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);
   }
 }