Implement MSC2175: remove the creator field from create events. (#15394)

8 files changed, 72 insertions, 24 deletions

diff --git a/changelog.d/15394.misc b/changelog.d/15394.misc
new file mode 100644
index 0000000000..91e6540438
--- /dev/null
+++ b/changelog.d/15394.misc
@@ -0,0 +1 @@
+Implement [MSC2175](https://github.com/matrix-org/matrix-doc/pull/2175) to stop adding `creator` to create events.
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 0f224b34cd..c56b2f2561 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -215,6 +215,8 @@ class EventContentFields:
     FEDERATE: Final = "m.federate"
 
     # The creator of the room, as used in `m.room.create` events.
+    #
+    # This is deprecated in MSC2175.
     ROOM_CREATOR: Final = "creator"
 
     # Used in m.room.guest_access events.
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index bc15f2d063..3dcae12161 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -78,6 +78,8 @@ class RoomVersion:
     # MSC2209: Check 'notifications' key while verifying
     # m.room.power_levels auth rules.
     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.
     msc2176_redaction_rules: bool
     # MSC3083: Support the 'restricted' join_rule.
@@ -118,6 +120,7 @@ class RoomVersions:
         special_case_aliases_auth=True,
         strict_canonicaljson=False,
         limit_notifications_power_levels=False,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -138,6 +141,7 @@ class RoomVersions:
         special_case_aliases_auth=True,
         strict_canonicaljson=False,
         limit_notifications_power_levels=False,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -158,6 +162,7 @@ class RoomVersions:
         special_case_aliases_auth=True,
         strict_canonicaljson=False,
         limit_notifications_power_levels=False,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -178,6 +183,7 @@ class RoomVersions:
         special_case_aliases_auth=True,
         strict_canonicaljson=False,
         limit_notifications_power_levels=False,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -198,6 +204,7 @@ class RoomVersions:
         special_case_aliases_auth=True,
         strict_canonicaljson=False,
         limit_notifications_power_levels=False,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -218,6 +225,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -238,6 +246,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=True,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -258,6 +267,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -278,6 +288,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=False,
@@ -298,6 +309,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=True,
@@ -318,6 +330,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=True,
@@ -338,6 +351,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=True,
@@ -358,6 +372,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=False,
         msc3375_redaction_rules=False,
@@ -379,6 +394,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=True,
@@ -399,6 +415,7 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
+        msc2175_implicit_room_creator=False,
         msc2176_redaction_rules=False,
         msc3083_join_rules=True,
         msc3375_redaction_rules=True,
diff --git a/synapse/event_auth.py b/synapse/event_auth.py
index af55874b5c..f95d00d472 100644
--- a/synapse/event_auth.py
+++ b/synapse/event_auth.py
@@ -455,8 +455,11 @@ def _check_create(event: "EventBase") -> None:
             "room appears to have unsupported version %s" % (room_version_prop,),
         )
 
-    # 1.4 If content has no creator field, reject.
-    if EventContentFields.ROOM_CREATOR not in event.content:
+    # 1.4 If content has no creator field, reject if the room version requires it.
+    if (
+        not event.room_version.msc2175_implicit_room_creator
+        and EventContentFields.ROOM_CREATOR not in event.content
+    ):
         raise AuthError(403, "Create event lacks a 'creator' property")
 
 
@@ -491,7 +494,11 @@ def _is_membership_change_allowed(
         key = (EventTypes.Create, "")
         create = auth_events.get(key)
         if create and event.prev_event_ids()[0] == create.event_id:
-            if create.content["creator"] == event.state_key:
+            if room_version.msc2175_implicit_room_creator:
+                creator = create.sender
+            else:
+                creator = create.content[EventContentFields.ROOM_CREATOR]
+            if creator == event.state_key:
                 return
 
     target_user_id = event.state_key
@@ -1004,10 +1011,14 @@ def get_user_power_level(user_id: str, auth_events: StateMap["EventBase"]) -> in
         # that.
         key = (EventTypes.Create, "")
         create_event = auth_events.get(key)
-        if create_event is not None and create_event.content["creator"] == user_id:
-            return 100
-        else:
-            return 0
+        if create_event is not None:
+            if create_event.room_version.msc2175_implicit_room_creator:
+                creator = create_event.sender
+            else:
+                creator = create_event.content[EventContentFields.ROOM_CREATOR]
+            if creator == user_id:
+                return 100
+        return 0
 
 
 def get_named_level(auth_events: StateMap["EventBase"], name: str, default: int) -> int:
diff --git a/synapse/handlers/federation_event.py b/synapse/handlers/federation_event.py
index 982c8d3b2f..8d5be81a92 100644
--- a/synapse/handlers/federation_event.py
+++ b/synapse/handlers/federation_event.py
@@ -1515,7 +1515,10 @@ class FederationEventHandler:
         # support it or the event is not from the room creator.
         room_version = await self._store.get_room_version(marker_event.room_id)
         create_event = await self._store.get_create_event_for_room(marker_event.room_id)
-        room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
+        if not room_version.msc2175_implicit_room_creator:
+            room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
+        else:
+            room_creator = create_event.sender
         if not room_version.msc2716_historical and (
             not self._config.experimental.msc2716_enabled
             or marker_event.sender != room_creator
diff --git a/synapse/handlers/message.py b/synapse/handlers/message.py
index 4c75433a63..a17fe3bf53 100644
--- a/synapse/handlers/message.py
+++ b/synapse/handlers/message.py
@@ -1909,7 +1909,12 @@ class EventCreationHandler:
                 room_version_obj = KNOWN_ROOM_VERSIONS[room_version]
 
                 create_event = await self.store.get_create_event_for_room(event.room_id)
-                room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
+                if not room_version_obj.msc2175_implicit_room_creator:
+                    room_creator = create_event.content.get(
+                        EventContentFields.ROOM_CREATOR
+                    )
+                else:
+                    room_creator = create_event.sender
 
                 # Only check an insertion event if the room version
                 # supports it or the event is from the room creator.
diff --git a/synapse/handlers/room.py b/synapse/handlers/room.py
index be120cb12f..2d69cabf43 100644
--- a/synapse/handlers/room.py
+++ b/synapse/handlers/room.py
@@ -567,6 +567,7 @@ class RoomCreationHandler:
         await self._send_events_for_new_room(
             requester,
             new_room_id,
+            new_room_version,
             # we expect to override all the presets with initial_state, so this is
             # somewhat arbitrary.
             room_config={"preset": RoomCreationPreset.PRIVATE_CHAT},
@@ -922,6 +923,7 @@ class RoomCreationHandler:
         ) = await self._send_events_for_new_room(
             requester,
             room_id,
+            room_version,
             room_config=config,
             invite_list=invite_list,
             initial_state=initial_state,
@@ -998,6 +1000,7 @@ class RoomCreationHandler:
         self,
         creator: Requester,
         room_id: str,
+        room_version: RoomVersion,
         room_config: JsonDict,
         invite_list: List[str],
         initial_state: MutableStateMap,
@@ -1020,6 +1023,8 @@ class RoomCreationHandler:
                 the user requesting the room creation
             room_id:
                 room id for the room being created
+            room_version:
+                The room version of the new room.
             room_config:
                 A dict of configuration options. This will be the body of
                 a /createRoom request; see
@@ -1053,14 +1058,6 @@ class RoomCreationHandler:
         # (as this info can't be pulled from the db)
         state_map: MutableStateMap[str] = {}
 
-        def create_event_dict(etype: str, content: JsonDict, **kwargs: Any) -> JsonDict:
-            e = {"type": etype, "content": content}
-
-            e.update(event_keys)
-            e.update(kwargs)
-
-            return e
-
         async def create_event(
             etype: str,
             content: JsonDict,
@@ -1083,7 +1080,10 @@ class RoomCreationHandler:
             nonlocal depth
             nonlocal prev_event
 
-            event_dict = create_event_dict(etype, content, **kwargs)
+            # Create the event dictionary.
+            event_dict = {"type": etype, "content": content}
+            event_dict.update(event_keys)
+            event_dict.update(kwargs)
 
             (
                 new_event,
@@ -1120,7 +1120,9 @@ class RoomCreationHandler:
                 400, f"'{preset_config}' is not a valid preset", errcode=Codes.BAD_JSON
             )
 
-        creation_content.update({"creator": creator_id})
+        # MSC2175 removes the creator field from the create event.
+        if not room_version.msc2175_implicit_room_creator:
+            creation_content["creator"] = creator_id
         creation_event, unpersisted_creation_context = await create_event(
             EventTypes.Create, creation_content, False
         )
diff --git a/synapse/storage/databases/main/room.py b/synapse/storage/databases/main/room.py
index 3825bd6079..dd7dbb6901 100644
--- a/synapse/storage/databases/main/room.py
+++ b/synapse/storage/databases/main/room.py
@@ -1998,6 +1998,9 @@ class RoomBackgroundUpdateStore(SQLBaseStore):
             for room_id, event_json in room_id_to_create_event_results:
                 event_dict = db_to_json(event_json)
 
+                # The creator property might not exist in newer room versions, but
+                # for those versions the creator column should be properly populate
+                # during room creation.
                 creator = event_dict.get("content").get(EventContentFields.ROOM_CREATOR)
 
                 self.db_pool.simple_update_txn(
@@ -2132,12 +2135,16 @@ class RoomStore(RoomBackgroundUpdateStore, RoomWorkerStore):
             # invalid, and it would fail auth checks anyway.
             raise StoreError(400, "No create event in state")
 
-        room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
+        # Before MSC2175, the room creator was a separate field.
+        if not room_version.msc2175_implicit_room_creator:
+            room_creator = create_event.content.get(EventContentFields.ROOM_CREATOR)
 
-        if not isinstance(room_creator, str):
-            # If the create event does not have a creator then the room is
-            # invalid, and it would fail auth checks anyway.
-            raise StoreError(400, "No creator defined on the create event")
+            if not isinstance(room_creator, str):
+                # If the create event does not have a creator then the room is
+                # invalid, and it would fail auth checks anyway.
+                raise StoreError(400, "No creator defined on the create event")
+        else:
+            room_creator = create_event.sender
 
         await self.db_pool.simple_upsert(
             desc="upsert_room_on_join",