Room creations denied by user_may_create_room should cause a M_FORBIDDEN error to be returned, not M_UNKNOWN

[jaller94]

Description:
0. Have a Synapse module that prevents you from creating a room.

1. Try to create a room.

Actual

HTTP 403 on POST /_matrix/client/r0/createRoom

{
  "errcode":"M_UNKNOWN",
  "error":"You are not permitted to create rooms"
}

The UI is unsure what to tell the user, making it seem like this could succeed on retry:

Expected

The Matrix errcode M_FORBIDDEN would be more specific and fits the error message.

[babolivier]

Slightly related (since it's around error handling): #11031

[babolivier]

@jaller94

Have a Synapse module that prevents you from creating a room.

There are a few module callbacks that prevent you from creating a room, could you be more specific here please?

[reivilibre]

iirc I saw this behaviour when working on https://github.com/matrix-org/synapse-user-restrictions/ with the user_may_create_room spam checker callback.

[babolivier]

As per DM, the module indeed uses user_may_create_room (and denies room creation by returning False upon getting called on that callback).

[lukasdenk]

I'll try to fix it 😃

[lukasdenk]

@jaller94 I fixed the issue in the above PR. Could you please close the issue?

[DMRobertson]

@lukasdenk For what it's worth, prefer linking the PR and the issue together as described here---that'll automatically close the issue if the PR is merged.