Add Admin API endpoint to change one's nickname only in a room

[Marc-Andrieu]

Description:

Create an endpoint for the Admin API of Synapse, so that a server admin can change a user's nickname that is displayed in a given room only. (Or change the behavior of one which already exists)

Motivation:

In different spaces or channels, corresponding to different communities, one given user may play different roles. We may want to change automatically this user's nickname to match their role in each community (or allow an admin to do so manually), without accessing an access token of this user.

• For instance, if a uni hosts a Synapse server for its student clubs, one student may play different roles if member of several clubs. The nicknames in the rooms could be changed automatically to emphasize one's role in the club that corresponds to this room.

Why it should be easily added:

The feature exists, it's just about allowing a server admin to change other people's nicknames in rooms.

• Indeed, a user can change their own nickname in a room using /myroomnick on Element. Likewise for the global nickname using /nick.
• An admin can already change a user's global nickname (there are even two ways, see below). The thing I'm asking for is analogous to it (thus the code should be analogous as well !).
• If an admin is allowed to get a user's access token, then the thing I'm asking works, using PUT /_matrix/client/v3/rooms/<roomId>/state/m.room.member/<userId> with a field "displayname" in the request json. However, we should avoid getting an access token for the users' accounts.

Current Contradictions:

• Although changing a user's global nickname should require more authorization than only in a room, the converse occurs in practice, since the latter currently requires an access token for the user.
• There is zero endpoint to do what I'm asking, whereas an admin can change the global nickname of a user... using any of these two endpoint that have an identical behavior, namely:
  • PUT /_matrix/client/v3/profile/<userId>/displayname
  • PUT /_synapse/admin/v2/users/<userId> Both with a field "displayname" in the request json.

[reivilibre]

The problem is that if we add an admin API command for everything you can do on the CS-API, this quickly gets out of hand :-)

You can use the 'Login as a User' API to get an access token (ideally use the valid_until_ms field so it expires), do the name change using the normal API and then log out the access token again.

• https://element-hq.github.io/synapse/latest/admin_api/user_admin_api.html#login-as-a-user

Although this takes more steps (which could be scripted away), this is functionally the same.