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).

2 files changed, 29 insertions, 0 deletions

diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py

index 5d9c13e3c3..e65b9a0287 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -96,6 +96,8 @@ class RoomVersion:
     msc2716_historical: bool
     # MSC2716: Adds support for redacting "insertion", "chunk", and "marker" events
     msc2716_redactions: bool
+    # MSC3389: Protect relation information from redaction.
+    msc3389_relation_redactions: bool
     # MSC3787: Adds support for a `knock_restricted` join rule, mixing concepts of
     # knocks and restricted join rules into the same join condition.
     msc3787_knock_restricted_join_rule: bool
@@ -128,6 +130,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -149,6 +152,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -170,6 +174,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -191,6 +196,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -212,6 +218,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -233,6 +240,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -254,6 +262,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -275,6 +284,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -296,6 +306,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -317,6 +328,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -338,6 +350,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -359,6 +372,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
         msc3931_push_features=(),
@@ -380,6 +394,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=True,
         msc2716_redactions=True,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=False,
         msc3667_int_only_power_levels=False,
         msc3931_push_features=(),
@@ -402,6 +417,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
         msc3931_push_features=(PushRuleRoomFlag.EXTENSIBLE_EVENTS,),
@@ -423,6 +439,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc3389_relation_redactions=False,
         msc3787_knock_restricted_join_rule=True,
         msc3667_int_only_power_levels=True,
         msc3931_push_features=(),
diff --git a/synapse/events/utils.py b/synapse/events/utils.py

index 0802eb1963..e540f1582a 100644
--- a/synapse/events/utils.py
+++ b/synapse/events/utils.py
@@ -171,6 +171,18 @@ def prune_event_dict(room_version: RoomVersion, event_dict: JsonDict) -> JsonDic
     elif room_version.msc2716_redactions and event_type == EventTypes.MSC2716_MARKER:
         add_fields(EventContentFields.MSC2716_INSERTION_EVENT_REFERENCE)
 
+    # Protect the rel_type and event_id fields under the m.relates_to field.
+    if room_version.msc3389_relation_redactions:
+        relates_to = event_dict["content"].get("m.relates_to")
+        if isinstance(relates_to, collections.abc.Mapping):
+            new_relates_to = {}
+            for field in ("rel_type", "event_id"):
+                if field in relates_to:
+                    new_relates_to[field] = relates_to[field]
+            # Only include a non-empty relates_to field.
+            if new_relates_to:
+                new_content["m.relates_to"] = new_relates_to
+
     allowed_fields = {k: v for k, v in event_dict.items() if k in allowed_keys}
 
     allowed_fields["content"] = new_content