Invalid username on appservice user creation due to underscore

[wlcx]

Background information

• Dendrite version or git SHA: 0.3.1
• Monolith or Polylith?: Monolith
• SQLite3 or Postgres?: Postgres
• Running in Docker?: Yes
• go version: Docker version

Description

When running the discord app service (https://github.com/Half-Shot/matrix-appservice-discord, v0.5.2), I get errors in the appservice's log regarding username registration:

discord-appsvc_1  | Nov-25 18:31:55.757 [UserSync] info: Creating new user @_discord_567871284180942858:hacksrus.uk
discord-appsvc_1  | Nov-25 18:31:55.772 [UserSync] error: Failed to update user's profile [M_INVALID_USERNAME: Username cannot start with a '_'] {
discord-appsvc_1  |   errcode: 'M_INVALID_USERNAME',
discord-appsvc_1  |   data: {
discord-appsvc_1  |     errcode: 'M_INVALID_USERNAME',
discord-appsvc_1  |     error: "Username cannot start with a '_'"
discord-appsvc_1  |   },
discord-appsvc_1  |   httpStatus: 400
discord-appsvc_1  | }

On discussion in #dendrite{,-dev}, it seems like is because dendrite calls validateUsername rather than validateApplicationServiceUsername : https://github.com/matrix-org/dendrite/blob/master/clientapi/routing/register.go#L484-L486

Steps to reproduce

• Run the discord appservice bridged to a discord guild
• send a message in a channel, causing the app service to try to create a user

[wlcx]

Somewhat related #630

[Half-Shot]

Oh missed this, we're using #1784