"Internal server error" when upgrading a room created with Conduit

[progval]

Description

Upgrading a room created with Conduit returns "Internal server error"

Steps to reproduce

1. Create a room with Conduit
2. Invite a Synapse user to it, make them admin
3. Make the Synapse user upgrade the room (eg. /upgraderoom 11 in Eleweb)

Synapse returns {"errcode":"M_UNKNOWN","error":"Internal server error"}

Homeserver

matrix.org and Conduit

Synapse Version

1.96.1 (b=matrix-org-hotfixes,9c3b906b3a)

Installation Method

I don't know

Database

postgresql

Workers

Multiple workers

Platform

n/a

Configuration

No response

Relevant log output

Nothing from Conduit.

I tried it on matrix.org, `synapse-trace-id: b6592675a1c241fa`

Anything else that would be useful to know?

No response

[erikjohnston]

Raw stack trace:

ValueError: max() arg is an empty sequence
  File "twisted/internet/defer.py", line 2000, in _inlineCallbacks
    result = context.run(gen.send, result)
  File "synapse/util/caches/response_cache.py", line 258, in cb
    return await callback(*args, **kwargs)
  File "synapse/handlers/room.py", line 306, in _upgrade_room
    await self.clone_existing_room(
  File "synapse/handlers/room.py", line 552, in clone_existing_room
    state_default_int, ban, max(event_power_levels.values())

Corresponds to:

synapse/synapse/handlers/room.py

Line 552 in 63d96bf

state_default_int, ban, max(event_power_levels.values())

Sentry link: https://sentry.tools.element.io/organizations/element/issues/85240/?project=2

The power level content is:

{"users":{"@<redacted>":100,"@<redacted>":100,"@<redacted>":100}}

So looks like we need to correctly handle an empty/missing events key. I think we can change the max(event_power_levels.values()) to include a default=0?