Failed handle request via 'RoomHierarchyRestServlet', KeyError: ('m.room.create', '')

[matrixbot]

This issue has been migrated from #12082.

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Description

Listing all the rooms belonging to some private Space on the "Manage & explore
rooms" page fails with an "Internal Server Error" from Element web and Android.

Filtering the rooms with the panel on the left outside of this page works fine
otherwise.

Steps to reproduce

This error only seems to happen in one particular private space, but not
other ones.

• List all the rooms in a space through "Manage & explore rooms".
• Scroll down the list, trying to load all elements, until the error shows up.

The following error appears in loop in Synapse's logs:

synapse.http.server: [GET-1572871] Failed handle request via 'RoomHierarchyRestServlet': <XForwardedForRequest at [REDACTED] method='GET' uri='/_matrix/client/unstable/org.matrix.msc2946/rooms/![REDACTED]/hierarchy?suggested_only=false&from=[REDACTED]&limit=20' clientproto='HTTP/1.1' site='8008'>
Traceback (most recent call last):
  File "/nix/store/dv9mv0fmmhkbxczqflhcq6ifvkcvyr72-python3.9-Twisted-21.7.0/lib/python3.9/site-packages/twisted/internet/defer.py", line 1661, in _inlineCallbacks
    result = current_context.run(gen.send, result)
  File "/nix/store/rp1cyyj9sbwq8xf41d334wxhlh5i2rvl-matrix-synapse-1.52.0/lib/python3.9/site-packages/synapse/util/caches/response_cache.py", line 246, in cb
    return await callback(*args, **kwargs)
  File "/nix/store/rp1cyyj9sbwq8xf41d334wxhlh5i2rvl-matrix-synapse-1.52.0/lib/python3.9/site-packages/synapse/handlers/room_summary.py", line 396, in _get_room_hierarchy
    room_entry = await self._summarize_local_room(
  File "/nix/store/rp1cyyj9sbwq8xf41d334wxhlh5i2rvl-matrix-synapse-1.52.0/lib/python3.9/site-packages/synapse/handlers/room_summary.py", line 654, in _summarize_local_room
    room_entry = await self._build_room_entry(room_id, for_federation=bool(origin))
  File "/nix/store/rp1cyyj9sbwq8xf41d334wxhlh5i2rvl-matrix-synapse-1.52.0/lib/python3.9/site-packages/synapse/handlers/room_summary.py", line 996, in _build_room_entry
    current_state_ids[(EventTypes.Create, "")]
KeyError: ('m.room.create', '')

I previously had issues with creating some room in that space due to another
server in the federation being offline. I speculate that some room might have
been partially created and partially added to the space.

The end of that traceback seems similar to the one of #10032.

Version information

If not matrix.org:

• Version: 1.52.0, Python 3.9.6
• Install method: distro package
• Platform: NixOS Linux, x86