Google API Service Account Authorization Error

Question

I am using the google-api-php-client client to connect to Google Drive using a Service Account. Most of the time it works, but every so often (say if I sit here an refresh the page over and over it's every 5th to 10th time), I receive a Google_Service_Exception with the message unauthorized_client: Client is unauthorized to retrieve access tokens using this method. The error only occurs if the $this->drive_service->files->listFiles() code is present. It does not occur if I use the Service Account directly rather than using setSubject() to impersonate another user.

        $this->client = new Google_Client();
        $this->client->useApplicationDefaultCredentials();
        $this->client->addScope("https://www.googleapis.com/auth/drive");
        $this->client->setSubject('xxxx');
        $this->drive_service = new Google_Service_Drive($this->client);
        $files = $this->drive_service->files->listFiles();

Any thoughts?

Update Feb 21, 2017: The error is no longer intermittent, it happens every time, so I can no longer connect at all to a regular account through a service account.

Update March 10, 2017: It seems as though this is a user misunderstanding. The service account must be given "domain-wide authority" in order to impersonate users of that domain - something I have not done. I had simply authorized a client to access the users account as described here.

Answer 1

I don't know why your error happens periodically but my understanding is that to impersonate an user, you must grant the service account access.