Cannot join mx-slack bridge room with more than 2 members : "CheckAuthEvents failed for event"

[matrixbot]

This issue was originally created by @Notmarrco at matrix-org/dendrite#1975.

Hello,

Background information

• Dendrite version or git SHA: 0.4.1
• Monolith or Polylith?: Monolyth
• SQLite3 or Postgres?: Postgres
• Running in Docker?: no
• go version: 1.15.9 (buster backports)

Description

I have a slack bridge, but dendrite refuse that I join some channels.

Steps to reproduce

• installing dendrite and slack bridge with sorunome mx-puppet-slack
• linking to a slack workspace
• listrooms for the slack bridge
• trying to access to rooms (by clicking on the listrooms result) yield a "user is not allowed to change their membership from leave to join" error

For some rooms/channel it works though but I cannot find why : some are private some are public, and it doesn't seem to affect my authorization to join or not. (fwiw I am admin of the workspace)

First I thought it was a bridge error but it's dendrite that refuse me to join the rooms. Sorry if I'm mistaken.

(edit : not related) On a side note, I saw in the github issues that there was a problem with federated rooms with the same error. I have tested my federated status with the matrix federation tester, and all seems good. However when I try to invite someone from my other dendrite (I have 2 , to test federation) the room is created, the user is invited, but it cannot accept the invitation ("fail to join the room" with "unkown error"). So it may be a bad configuration from me, or because one server can do ipv4 only and the other ipv6+ipv4.

Is there more info that I can provide ? I have no idea where / what to look for.

time="2021-08-13T10:36:11.739578496Z" level=error msg="helpers.CheckAuthEvents failed for event, rejecting event" func="github.com/matrix-org/dendrite/roomserver/internal/input.(*Inputer).processRoomEvent" file="/opt/dendrite/roomserver/internal/input/input_events.go:106" auth_event_ids="[$FC9SthiWVNSL4H1QSE4o0XQBXBGfAMcHeEMp7kLRn_s $aSkhwXnolPYpuWMs4d_Tjh8CoLVRv9pMnunUmgjiHSU $l9CIiUDrLfOQMr1L8INspFY9Hc5GJUW3BYXE0ksfjpE]" error="eventauth: \"@myuser:mydomain.fr\" is not allowed to change their membership from \"leave\" to \"join\"" event_id="$8SxnJU6XZM8NpwXuAWF2btfRnUOFtVDYCQsA988xUbY"
time="2021-08-13T10:36:11.741465877Z" level=info msg="Error authing soft-failed event" func="github.com/matrix-org/dendrite/roomserver/internal/input.(*Inputer).processRoomEvent" file="/opt/dendrite/roomserver/internal/input/input_events.go:120" error="eventauth: \"@myuser:mydomain.fr\" is not allowed to change their membership from \"leave\" to \"join\"" event_id="$8SxnJU6XZM8NpwXuAWF2btfRnUOFtVDYCQsA988xUbY" room="!FcZUHCcUaHycxdDe:mydomain.fr" type=m.room.member
time="2021-08-13T10:36:11.750148536Z" level=debug msg="Stored rejected event" func="github.com/matrix-org/dendrite/roomserver/internal/input.(*Inputer).processRoomEvent" file="/opt/dendrite/roomserver/internal/input/input_events.go:189" event_id="$8SxnJU6XZM8NpwXuAWF2btfRnUOFtVDYCQsA988xUbY" room="!FcZUHCcUaHycxdDe:mydomain.fr" sender="@myuser:mydomain.fr" soft_fail=true type=m.room.member

EDIT

Found out that the issue seems to be when joining slack channels with more than 2 members (>= 3). I opened an issue in the mx-slack github but since it seems to be working with synapse, it's possible that it's a dendrite issue.

Thank you

[matrixbot]

This comment was originally posted by @kegsay at matrix-org/dendrite#1975 (comment).

I think this is due to /createRoom's default public|private room value. Dendrite follows the spec:

Whereas synapse does not. matrix-org/synapse#11498 - This could cause a problem if the bridge is calling this endpoint without an explicit visibility and the behaviour will vary between server implementations. Please re-open this if you have relevant logs to show that the bridge is in fact sending visibility.