Remove redundant length check

> if `len(auth_event_ids) == 0` then `auth_event_ids` will be falsey, so the extra check is redundant.
>
> -- https://github.com/matrix-org/synapse/pull/11243#discussion_r745408186

changelog.d/11243.feature

@@ -0,0 +1 @@
+Add experimental room version `org.matrix.msc2716v4` to allow events to be created without `prev_events` (only `auth_events`).

synapse/api/room_versions.py

@@ -81,6 +81,8 @@ class RoomVersion:
     msc2716_historical = attr.ib(type=bool)
     # MSC2716: Adds support for redacting "insertion", "chunk", and "marker" events
     msc2716_redactions = attr.ib(type=bool)
+    # MSC2716: Adds support for events with no `prev_events` but with some `auth_events`
+    msc2716_empty_prev_events = attr.ib(type=bool)
 
 
 class RoomVersions:
@@ -99,6 +101,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V2 = RoomVersion(
         "2",
@@ -115,6 +118,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V3 = RoomVersion(
         "3",
@@ -131,6 +135,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V4 = RoomVersion(
         "4",
@@ -147,6 +152,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V5 = RoomVersion(
         "5",
@@ -163,6 +169,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V6 = RoomVersion(
         "6",
@@ -179,6 +186,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     MSC2176 = RoomVersion(
         "org.matrix.msc2176",
@@ -195,6 +203,7 @@ class RoomVersions:
         msc2403_knocking=False,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V7 = RoomVersion(
         "7",
@@ -211,6 +220,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V8 = RoomVersion(
         "8",
@@ -227,6 +237,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     V9 = RoomVersion(
         "9",
@@ -243,6 +254,7 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=False,
         msc2716_redactions=False,
+        msc2716_empty_prev_events=False,
     )
     MSC2716v3 = RoomVersion(
         "org.matrix.msc2716v3",
@@ -259,6 +271,24 @@ class RoomVersions:
         msc2403_knocking=True,
         msc2716_historical=True,
         msc2716_redactions=True,
+        msc2716_empty_prev_events=False,
+    )
+    MSC2716v4 = RoomVersion(
+        "org.matrix.msc2716v4",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.V3,
+        StateResolutionVersions.V2,
+        enforce_key_validity=True,
+        special_case_aliases_auth=False,
+        strict_canonicaljson=True,
+        limit_notifications_power_levels=True,
+        msc2176_redaction_rules=False,
+        msc3083_join_rules=False,
+        msc3375_redaction_rules=False,
+        msc2403_knocking=True,
+        msc2716_historical=True,
+        msc2716_redactions=True,
+        msc2716_empty_prev_events=True,
     )
 
 
@@ -276,6 +306,7 @@ KNOWN_ROOM_VERSIONS: Dict[str, RoomVersion] = {
         RoomVersions.V8,
         RoomVersions.V9,
         RoomVersions.MSC2716v3,
+        RoomVersions.MSC2716v4,
     )
 }
 

synapse/handlers/message.py

@@ -918,6 +918,7 @@ class EventCreationHandler:
         full_state_ids_at_event = None
         if auth_event_ids is not None:
             # If auth events are provided, prev events must be also.
+            # prev_event_ids could be an empty array though.
             assert prev_event_ids is not None
 
             # Copy the full auth state before it stripped down
@@ -949,14 +950,24 @@ class EventCreationHandler:
         else:
             prev_event_ids = await self.store.get_prev_events_for_room(builder.room_id)
 
-        # we now ought to have some prev_events (unless it's a create event).
-        #
-        # do a quick sanity check here, rather than waiting until we've created the
+        # Do a quick sanity check here, rather than waiting until we've created the
         # event and then try to auth it (which fails with a somewhat confusing "No
         # create event in auth events")
-        assert (
-            builder.type == EventTypes.Create or len(prev_event_ids) > 0
-        ), "Attempting to create an event with no prev_events"
+        room_version_obj = await self.store.get_room_version(builder.room_id)
+        if room_version_obj.msc2716_empty_prev_events:
+            # We allow events with no `prev_events` but it better have some `auth_events`
+            assert (
+                builder.type == EventTypes.Create
+                or len(prev_event_ids) > 0
+                # Allow an event to have empty list of prev_event_ids
+                # only if it has auth_event_ids.
+                or auth_event_ids
+            ), "Attempting to create a non-m.room.create event with no prev_events or auth_event_ids"
+        else:
+            # we now ought to have some prev_events (unless it's a create event).
+            assert (
+                builder.type == EventTypes.Create or len(prev_event_ids) > 0
+            ), "Attempting to create a non-m.room.create event with no prev_events"
 
         event = await builder.build(
             prev_event_ids=prev_event_ids,

synapse/handlers/room_member.py

@@ -644,7 +644,8 @@ class RoomMemberHandler(metaclass=abc.ABCMeta):
             if block_invite:
                 raise SynapseError(403, "Invites have been disabled on this server")
 
-        if prev_event_ids:
+        # An empty prev_events list is allowed by room version "org.matrix.msc2716v4"
+        if prev_event_ids is not None:
             return await self._local_membership_update(
                 requester=requester,
                 target=target,

tests/handlers/test_message.py

@@ -15,6 +15,7 @@ import logging
 from typing import Tuple
 
 from synapse.api.constants import EventTypes
+from synapse.api.room_versions import RoomVersions
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
 from synapse.rest import admin
@@ -23,6 +24,7 @@ from synapse.types import create_requester
 from synapse.util.stringutils import random_string
 
 from tests import unittest
+from tests.test_utils.event_injection import create_event
 
 logger = logging.getLogger(__name__)
 
@@ -156,6 +158,80 @@ class EventCreationTestCase(unittest.HomeserverTestCase):
         self.assertEqual(len(events), 2)
         self.assertEqual(events[0].event_id, events[1].event_id)
 
+    def test_create_empty_prev_events_in_msc2716_room_version(self):
+        """Try to create an event without any prev_events (only auth_events).
+
+        This is currently only supported in the experimental MSC2716v4+ room versions.
+        """
+        room_id = self.helper.create_room_as(
+            self.user_id,
+            tok=self.access_token,
+            room_version=RoomVersions.MSC2716v4.identifier,
+        )
+
+        # Create a member event we can use as an auth_event
+        memberEvent, memberEventContext = self.get_success(
+            create_event(
+                self.hs,
+                room_id=room_id,
+                type="m.room.member",
+                sender=self.requester.user.to_string(),
+                state_key=self.requester.user.to_string(),
+                content={"membership": "join"},
+            )
+        )
+        self.get_success(
+            self.persist_event_storage.persist_event(memberEvent, memberEventContext)
+        )
+
+        # Try to create the event with empty prev_events (only auth_events)
+        event, _ = self.get_success(
+            self.handler.create_event(
+                self.requester,
+                {
+                    "type": EventTypes.Message,
+                    "room_id": room_id,
+                    "sender": self.requester.user.to_string(),
+                    "content": {"msgtype": "m.text", "body": random_string(5)},
+                },
+                # Empty prev_events is the key thing we're testing here
+                prev_event_ids=[],
+                auth_event_ids=[memberEvent.event_id],
+            )
+        )
+        self.assertIsNotNone(event)
+
+    def test_reject_empty_prev_events_and_auth_events_in_msc2716_room_version(
+        self,
+    ):
+        """Try to create an event without any prev_events or auth_events.
+        Expect an exception to be raised.
+
+        This is currently only supported in the experimental MSC2716v4 room versions.
+        """
+        room_id = self.helper.create_room_as(
+            self.user_id,
+            tok=self.access_token,
+            room_version=RoomVersions.MSC2716v4.identifier,
+        )
+
+        # Try to create the event with empty prev_events and empty auth_events
+        self.get_failure(
+            self.handler.create_event(
+                self.requester,
+                {
+                    "type": EventTypes.Message,
+                    "room_id": room_id,
+                    "sender": self.requester.user.to_string(),
+                    "content": {"msgtype": "m.text", "body": random_string(5)},
+                },
+                prev_event_ids=[],
+                # The event should be rejected when there are no auth_events
+                auth_event_ids=[],
+            ),
+            AssertionError,
+        )
+
 
 class ServerAclValidationTestCase(unittest.HomeserverTestCase):
     servlets = [