Registration: Allow registration without a username

[anoadragon453]

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

If omitted, the homeserver MUST generate a Matrix ID local part.

Phase: 1

We currently do not generate a username if username is not set as part of a POST request to /register.

Sent Request

{
    "password": "asd",
    "auth": {
        "type": "m.login.dummy",
        "session": "Mxa1dMHgfP2VTIFNNgSkDL5n"
    }
}

Received Response

Synapse:

{
  "access_token": "...",
  "home_server": "amorgan.xyz",
  "user_id": "@15:amorgan.xyz",
  "device_id": "IGKRGSXVFW"
}

Dendrite:

{
  "errcode": "M_INVALID_USERNAME",
  "error": "User ID can only contain characters a-z, 0-9, or '_-./'"
}

I believe Synapse will just set the new username to the next available user ID. So for instance, if my homeserver has registered 3 users, the newly generated username would be @4:homeserver.xyz. We can either use this method or some other method that generates ID in an increasing manner.

Summary:

• Don't error out if a username is not provided as part of registration
• Generate and return a new username if one was not provided
• Create test case(s) that checks for a successful registration without a username, and that the returned generated username is valid

[jsinghrai]

Mind If I take this issue.

[anoadragon453]

@jsinghrai Go for it!

[anoadragon453]

We currently validate usernames here:

dendrite/src/github.com/matrix-org/dendrite/clientapi/routing/register.go

Lines 373 to 378 in dfcf31f

	// Squash username to all lowercase letters
	r.Username = strings.ToLower(r.Username)
	if resErr = validateUserName(r.Username); resErr != nil {
	return *resErr
	}

If there is no username, then we should generate one instead and use that.