Refactor FAPI getCache()/setCache() into a standalone class

1. +++ b/core/core.services.yml
   @@ -160,13 +160,16 @@ services:
      form_validator:
        class: Drupal\Core\Form\FormValidator
        arguments: ['@request_stack', '@string_translation', '@csrf_token', '@logger.channel.form']
      form_submitter:
        class: Drupal\Core\Form\FormSubmitter
        arguments: ['@request_stack', '@url_generator']
   +  form_cache:
   +    class: Drupal\Core\Form\FormCache
   +    arguments: ['@keyvalue.expirable', '@module_handler', '@?csrf_token']
   

   Did you considered to mark them as private?

2. +++ b/core/lib/Drupal/Core/Form/FormBuilder.php
   @@ -108,16 +99,21 @@ class FormBuilder implements FormBuilderInterface, FormValidatorInterface, FormS
      /**
   +   * @var \Drupal\Core\Form\FormCacheInterface
   +   */
   +  protected $formCache;
   +
   

   Let's put some documentation on there.

3. +++ b/core/lib/Drupal/Core/Form/FormCache.php
   @@ -0,0 +1,152 @@
   +      if ($this->currentUser()->isAuthenticated()) {
   ...
   +  /**
   +   * Gets the current active user.
   +   *
   +   * @return \Drupal\Core\Session\AccountInterface
   +   */
   +  protected function currentUser() {
   +    if (!$this->currentUser && \Drupal::hasService('current_user')) {
   +      $this->currentUser = \Drupal::currentUser();
   +    }
   +    return $this->currentUser;
   +  }
   

   Can't you instead provide an optional constructor dependency? How does that work, to not fail? This code assumes that the current user always exists.

It is simple re-factoring so +1 but I must say new test coverage is amazing. Just minor issues need some clarification other then that RTBC.

1. +++ b/core/core.services.yml
   @@ -160,13 +160,17 @@ services:
   +    public: false  # Private to form_builder
   

   Please move this above the line like all other inline comments.

2. +++ b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
   @@ -0,0 +1,402 @@
   +    $this->keyValueExpirableFactory = $this->getMockBuilder('Drupal\Core\KeyValueStore\KeyValueExpirableFactory')
   +      ->disableOriginalConstructor()
   +      ->getMock();
   

   Why not use simple getMock()?

Thank you for explaining that.

Committed and pushed to 8.x. Thanks!

Getting those errors now when running unit tests:

There were 12 errors:

1) Drupal\Tests\Core\Form\FormCacheTest::testGetCacheValidToken
serialize(): __sleep should return an array only containing the names of instance-variables to serialize

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@Berdir: See #2329765: Remove all @backup* annotations from (pure) unit tests