getAccessToken() does not return access token on first call in client_credentials grant type

Problem/Motivation

Method getAccessToken($client_id) in Oauth2ClientService.php for client_credentials grant type does not return the access token on first call to create token. On first call to create token (no token stored or token expired), the token is only stored into state but not returned directly. This is the current code for client_credentials in Oauth2ClientService.php:

  private function getClientCredentialsAccessToken($clientId) {
    $access_token = $this->retrieveAccessToken($clientId);

    if (!$access_token) {
      $access_token = $this->grantServices['client_credentials']->getAccessToken($clientId);
    }

    return $access_token;
  }

$this->grantServices['client_credentials']->getAccessToken($clientId); does not return anything, see ClientCredentialsGrantService.php

This might be intentional but it will be helpful to also return the access token there directly after it is stored into State. This will allow some integration work to seamlessly obtain and renew token efficiently.

This could also be useful on other grant type too but at the moment, I'm only doing some work on client_credentials

Proposed resolution

Return access token in getAccessToken in ClientCredentialsGrantService.php