system_form_alter

system_form_alter(&$form, FormStateInterface $form_state)

function system_form_alter(&$form, FormStateInterface $form_state) {
  // If the page that's being built is cacheable, set the 'immutable' flag, to
  // ensure that when the form is used, a new form build ID is generated when
  // appropriate, to prevent information disclosure.

  // Note: This code just wants to know whether cache response headers are set,
  // not whether page_cache module will be active.
  // \Drupal\Core\EventSubscriber\FinishResponseSubscriber::onRespond will
  // send those headers, in case $request_policy->check($request) succeeds. In
  // that case we need to ensure that the immutable flag is sot, so future POST
  // request won't take over the form state of another user.
  /** @var \Drupal\Core\PageCache\RequestPolicyInterface $request_policy */
  $request_policy = \Drupal::service('page_cache_request_policy');
  $request = \Drupal::requestStack()->getCurrentRequest();
  $request_is_cacheable = $request_policy->check($request) === RequestPolicyInterface::ALLOW;
  if ($request_is_cacheable) {
    $form_state->addBuildInfo('immutable', TRUE);
  }
}