Registration: Don't create a new device_id during login if one is supplied

[anoadragon453]

Spec URL: https://matrix.org/docs/spec/client_server/r0.3.0.html#post-matrix-client-r0-login

Under "Response Format":

device_id | string | ID of the logged-in device. Will be the same as the corresponding parameter in the request, if one was specified.

Phase: 1

Difficulty: Easy

When we login, we supply a device parameter so that we can tell the homeserver what device we're logging in from. If we don't supply this parameter, the homeserver presumes we are logging in from a new device and creates a new device ID for us. If we supply one, that ID should just be returned in the response. The problem is that a new ID is being generated whether we supply one or not.

Request

Note: "ClaaatBG" is a valid device ID
{
    "user": "@supercoolname:localhost",
    "password": "greatpassword123",
    "type": "m.login.password",
    "device_id": "ClaaatBG"
}

Response

{
  "user_id": "@supercoolname:localhost",
  "access_token": "HUUXAhG8eU6qrN1MiJ-3-1hCH4kotGE2q2uD28D7VIQ",
  "home_server": "localhost",
  "device_id": "8KRmr3IY" # This should have been "ClaaatBG" again.
}

We got back "device_id": "8KRmr3IY", a new device ID even though we supplied a valid ID already. It should have responded with the original device ID.

Summary:

• Prevent a new Device ID from being generated if we supply one on login
• Make sure that if a Device ID is supplied in the request, it is returned in the response.
• Create test case(s) to verify a new Device ID is created, and that the same Device ID is sent back if we supply one to /login.

[MohitKS5]

Although I implemented it, I guess its not a necessary thing to do because we remove device at logout. So unless you want to login twice with same device simultaneously its no use at all.

[anoadragon453]

Spec mandates it, so ¯\_(ツ)_/¯