Implement MSC3389 to protect relations from redaction. (#15565)

MSC3389 proposes protecting the relation type & parent event ID
from redaction. This keeps the relation information intact after
redaction which helps with some UX flaws (e.g. deleting an
event causes it to no longer be in a thread, which is confusing).

1 files changed, 90 insertions, 0 deletions

diff --git a/tests/events/test_utils.py b/tests/events/test_utils.py

index 1b179acb20..02f0800a31 100644
--- a/tests/events/test_utils.py
+++ b/tests/events/test_utils.py
@@ -15,6 +15,8 @@
 import unittest as stdlib_unittest
 from typing import Any, List, Mapping, Optional
 
+import attr
+
 from synapse.api.constants import EventContentFields
 from synapse.api.room_versions import RoomVersions
 from synapse.events import EventBase, make_event_from_dict
@@ -435,6 +437,94 @@ class PruneEventTestCase(stdlib_unittest.TestCase):
             room_version=RoomVersions.V9,
         )
 
+    def test_relations(self) -> None:
+        """Event relations get redacted until MSC3389."""
+        # Normally the m._relates_to field is redacted.
+        self.run_test(
+            {
+                "type": "m.room.message",
+                "content": {
+                    "body": "foo",
+                    "m.relates_to": {
+                        "rel_type": "rel_type",
+                        "event_id": "$parent:domain",
+                        "other": "stripped",
+                    },
+                },
+            },
+            {
+                "type": "m.room.message",
+                "content": {},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=RoomVersions.V10,
+        )
+
+        # Create a new room version.
+        msc3389_room_ver = attr.evolve(
+            RoomVersions.V10, msc3389_relation_redactions=True
+        )
+
+        self.run_test(
+            {
+                "type": "m.room.message",
+                "content": {
+                    "body": "foo",
+                    "m.relates_to": {
+                        "rel_type": "rel_type",
+                        "event_id": "$parent:domain",
+                        "other": "stripped",
+                    },
+                },
+            },
+            {
+                "type": "m.room.message",
+                "content": {
+                    "m.relates_to": {
+                        "rel_type": "rel_type",
+                        "event_id": "$parent:domain",
+                    },
+                },
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=msc3389_room_ver,
+        )
+
+        # If the field is not an object, redact it.
+        self.run_test(
+            {
+                "type": "m.room.message",
+                "content": {
+                    "body": "foo",
+                    "m.relates_to": "stripped",
+                },
+            },
+            {
+                "type": "m.room.message",
+                "content": {},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=msc3389_room_ver,
+        )
+
+        # If the m.relates_to property would be empty, redact it.
+        self.run_test(
+            {
+                "type": "m.room.message",
+                "content": {"body": "foo", "m.relates_to": {"foo": "stripped"}},
+            },
+            {
+                "type": "m.room.message",
+                "content": {},
+                "signatures": {},
+                "unsigned": {},
+            },
+            room_version=msc3389_room_ver,
+        )
+
 
 class SerializeEventTestCase(stdlib_unittest.TestCase):
     def serialize(self, ev: EventBase, fields: Optional[List[str]]) -> JsonDict: