MSC4242: State DAGs (Federation)

changelog.d/19424.feature

@@ -0,0 +1 @@
+Add experimental support for MSC4242: State DAGs. Excludes federation support.

rust/src/events/internal_metadata.rs

@@ -59,6 +59,7 @@ enum EventInternalMetadataData {
     TxnId(Box<str>),
     TokenId(i64),
     DeviceId(Box<str>),
+    CalculatedAuthEventIDs(Vec<String>),
 }
 
 impl EventInternalMetadataData {
@@ -123,6 +124,10 @@ impl EventInternalMetadataData {
                 pyo3::intern!(py, "device_id"),
                 o.into_pyobject(py).unwrap_infallible().into_any(),
             ),
+            EventInternalMetadataData::CalculatedAuthEventIDs(o) => (
+                pyo3::intern!(py, "calculated_auth_event_ids"),
+                o.into_pyobject(py).unwrap().into_any(),
+            ),
         }
     }
 
@@ -190,6 +195,11 @@ impl EventInternalMetadataData {
                     .map(String::into_boxed_str)
                     .with_context(|| format!("'{key_str}' has invalid type"))?,
             ),
+            "calculated_auth_event_ids" => EventInternalMetadataData::CalculatedAuthEventIDs(
+                value
+                    .extract()
+                    .with_context(|| format!("'{key_str}' has invalid type"))?,
+            ),
             _ => return Ok(None),
         };
 
@@ -472,6 +482,17 @@ impl EventInternalMetadata {
         set_property!(self, TxnId, obj.into_boxed_str());
     }
 
+    /// The calculated auth event IDs, if it was set when the event was created.
+    #[getter]
+    fn get_calculated_auth_event_ids(&self) -> PyResult<&Vec<String>> {
+        let s = get_property!(self, CalculatedAuthEventIDs)?;
+        Ok(s)
+    }
+    #[setter]
+    fn set_calculated_auth_event_ids(&mut self, obj: Vec<String>) {
+        set_property!(self, CalculatedAuthEventIDs, obj);
+    }
+
     /// The access token ID of the user who sent this event, if any.
     #[getter]
     fn get_token_id(&self) -> PyResult<i64> {

synapse/api/room_versions.py

@@ -36,13 +36,15 @@ class EventFormatVersions:
     ROOM_V3 = 2  # MSC1659-style $hash event id format: used for room v3
     ROOM_V4_PLUS = 3  # MSC1884-style $hash format: introduced for room v4
     ROOM_V11_HYDRA_PLUS = 4  # MSC4291 room IDs as hashes: introduced for room HydraV11
+    ROOM_VMSC4242 = 5  # MSC4242 state dags: adds prev_state_events, removes auth_events
 
 
 KNOWN_EVENT_FORMAT_VERSIONS = {
     EventFormatVersions.ROOM_V1_V2,
     EventFormatVersions.ROOM_V3,
     EventFormatVersions.ROOM_V4_PLUS,
     EventFormatVersions.ROOM_V11_HYDRA_PLUS,
+    EventFormatVersions.ROOM_VMSC4242,
 }
 
 
@@ -111,6 +113,8 @@ class RoomVersion:
     msc3931_push_features: tuple[str, ...]  # values from PushRuleRoomFlag
     # MSC3757: Restricting who can overwrite a state event
     msc3757_enabled: bool
+    # MSC4242: Creates events with prev_state_events instead of auth_events and derives state from it.
+    msc4242_state_dags: bool
     # MSC4289: Creator power enabled
     msc4289_creator_power_enabled: bool
     # MSC4291: Room IDs as hashes of the create event
@@ -137,6 +141,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -159,6 +164,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -181,6 +187,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -203,6 +210,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -225,6 +233,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -247,6 +256,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -269,6 +279,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -291,6 +302,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -313,6 +325,7 @@ class RoomVersions:
         enforce_int_power_levels=False,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -335,6 +348,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -358,6 +372,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(PushRuleRoomFlag.EXTENSIBLE_EVENTS,),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -381,6 +396,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=True,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -403,6 +419,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -426,6 +443,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=True,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=False,
         msc4291_room_ids_as_hashes=False,
     )
@@ -438,8 +456,8 @@ class RoomVersions:
         special_case_aliases_auth=False,
         strict_canonicaljson=True,
         limit_notifications_power_levels=True,
-        implicit_room_creator=True,  # Used by MSC3820
-        updated_redaction_rules=True,  # Used by MSC3820
+        implicit_room_creator=True,
+        updated_redaction_rules=True,
         restricted_join_rule=True,
         restricted_join_rule_fix=True,
         knock_join_rule=True,
@@ -448,6 +466,7 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=True,  # Changed from v11
         msc4291_room_ids_as_hashes=True,  # Changed from v11
     )
@@ -470,9 +489,33 @@ class RoomVersions:
         enforce_int_power_levels=True,
         msc3931_push_features=(),
         msc3757_enabled=False,
+        msc4242_state_dags=False,
         msc4289_creator_power_enabled=True,  # Changed from v11
         msc4291_room_ids_as_hashes=True,  # Changed from v11
     )
+    MSC4242v12 = RoomVersion(
+        "org.matrix.msc4242.12",  # MSC4242 (State DAGs) based on room version "12"
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.ROOM_VMSC4242,
+        StateResolutionVersions.V2_1,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        implicit_room_creator=True,
+        updated_redaction_rules=True,
+        restricted_join_rule=True,
+        restricted_join_rule_fix=True,
+        knock_join_rule=True,
+        msc3389_relation_redactions=False,
+        knock_restricted_join_rule=True,
+        enforce_int_power_levels=True,
+        msc3931_push_features=(),
+        msc3757_enabled=False,
+        msc4289_creator_power_enabled=True,
+        msc4291_room_ids_as_hashes=True,
+        msc4242_state_dags=True,
+    )
 
 
 KNOWN_ROOM_VERSIONS: dict[str, RoomVersion] = {
@@ -492,6 +535,7 @@ class RoomVersions:
         RoomVersions.V12,
         RoomVersions.MSC3757v10,
         RoomVersions.MSC3757v11,
+        RoomVersions.MSC4242v12,
         RoomVersions.HydraV11,
     )
 }

synapse/event_auth.py

@@ -62,7 +62,7 @@
     RoomVersion,
     RoomVersions,
 )
-from synapse.events import is_creator
+from synapse.events import FrozenEventVMSC4242, is_creator
 from synapse.state import CREATE_KEY
 from synapse.storage.databases.main.events_worker import EventRedactBehaviour
 from synapse.types import (
@@ -187,6 +187,73 @@ async def check_state_independent_auth_rules(
         # 1.5 Otherwise, allow
         return
 
+    # State DAGs 2. Considering the event's prev_state_events:
+    if event.room_version.msc4242_state_dags:
+        prev_state_events_ids = set(cast(FrozenEventVMSC4242, event).prev_state_events)
+        needed_prev_state_event_ids = {
+            event_id
+            for event_id in prev_state_events_ids
+            if not batched_auth_events or event_id not in batched_auth_events
+        }
+        # We need to do some checks on the events provided as prev_state_events, so we need to load them.
+        # Try to load the `prev_state_events` from `batched_auth_events` initially as that can save us
+        # a database hit.
+        prev_state_events = (
+            {}
+            if not batched_auth_events
+            else {
+                e: batched_auth_events[e]
+                for e in prev_state_events_ids - needed_prev_state_event_ids
+            }
+        )
+        if len(needed_prev_state_event_ids) > 0:
+            needed_prev_state_events = await store.get_events(
+                needed_prev_state_event_ids,
+                redact_behaviour=EventRedactBehaviour.as_is,
+                allow_rejected=True,
+            )
+            prev_state_events.update(needed_prev_state_events)
+        if len(prev_state_events) != len(prev_state_events_ids):
+            # we should have all the prev state events by now, so if we do not, that suggests
+            # a synapse programming error
+            known_prev_state_event_ids = set(prev_state_events)
+            raise AssertionError(
+                f"Event {event.event_id} has unknown prev_state_events "
+                + f"{len(prev_state_events)} != {len(prev_state_events_ids)} "
+                + f"{prev_state_events_ids - known_prev_state_event_ids} missing "
+                + f"out of {prev_state_events_ids}"
+            )
+        for prev_state_event in prev_state_events.values():
+            # 2.1 If there are entries which do not belong in the same room, reject.
+            if prev_state_event.room_id != event.room_id:
+                raise AuthError(
+                    403,
+                    "During auth for event %s in room %s, found event %s in prev_state_events "
+                    "which belongs to a different room %s"
+                    % (
+                        event.event_id,
+                        event.room_id,
+                        prev_state_event.event_id,
+                        prev_state_event.room_id,
+                    ),
+                )
+            # 2.2 If there are entries which do not have a state_key, reject.
+            if not prev_state_event.is_state():
+                raise AuthError(
+                    403,
+                    f"During auth for event {event.event_id} in room {event.room_id}, event has a "
+                    + f"prev_state_event which is not state: {prev_state_event.event_id}",
+                )
+            # 2.3 If there are entries which were themselves rejected under the checks performed on
+            # receipt of a PDU, reject.
+            if prev_state_event.rejected_reason is not None:
+                raise AuthError(
+                    403,
+                    f"During auth for event {event.event_id} in room {event.room_id}, event has a "
+                    + f"prev_state_event which is rejected ({prev_state_event.rejected_reason}): "
+                    + f"{prev_state_event.event_id}",
+                )
+
     # 2. Reject if event has auth_events that: ...
     auth_events: ChainMap[str, EventBase] = ChainMap()
     if batched_auth_events:
@@ -472,6 +539,12 @@ def _check_create(event: "EventBase") -> None:
     if event.prev_event_ids():
         raise AuthError(403, "Create event has prev events")
 
+    # State DAGs 1.2 If it has any prev_state_events, reject.
+    if event.room_version.msc4242_state_dags:
+        assert isinstance(event, FrozenEventVMSC4242)
+        if len(event.prev_state_events) > 0:
+            raise AuthError(403, "Create event has prev state events")
+
     if event.room_version.msc4291_room_ids_as_hashes:
         # 1.2 If the create event has a room_id, reject
         if "room_id" in event: