assertion failed when trying to leave room prevents leaving room

[emdete]

Description

when trying to leave a room this fails. the homeserver log shows an exception:

Steps to reproduce

• list the steps
• that reproduce the bug
• using hyphens as bullet points

Homeserver

emdete.de

Synapse Version

1.144.0

Installation Method

Debian packages from packages.matrix.org

Database

i use a single pg, i migrated from sqlite long, long ago

Workers

I don't know

Platform

devuan excalibur/ceres, no container or vm

Configuration

no

Relevant log output

2026-02-19 19:04:50,880 - synapse.http.server - 147 - ERROR - POST-8727960- Failed handle request via 'RoomMembershipRestServlet': <XForwardedForRequest at 0x7f8bec1ec550 method='POST' uri='/_matrix/client/v3/rooms/!bDwQOcrZFaoJMOjDcu:matrix.keinhorn.de/leave' clientproto='
HTTP/1.1' site='8008'>
Traceback (most recent call last):
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/http/server.py", line 335, in _async_render_wrapper
    callback_return = await self._async_render(request)
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/http/server.py", line 576, in _async_render
    callback_return = await raw_callback_return
                      ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/rest/client/room.py", line 1295, in on_POST
    return await self._do(request, requester, room_id, membership_action, None)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/rest/client/room.py", line 1268, in _do
    await self.room_member_handler.update_membership(
    ...<7 lines>...
    )
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/handlers/room_member.py", line 663, in update_membership
    result = await self.update_membership_locked(
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<16 lines>...
    )
    ^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/handlers/room_member.py", line 1188, in update_membership_locked
    return await self._local_membership_update(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<13 lines>...
    )
    ^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/handlers/room_member.py", line 475, in _local_membership_update
    ) = await self.event_creation_handler.create_event(
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<17 lines>...
    )
    ^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/handlers/message.py", line 731, in create_event
    event, unpersisted_context = await self.create_new_client_event(
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<9 lines>...
    )
    ^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/util/metrics.py", line 173, in measured_func
    r = await func(self, *args, **kwargs)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/venvs/matrix-synapse/lib/python3.13/site-packages/synapse/handlers/message.py", line 1336, in create_new_client_event
    assert builder.type == EventTypes.Create or len(prev_event_ids) > 0, (
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: Attempting to create an event with no prev_events

(btw: why does synapse run with assertions enabled? shouldnt important validations be usual code and prod-runs with -o?)

Anything else that would be useful to know?

this room was a DM room where the other party left