function mymodule_form_alter(&$form, $form_state, $form_id) {

  if ($form_id == 'user_login_block' || $form_id == 'user_login') {

    $form['#validate'] = array('user_login_name_validate', 'mymodule_login_validate', 'user_login_final_validate');

  }
}

function mymodule_login_validate( $form, &$form_state )
{
     global $user;


     $username = $form_state['values']['name'];
     $password = $form_state['values']['pass'];


     $ch = curl_init();

    curl_setopt($ch,CURLOPT_URL,"http://localhost/MYAPI/oauth/authenticate");

    curl_setopt($ch, CURLOPT_POST, 1);
    curl_setopt($ch, CURLOPT_POSTFIELDS,"username=".$username."&grant_type=password");
    curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/x-www-form-urlencoded","Authorization:Basic xxxxxxxxxxxxxxx"));


   // receive server response ...
   curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

   $server_output = curl_exec ($ch);

   curl_close ($ch);


   $postData = drupal_json_decode($server_output);

   $count = db_query("SELECT COUNT(*) FROM {users} WHERE name = :name;", array(':name' =>$username))->fetchField();

  if($count == 0 )
  {
    if (!empty($postData['access_token']))
    {

       $ch = curl_init();

       curl_setopt($ch, CURLOPT_URL,"http://localhost/MYAPI/rest/user/1/get_user_details");
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array("Content-Type: application/x-www-form-urlencoded","Authorization:Bearer ".$postData['access_token']));


         // receive server response ...
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);

        $server_output = curl_exec ($ch);

        curl_close ($ch);

        $postData = drupal_json_decode($server_output);


        $resRoleArr = $postData['userDetail']['role'];
        $resMail = $postData['userDetail']['email'];

        $roleArr = user_roles();

        $common_roles = array_intersect($resRoleArr,$roleArr);


         if (count($common_roles) > 0) {

               user_external_login_register($username, 'mymodule');

       $user_fields = [
         'pass' => $password,
         'status' => 1,
         'mail' => $resMail,
         'init' => $resMail,
         'roles' =>
        [DRUPAL_AUTHENTICATED_RID => 'authenticated user'],

       ];


     user_save((object) ['uid' => $user->uid], (array) $user_fields);



    $account = user_external_load($username);
    $form_state['uid'] = $account->uid;

            if ($role = user_role_load_by_name($common_roles)) {
              user_multiple_role_edit(array($user->uid), 'add_role', $role->rid);
            }



    }
  }
}
 else {

  user_login_authenticate_validate($form,$form_state);

   }

}

user_external_login_register($username, 'mymodule');

user_login_authenticate_validate($form,$form_state);