Implement MSC2174: move redacts to a content property. (#15395)

This moves `redacts` from being a top-level property to
a `content` property in a new room version.

MSC2176 (which was previously implemented) states to not
`redact` this property.

8 files changed, 87 insertions, 17 deletions

diff --git a/changelog.d/15395.misc b/changelog.d/15395.misc
new file mode 100644
index 0000000000..ee93845241
--- /dev/null
+++ b/changelog.d/15395.misc
@@ -0,0 +1 @@
+Implement [MSC2174](https://github.com/matrix-org/matrix-spec-proposals/pull/2174) to move the `redacts` key to a `content` property.
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index 3dcae12161..5d9c13e3c3 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -80,7 +80,8 @@ class RoomVersion:
     limit_notifications_power_levels: bool
     # MSC2175: No longer include the creator in m.room.create events.
     msc2175_implicit_room_creator: bool
-    # MSC2174/MSC2176: Apply updated redaction rules algorithm.
+    # MSC2174/MSC2176: Apply updated redaction rules algorithm, move redacts to
+    # content property.
     msc2176_redaction_rules: bool
     # MSC3083: Support the 'restricted' join_rule.
     msc3083_join_rules: bool
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index f95d00d472..25898b95a5 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -793,7 +793,7 @@ def check_redaction(
     """Check whether the event sender is allowed to redact the target event.
 
     Returns:
-        True if the the sender is allowed to redact the target event if the
+        True if the sender is allowed to redact the target event if the
         target event was created by them.
         False if the sender is allowed to redact the target event with no
         further checks.
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index d475fe7ae5..4501518cf0 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -326,7 +326,6 @@ class EventBase(metaclass=abc.ABCMeta):
     hashes: DictProperty[Dict[str, str]] = DictProperty("hashes")
     origin: DictProperty[str] = DictProperty("origin")
     origin_server_ts: DictProperty[int] = DictProperty("origin_server_ts")
-    redacts: DefaultDictProperty[Optional[str]] = DefaultDictProperty("redacts", None)
     room_id: DictProperty[str] = DictProperty("room_id")
     sender: DictProperty[str] = DictProperty("sender")
     # TODO state_key should be Optional[str]. This is generally asserted in Synapse
@@ -346,6 +345,13 @@ class EventBase(metaclass=abc.ABCMeta):
     def membership(self) -> str:
         return self.content["membership"]
 
+    @property
+    def redacts(self) -> Optional[str]:
+        """MSC2176 moved the redacts field into the content."""
+        if self.room_version.msc2176_redaction_rules:
+            return self.content.get("redacts")
+        return self.get("redacts")
+
     def is_state(self) -> bool:
         return self.get_state_key() is not None
 
diff --git a/synapse/events/builder.py b/synapse/events/builder.py
index c82745275f..a254548c6c 100644
--- a/synapse/events/builder.py
+++ b/synapse/events/builder.py
@@ -173,7 +173,9 @@ class EventBuilder:
         if self.is_state():
             event_dict["state_key"] = self._state_key
 
-        if self._redacts is not None:
+        # MSC2174 moves the redacts property to the content, it is invalid to
+        # provide it as a top-level property.
+        if self._redacts is not None and not self.room_version.msc2176_redaction_rules:
             event_dict["redacts"] = self._redacts
 
         if self._origin_server_ts is not None:
diff --git a/synapse/rest/client/room.py b/synapse/rest/client/room.py
index c0705d4291..7699cc8d1b 100644
--- a/synapse/rest/client/room.py
+++ b/synapse/rest/client/room.py
@@ -1096,6 +1096,7 @@ class RoomRedactEventRestServlet(TransactionRestServlet):
         super().__init__(hs)
         self.event_creation_handler = hs.get_event_creation_handler()
         self.auth = hs.get_auth()
+        self._store = hs.get_datastores().main
         self._relation_handler = hs.get_relations_handler()
         self._msc3912_enabled = hs.config.experimental.msc3912_enabled
 
@@ -1113,6 +1114,19 @@ class RoomRedactEventRestServlet(TransactionRestServlet):
     ) -> Tuple[int, JsonDict]:
         content = parse_json_object_from_request(request)
 
+        # Ensure the redacts property in the content matches the one provided in
+        # the URL.
+        room_version = await self._store.get_room_version(room_id)
+        if room_version.msc2176_redaction_rules:
+            if "redacts" in content and content["redacts"] != event_id:
+                raise SynapseError(
+                    400,
+                    "Cannot provide a redacts value incoherent with the event_id of the URL parameter",
+                    Codes.INVALID_PARAM,
+                )
+            else:
+                content["redacts"] = event_id
+
         try:
             with_relations = None
             if self._msc3912_enabled and "org.matrix.msc3912.with_relations" in content:
@@ -1128,20 +1142,23 @@ class RoomRedactEventRestServlet(TransactionRestServlet):
                     requester, txn_id, room_id
                 )
 
+            # Event is not yet redacted, create a new event to redact it.
             if event is None:
+                event_dict = {
+                    "type": EventTypes.Redaction,
+                    "content": content,
+                    "room_id": room_id,
+                    "sender": requester.user.to_string(),
+                }
+                # Earlier room versions had a top-level redacts property.
+                if not room_version.msc2176_redaction_rules:
+                    event_dict["redacts"] = event_id
+
                 (
                     event,
                     _,
                 ) = await self.event_creation_handler.create_and_send_nonmember_event(
-                    requester,
-                    {
-                        "type": EventTypes.Redaction,
-                        "content": content,
-                        "room_id": room_id,
-                        "sender": requester.user.to_string(),
-                        "redacts": event_id,
-                    },
-                    txn_id=txn_id,
+                    requester, event_dict, txn_id=txn_id
                 )
 
                 if with_relations:
diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py
index c35f58f462..1b179acb20 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -318,7 +318,11 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
         """Redaction events have no special behaviour until MSC2174/MSC2176."""
 
         self.run_test(
-            {"type": "m.room.redaction", "content": {"redacts": "$test2:domain"}},
+            {
+                "type": "m.room.redaction",
+                "content": {"redacts": "$test2:domain"},
+                "redacts": "$test2:domain",
+            },
             {
                 "type": "m.room.redaction",
                 "content": {},
@@ -330,7 +334,11 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
 
         # After MSC2174, redaction events keep the redacts content key.
         self.run_test(
-            {"type": "m.room.redaction", "content": {"redacts": "$test2:domain"}},
+            {
+                "type": "m.room.redaction",
+                "content": {"redacts": "$test2:domain"},
+                "redacts": "$test2:domain",
+            },
             {
                 "type": "m.room.redaction",
                 "content": {"redacts": "$test2:domain"},
diff --git a/tests/rest/client/test_redactions.py b/tests/rest/client/test_redactions.py
index 5dfe44defb..84a60c0b07 100644
--- a/tests/rest/client/test_redactions.py
+++ b/tests/rest/client/test_redactions.py
@@ -16,6 +16,7 @@ from typing import List, Optional
 from twisted.test.proto_helpers import MemoryReactor
 
 from synapse.api.constants import EventTypes, RelationTypes
+from synapse.api.room_versions import RoomVersions
 from synapse.rest import admin
 from synapse.rest.client import login, room, sync
 from synapse.server import HomeServer
@@ -74,6 +75,7 @@ class RedactionsTestCase(HomeserverTestCase):
         event_id: str,
         expect_code: int = 200,
         with_relations: Optional[List[str]] = None,
+        content: Optional[JsonDict] = None,
     ) -> JsonDict:
         """Helper function to send a redaction event.
 
@@ -81,7 +83,7 @@ class RedactionsTestCase(HomeserverTestCase):
         """
         path = "/_matrix/client/r0/rooms/%s/redact/%s" % (room_id, event_id)
 
-        request_content = {}
+        request_content = content or {}
         if with_relations:
             request_content["org.matrix.msc3912.with_relations"] = with_relations
 
@@ -92,7 +94,7 @@ class RedactionsTestCase(HomeserverTestCase):
         return channel.json_body
 
     def _sync_room_timeline(self, access_token: str, room_id: str) -> List[JsonDict]:
-        channel = self.make_request("GET", "sync", access_token=self.mod_access_token)
+        channel = self.make_request("GET", "sync", access_token=access_token)
         self.assertEqual(channel.code, 200)
         room_sync = channel.json_body["rooms"]["join"][room_id]
         return room_sync["timeline"]["events"]
@@ -466,3 +468,36 @@ class RedactionsTestCase(HomeserverTestCase):
         )
         self.assertIn("body", event_dict["content"], event_dict)
         self.assertEqual("I'm in a thread!", event_dict["content"]["body"])
+
+    def test_content_redaction(self) -> None:
+        """MSC2174 moved the redacts property to the content."""
+        # Create a room with the newer room version.
+        room_id = self.helper.create_room_as(
+            self.mod_user_id,
+            tok=self.mod_access_token,
+            room_version=RoomVersions.MSC2176.identifier,
+        )
+
+        # Create an event.
+        b = self.helper.send(room_id=room_id, tok=self.mod_access_token)
+        event_id = b["event_id"]
+
+        # Attempt to redact it with a bogus event ID.
+        self._redact_event(
+            self.mod_access_token,
+            room_id,
+            event_id,
+            expect_code=400,
+            content={"redacts": "foo"},
+        )
+
+        # Redact it for real.
+        self._redact_event(self.mod_access_token, room_id, event_id)
+
+        # Sync the room, to get the id of the create event
+        timeline = self._sync_room_timeline(self.mod_access_token, room_id)
+        redact_event = timeline[-1]
+        self.assertEqual(redact_event["type"], EventTypes.Redaction)
+        # The redacts key should be in the content.
+        self.assertNotIn("redacts", redact_event)
+        self.assertEquals(redact_event["content"]["redacts"], event_id)