error 500 if room in space destroyed, but still in space, and then viewed over federation

[cleverca22]

Description

error 500 on the server-to-server protocol, when listing rooms in a space, after one has been partially destroyed

Steps to reproduce

• create a space with a public room on HS-1
• join the space from a user on HS-2
• delete the public room
• try to list the rooms, from the HS-2 user

HS-2 will get:

synapse.http.matrixfederationclient: [POST-7202] {POST-O-3371} [matrix.org] Got response headers: 500 Internal Server Error
synapse.http.matrixfederationclient: [POST-7202] {POST-O-3371} [matrix.org] Request failed: POST matrix://matrix.org/_matrix/federation/unstable/org.matrix.msc2946/spaces/%21EDITED%3Amatrix.org: HttpResponseException('500: Internal Server Error')

HS-1 will get:

synapse.http.server: [POST-8610] Failed handle request via 'FederationSpaceSummaryServlet': <XForwardedForRequest at 0x7f1d5151e520 method='POST' uri='/_matrix/federation/unstable/org.matrix.msc2946/spaces/%21EDITED%3AEDITED' clientproto='HTTP/1.0' site='8448'>
Traceback (most recent call last):
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/http/server.py", line 258, in _async_render_wrapper
    callback_return = await self._async_render(request)
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/http/server.py", line 446, in _async_render
    callback_return = await raw_callback_return
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/federation/transport/server.py", line 347, in new_func
    response = await func(
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/federation/transport/server.py", line 1430, in on_POST
    return 200, await self.handler.federation_space_summary(
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/handlers/space_summary.py", line 209, in federation_space_summary
    rooms, events = await self._summarize_local_room(
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/handlers/space_summary.py", line 250, in _summarize_local_room
    room_entry = await self._build_room_entry(room_id)
  File "/nix/store/pq9bx9ka6jpjp00k99vhhxdwlp9dz3rw-matrix-synapse-1.34.0/lib/python3.8/site-packages/synapse/handlers/space_summary.py", line 349, in _build_room_entry
    current_state_ids[(EventTypes.Create, "")]
KeyError: ('m.room.create', '')

note, the server addresses in the errors dont match up, i flipped HS-1 and HS-2 and reproduced it twice, to get logs for each end, without running 2 HS's

Version information

issue happens when matrix.org and a local 1.34.0 HS interact together, both ends fail with error 500

• Platform:
  nixos

[clokep]

Can you describe how you "delete the public room"? Do you mean remove it from the space or something else?

[clokep]

I wonder if this was due to using StateHandler.get_current_state() and store.get_current_state_ids() in different places. This was changed (for other reasons) in #9922, so will be interesting to try to reproduce on develop / once 1.35.0rc1 comes out.

[cleverca22]

Can you describe how you "delete the public room"? Do you mean remove it from the space or something else?

being the last person in the room, i simply leave it
then attempting to view the list of spaces, from a different homeserver, causes that error in the logs

the state on the space claims it still exists, but the homeserver for the room claims it doesnt

[clokep]

Thanks! I'll take a look at this, I think I have an idea of the issue. 😄

[clokep]

I attempted to reproduce this and was unable to (on the latest develop)s, so it is possible that #9922 fixed this as a side-effect or that I'm doing something wrong. :) I'll need to revert to an older version and check again.

[clokep]

I confirmed this was fixed by #9922 and was released in Synapse v1.35.0rc1.