page_cache_form_test.module

<?php

/**
 * @file
 * Provides functionality for testing form caching.
 */
use Drupal\Core\Form\FormStateInterface;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function page_cache_form_test_form_page_cache_form_test_alter(&$form, FormStateInterface $form_state, $form_id) {

  // This runs earlier than system_form_alter() so we fore-go the immutability
  // check to the process callback, by which time system_form_alter() has run.
  $form['#process'][] = 'page_cache_form_test_form_page_cache_form_test_process';
}

/**
 * Process callback to check immutability.
 */
function page_cache_form_test_form_page_cache_form_test_process($form, FormStateInterface $form_state) {
  if (isset($form_state
    ->getBuildInfo()['immutable']) && $form_state
    ->getBuildInfo()['immutable']) {
    $form['#suffix'] = 'Immutable: TRUE';
  }
  else {
    $form['#suffix'] = 'Immutable: FALSE';
  }
  return $form;
}

/**
 * Implements hook_module_implements_alter().
 */
function page_cache_form_test_module_implements_alter(&$implementations, $hook) {
  if ($hook === 'form_alter' && \Drupal::state()
    ->get('page_cache_bypass_form_immutability', FALSE)) {

    // Disable system_form_alter
    unset($implementations['system']);
  }
}