get("redact")
-            if not redact_level:
-                redact_level = pl_event.content.get("events_default", 0)
-
-            room_redaction_level = pl_event.content.get("events", {}).get(
-                "m.room.redaction"
-            )
-            if room_redaction_level:
-                if sender_level < room_redaction_level:
-                    return False
-
-            if sender_level >= redact_level:
+        pl_event = await self.store.get_event(pl_id, allow_none=True)
+
+        if pl_event is None:
+            # per the spec, if a power level event isn't in the room, grant the creator
+            # level 100 and all other users 0
+            create_id = state[(EventTypes.Create, "")]
+            create_event = await self.store.get_event(create_id, allow_none=True)
+            if create_event is None:
+                # not sure how this would happen but if it does then just deny the redaction
+                return False
+            if create_event.sender == event.sender:
                 return True
 
+        assert pl_event is not None
+        sender_level = pl_event.content.get("users", {}).get(event.sender)
+        if sender_level is None:
+            sender_level = pl_event.content.get("users_default", 0)
+
+        redact_level = pl_event.content.get("redact")
+        if not redact_level:
+            redact_level = pl_event.content.get("events_default", 0)
+
+        room_redaction_level = pl_event.content.get("events", {}).get(
+            "m.room.redaction"
+        )
+        if room_redaction_level:
+            if sender_level < room_redaction_level:
+                return False
+
+        if sender_level >= redact_level:
+            return True
+
         return False
 
     async def _calculate_sliding_sync_table_changes(

synapse/storage/databases/main/events_worker.py

@@ -1614,7 +1614,10 @@ def _fetch_event_rows(
                         continue
                     redacting_event_id, redact_end_ordering = res
                     if redact_end_ordering:
-                        if e_row.stream_ordering > redact_end_ordering:
+                        # Avoid redacting any events arriving *after* the membership event which
+                        # ends an active redaction - note that this will always redact
+                        # backfilled events, as they have a negative stream ordering
+                        if e_row.stream_ordering >= redact_end_ordering:
                             continue
                     e_row.redactions.append(redacting_event_id)
         return event_dict

tests/rest/client/test_rooms.py

@@ -4590,7 +4590,7 @@ def _check_redactions(
     def test_banning_local_member_with_flag_redacts_their_events(self) -> None:
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         originals = []
         for i in range(5):
             event = {"body": f"bothersome noise {i}", "msgtype": "m.text"}
@@ -4916,7 +4916,7 @@ def test_redaction_flag_ignored_for_user_if_banner_lacks_redaction_power(
         )
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         original_ids = []
         for i in range(15):
             event = {"body": f"being a menace {i}", "msgtype": "m.text"}
@@ -4955,7 +4955,7 @@ def test_redaction_flag_ignored_for_user_if_banner_lacks_redaction_power(
     def test_kicking_local_member_with_flag_redacts_their_events(self) -> None:
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         originals = []
         for i in range(5):
             event = {"body": f"bothersome noise {i}", "msgtype": "m.text"}
@@ -5273,7 +5273,7 @@ def test_redaction_flag_ignored_for_user_if_kicker_lacks_redaction_power(
         )
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         original_ids = []
         for i in range(15):
             event = {"body": f"being a menace {i}", "msgtype": "m.text"}
@@ -5312,7 +5312,7 @@ def test_redaction_flag_ignored_for_user_if_kicker_lacks_redaction_power(
     def test_MSC4293_flag_ignored_in_other_membership_events(self) -> None:
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         original_ids = []
         for i in range(15):
             event = {"body": f"being a menace {i}", "msgtype": "m.text"}
@@ -5399,7 +5399,7 @@ def test_MSC4293_redaction_applied_via_kick_api(self) -> None:
         """
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         original_ids = []
         for i in range(15):
             event = {"body": f"being a menace {i}", "msgtype": "m.text"}
@@ -5444,7 +5444,7 @@ def test_MSC4293_redaction_applied_via_ban_api(self) -> None:
         """
         self.helper.join(self.room_id, self.bad_user_id, tok=self.bad_tok)
 
-        # bad user send some messages
+        # bad user sends some messages
         original_ids = []
         for i in range(15):
             event = {"body": f"being a menace {i}", "msgtype": "m.text"}