diff --git a/core/core.services.yml b/core/core.services.yml
index 6316b1f25c2a4243d080e1703f14026901315be2..655bbe94e6e7a03fa325242ab3236c0ba6c913ef 100644
--- a/core/core.services.yml
+++ b/core/core.services.yml
@@ -182,7 +182,7 @@ services:
     arguments: ['@request_stack', '@url_generator']
   form_cache:
     class: Drupal\Core\Form\FormCache
-    arguments: ['@keyvalue.expirable', '@module_handler', '@current_user', '@csrf_token']
+    arguments: ['@keyvalue.expirable', '@module_handler', '@current_user', '@csrf_token', '@logger.channel.form', '@config.factory', '@request_stack', '@page_cache_request_policy']
     public: false  # Private to form_builder
   keyvalue:
     class: Drupal\Core\KeyValueStore\KeyValueFactory
diff --git a/core/lib/Drupal/Core/Ajax/UpdateBuildIdCommand.php b/core/lib/Drupal/Core/Ajax/UpdateBuildIdCommand.php
new file mode 100644
index 0000000000000000000000000000000000000000..2ce74db33c06b8cdc9e078119ac86b1d418c7197
--- /dev/null
+++ b/core/lib/Drupal/Core/Ajax/UpdateBuildIdCommand.php
@@ -0,0 +1,65 @@
+<?php
+
+/**
+ * @file
+ * Definition of Drupal\Core\Ajax\UpdateBuildIdCommand.
+ */
+
+namespace Drupal\Core\Ajax;
+
+/**
+ * AJAX command for updating the value of a hidden form_build_id input element
+ * on a form. It requires the form passed in to have keys for both the old build
+ * ID in #build_id_old and the new build ID in #build_id.
+ *
+ * The primary use case for this Ajax command is to serve a new build ID to a
+ * form served from the cache to an anonymous user, preventing one anonymous
+ * user from accessing the form state of another anonymous user on Ajax enabled
+ * forms.
+ *
+ * This command is implemented by
+ * Drupal.AjaxCommands.prototype.update_build_id() defined in misc/ajax.js.
+ *O
+ * @ingroup ajax
+ */
+class UpdateBuildIdCommand implements CommandInterface {
+
+  /**
+   * Old build id.
+   *
+   * @var string
+   */
+  protected $old;
+
+  /**
+   * New build id.
+   *
+   * @var string
+   */
+  protected $new;
+
+  /**
+   * Constructs a UpdateBuildIdCommand object.
+   *
+   * @param string $old
+   *   The old build_id.
+   * @param string $new
+   *   The new build_id.
+   */
+  public function __construct($old, $new) {
+    $this->old = $old;
+    $this->new = $new;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return [
+      'command' => 'update_build_id',
+      'old' => $this->old,
+      'new' => $this->new,
+    ];
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Form/FormBuilder.php b/core/lib/Drupal/Core/Form/FormBuilder.php
index ebab63f5c9000dc118133a23f44df63aaf5741b4..dd2bcd82374a603502485a0f4aa1da91c2475a14 100644
--- a/core/lib/Drupal/Core/Form/FormBuilder.php
+++ b/core/lib/Drupal/Core/Form/FormBuilder.php
@@ -271,8 +271,8 @@ public function buildForm($form_id, FormStateInterface &$form_state) {
       exit;
     }
 
-    // If this was a successful submission of a single-step form or the last step
-    // of a multi-step form, then self::processForm() issued a redirect to
+    // If this was a successful submission of a single-step form or the last
+    // step of a multi-step form, then self::processForm() issued a redirect to
     // another page, or back to this page, but as a new request. Therefore, if
     // we're here, it means that this is either a form being viewed initially
     // before any user input, or there was a validation error requiring the form
@@ -291,17 +291,25 @@ public function rebuildForm($form_id, FormStateInterface &$form_state, $old_form
     $form_state->setCached();
 
     // If only parts of the form will be returned to the browser (e.g., Ajax or
-    // RIA clients), re-use the old #build_id to not require client-side code to
-    // manually update the hidden 'build_id' input element.
+    // RIA clients), or if the form already had a new build ID regenerated when
+    // it was retrieved from the form cache, reuse the existing #build_id.
     // Otherwise, a new #build_id is generated, to not clobber the previous
     // build's data in the form cache; also allowing the user to go back to an
     // earlier build, make changes, and re-submit.
     // @see self::prepareForm()
     $rebuild_info = $form_state->getRebuildInfo();
-    if (isset($old_form['#build_id']) && !empty($rebuild_info['copy']['#build_id'])) {
+    $enforce_old_build_id = isset($old_form['#build_id']) && !empty($rebuild_info['copy']['#build_id']);
+    $old_form_is_mutable_copy = isset($old_form['#build_id_old']);
+    if ($enforce_old_build_id || $old_form_is_mutable_copy) {
       $form['#build_id'] = $old_form['#build_id'];
+      if ($old_form_is_mutable_copy) {
+        $form['#build_id_old'] = $old_form['#build_id_old'];
+      }
     }
     else {
+      if (isset($old_form['#build_id'])) {
+        $form['#build_id_old'] = $old_form['#build_id'];
+      }
       $form['#build_id'] = 'form-' . Crypt::randomBytesBase64();
     }
 
@@ -486,17 +494,17 @@ public function processForm($form_id, &$form, FormStateInterface &$form_state) {
         return;
       }
 
-      // If $form_state->isRebuilding() has been set and input has been processed
-      // without validation errors, we are in a multi-step workflow that is not
-      // yet complete. A new $form needs to be constructed based on the changes
-      // made to $form_state during this request. Normally, a submit handler
-      // sets $form_state->isRebuilding() if a fully executed form requires
-      // another step. However, for forms that have not been fully executed
-      // (e.g., Ajax submissions triggered by non-buttons), there is no submit
-      // handler to set $form_state->isRebuilding(). It would not make sense to
-      // redisplay the identical form without an error for the user to correct,
-      // so we also rebuild error-free non-executed forms, regardless of
-      // $form_state->isRebuilding().
+      // If $form_state->isRebuilding() has been set and input has been
+      // processed without validation errors, we are in a multi-step workflow
+      // that is not yet complete. A new $form needs to be constructed based on
+      // the changes made to $form_state during this request. Normally, a submit
+      // handler sets $form_state->isRebuilding() if a fully executed form
+      // requires another step. However, for forms that have not been fully
+      // executed (e.g., Ajax submissions triggered by non-buttons), there is no
+      // submit handler to set $form_state->isRebuilding(). It would not make
+      // sense to redisplay the identical form without an error for the user to
+      // correct, so we also rebuild error-free non-executed forms, regardless
+      // of $form_state->isRebuilding().
       // @todo Simplify this logic; considering Ajax and non-HTML front-ends,
       //   along with element-level #submit properties, it makes no sense to
       //   have divergent form execution based on whether the triggering element
diff --git a/core/lib/Drupal/Core/Form/FormCache.php b/core/lib/Drupal/Core/Form/FormCache.php
index 93da48c457028da6faa6f51ba2fa692e5814dcac..c399f1ad07ddffed7c70d8883878e88220734a5c 100644
--- a/core/lib/Drupal/Core/Form/FormCache.php
+++ b/core/lib/Drupal/Core/Form/FormCache.php
@@ -7,11 +7,16 @@
 
 namespace Drupal\Core\Form;
 
+use Drupal\Component\Utility\Crypt;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Access\CsrfTokenGenerator;
+use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Extension\ModuleHandlerInterface;
 use Drupal\Core\KeyValueStore\KeyValueExpirableFactoryInterface;
+use Drupal\Core\PageCache\RequestPolicyInterface;
 use Drupal\Core\Session\AccountInterface;
+use Psr\Log\LoggerInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
 
 /**
  * Encapsulates the caching of a form and its form state.
@@ -48,6 +53,34 @@ class FormCache implements FormCacheInterface {
    */
   protected $moduleHandler;
 
+  /**
+   * Logger channel.
+   *
+   * @var \Drupal\Core\Logger\LoggerChannelInterface
+   */
+  protected $logger;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * A policy rule determining the cacheability of a request.
+   *
+   * @var \Drupal\Core\PageCache\RequestPolicyInterface
+   */
+  protected $requestPolicy;
+
   /**
    * Constructs a new FormCache.
    *
@@ -60,12 +93,24 @@ class FormCache implements FormCacheInterface {
    *   The current user.
    * @param \Drupal\Core\Access\CsrfTokenGenerator $csrf_token
    *   The CSRF token generator.
+   * @param \Psr\Log\LoggerInterface $logger
+   *   A logger instance.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack.
+   * @param \Drupal\Core\PageCache\RequestPolicyInterface $request_policy
+   *   A policy rule determining the cacheability of a request.
    */
-  public function __construct(KeyValueExpirableFactoryInterface $key_value_expirable_factory, ModuleHandlerInterface $module_handler, AccountInterface $current_user, CsrfTokenGenerator $csrf_token = NULL) {
+  public function __construct(KeyValueExpirableFactoryInterface $key_value_expirable_factory, ModuleHandlerInterface $module_handler, AccountInterface $current_user, CsrfTokenGenerator $csrf_token, LoggerInterface $logger, ConfigFactoryInterface $config_factory, RequestStack $request_stack, RequestPolicyInterface $request_policy) {
     $this->keyValueExpirableFactory = $key_value_expirable_factory;
     $this->moduleHandler = $module_handler;
     $this->currentUser = $current_user;
+    $this->logger = $logger;
+    $this->configFactory = $config_factory;
     $this->csrfToken = $csrf_token;
+    $this->requestStack = $request_stack;
+    $this->requestPolicy = $request_policy;
   }
 
   /**
@@ -75,6 +120,18 @@ public function getCache($form_build_id, FormStateInterface $form_state) {
     if ($form = $this->keyValueExpirableFactory->get('form')->get($form_build_id)) {
       if ((isset($form['#cache_token']) && $this->csrfToken->validate($form['#cache_token'])) || (!isset($form['#cache_token']) && $this->currentUser->isAnonymous())) {
         $this->loadCachedFormState($form_build_id, $form_state);
+
+        // Generate a new #build_id if the cached form was rendered on a
+        // cacheable page.
+        $build_info = $form_state->getBuildInfo();
+        if (!empty($build_info['immutable'])) {
+          $form['#build_id_old'] = $form['#build_id'];
+          $form['#build_id'] = 'form-' . Crypt::randomBytesBase64();
+          $form['form_build_id']['#value'] = $form['#build_id'];
+          $form['form_build_id']['#id'] = $form['#build_id'];
+          unset($build_info['immutable']);
+          $form_state->setBuildInfo($build_info);
+        }
         return $form;
       }
     }
@@ -106,8 +163,8 @@ protected function loadCachedFormState($form_build_id, FormStateInterface $form_
           require_once DRUPAL_ROOT . '/' . $file;
         }
       }
-      // Retrieve the list of previously known safe strings and store it
-      // for this request.
+      // Retrieve the list of previously known safe strings and store it for
+      // this request.
       // @todo Ensure we are not storing an excessively large string list
       //   in: https://www.drupal.org/node/2295823
       $build_info += ['safe_strings' => []];
@@ -124,15 +181,29 @@ public function setCache($form_build_id, $form, FormStateInterface $form_state)
     // 6 hours cache life time for forms should be plenty.
     $expire = 21600;
 
+    // Ensure that the form build_id embedded in the form structure is the same
+    // as the one passed in as a parameter. This is an additional safety measure
+    // to prevent legacy code operating directly with form_get_cache and
+    // form_set_cache from accidentally overwriting immutable form state.
+    if (isset($form['#build_id']) && $form['#build_id'] != $form_build_id) {
+      $this->logger->error('Form build-id mismatch detected while attempting to store a form in the cache.');
+      return;
+    }
+
     // Cache form structure.
     if (isset($form)) {
       if ($this->currentUser->isAuthenticated()) {
         $form['#cache_token'] = $this->csrfToken->get();
       }
+      unset($form['#build_id_old']);
       $this->keyValueExpirableFactory->get('form')->setWithExpire($form_build_id, $form, $expire);
     }
 
     // Cache form state.
+    if ($this->configFactory->get('system.performance')->get('cache.page.use_internal') && $this->isPageCacheable()) {
+      $form_state->addBuildInfo('immutable', TRUE);
+    }
+
     // Store the known list of safe strings for form re-use.
     // @todo Ensure we are not storing an excessively large string list in:
     //   https://www.drupal.org/node/2295823
@@ -143,4 +214,14 @@ public function setCache($form_build_id, $form, FormStateInterface $form_state)
     }
   }
 
+  /**
+   * Checks if the page is cacheable.
+   *
+   * @return bool
+   *   TRUE is the page is cacheable, FALSE if not.
+   */
+  protected function isPageCacheable() {
+    return ($this->requestPolicy->check($this->requestStack->getCurrentRequest()) === RequestPolicyInterface::ALLOW);
+  }
+
 }
diff --git a/core/lib/Drupal/Core/Form/FormState.php b/core/lib/Drupal/Core/Form/FormState.php
index 1c469d03cb3d0ce7cf1cf0a928b359ba3ad8924a..09db977d9b9ccc757cbdacaaec709bdd4f22ee7f 100644
--- a/core/lib/Drupal/Core/Form/FormState.php
+++ b/core/lib/Drupal/Core/Form/FormState.php
@@ -57,6 +57,12 @@ class FormState implements FormStateInterface {
    *     processed.
    *   - base_form_id: Identification for a base form, as declared in the form
    *     class's \Drupal\Core\Form\BaseFormIdInterface::getBaseFormId() method.
+   *   - immutable: If this flag is set to TRUE, a new form build id is
+   *     generated when the form is loaded from the cache. If it is subsequently
+   *     saved to the cache again, it will have another cache id and therefore
+   *     the original form and form-state will remain unaltered. This is
+   *     important when page caching is enabled in order to prevent form state
+   *     from leaking between anonymous users.
    *
    * @var array
    */
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 705d592e2d99b225499d96c707969a9ba9d7273c..c432706c83afe745f0eeec9f23b8b3e922926f47 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -718,6 +718,13 @@
         .filter(':odd').addClass('even');
     },
 
+    /**
+     * Command to update a form's build ID.
+     */
+    update_build_id: function(ajax, response, status) {
+      $('input[name="form_build_id"][value="' + response.old + '"]').val(response.new);
+    },
+
     /**
      * Command to add css.
      *
diff --git a/core/modules/file/src/Controller/FileWidgetAjaxController.php b/core/modules/file/src/Controller/FileWidgetAjaxController.php
index 119867249d9a0b79bd2282a71338b28b8dd06442..8c136f137c7d0fe3aa1268c45dd677de2dc1d231 100644
--- a/core/modules/file/src/Controller/FileWidgetAjaxController.php
+++ b/core/modules/file/src/Controller/FileWidgetAjaxController.php
@@ -43,7 +43,11 @@ public function upload(Request $request) {
     }
 
     try {
-      list($form, $form_state) = $this->getForm($request);
+      /** @var $ajaxForm \Drupal\system\FileAjaxForm */
+      $ajaxForm = $this->getForm($request);
+      $form = $ajaxForm->getForm();
+      $form_state = $ajaxForm->getFormState();
+      $commands = $ajaxForm->getCommands();
     }
     catch (HttpExceptionInterface $e) {
       // Invalid form_build_id.
@@ -80,6 +84,9 @@ public function upload(Request $request) {
     $settings = drupal_merge_js_settings($js['settings']['data']);
 
     $response = new AjaxResponse();
+    foreach ($commands as $command) {
+      $response->addCommand($command, TRUE);
+    }
     return $response->addCommand(new ReplaceCommand(NULL, $output, $settings));
   }
 
diff --git a/core/modules/simpletest/src/WebTestBase.php b/core/modules/simpletest/src/WebTestBase.php
index 98f68fcc84a0381bfe783b58fc87e5d12bd70fb8..686b9bcdb16ab3d465672e6670b0932d508fb745 100644
--- a/core/modules/simpletest/src/WebTestBase.php
+++ b/core/modules/simpletest/src/WebTestBase.php
@@ -1897,6 +1897,12 @@ protected function drupalProcessAjaxResponse($content, array $ajax_response, arr
           break;
         case 'add_css':
           break;
+        case 'update_build_id':
+          $buildId = $xpath->query('//input[@name="form_build_id" and @value="' . $command['old'] . '"]')->item(0);
+          if ($buildId) {
+            $buildId->setAttribute('value', $command['new']);
+          }
+          break;
       }
     }
     $content = $dom->saveHTML();
diff --git a/core/modules/system/src/Controller/FormAjaxController.php b/core/modules/system/src/Controller/FormAjaxController.php
index c3c678c38c1c6202ab00d3b2a04adb64fbb75944..c0a54e642a9df6d78487383d36b9d49e6e3ab93d 100644
--- a/core/modules/system/src/Controller/FormAjaxController.php
+++ b/core/modules/system/src/Controller/FormAjaxController.php
@@ -7,8 +7,10 @@
 
 namespace Drupal\system\Controller;
 
+use Drupal\Core\Ajax\UpdateBuildIdCommand;
 use Drupal\Core\Form\FormState;
 use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\system\FileAjaxForm;
 use Psr\Log\LoggerInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\HttpFoundation\Request;
@@ -63,7 +65,12 @@ public static function create(ContainerInterface $container) {
    * @throws \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface
    */
   public function content(Request $request) {
-    list($form, $form_state) = $this->getForm($request);
+    /** @var $ajaxForm \Drupal\system\FileAjaxForm */
+    $ajaxForm = $this->getForm($request);
+    $form = $ajaxForm->getForm();
+    $form_state = $ajaxForm->getFormState();
+    $commands = $ajaxForm->getCommands();
+
     drupal_process_form($form['#form_id'], $form, $form_state);
 
     // We need to return the part of the form (or some other content) that needs
@@ -81,7 +88,12 @@ public function content(Request $request) {
     if (empty($callback) || !is_callable($callback)) {
       throw new HttpException(500, t('Internal Server Error'));
     }
-    return call_user_func_array($callback, array(&$form, &$form_state));
+    /** @var \Drupal\Core\Ajax\AjaxResponse $response */
+    $response = call_user_func_array($callback, [&$form, &$form_state]);
+    foreach ($commands as $command) {
+      $response->addCommand($command, TRUE);
+    }
+    return $response;
   }
 
   /**
@@ -93,14 +105,11 @@ public function content(Request $request) {
    * @param \Symfony\Component\HttpFoundation\Request $request
    *   The current request object.
    *
-   * @return array
-   *   An array containing the $form and $form_state. Use the list() function
-   *   to break these apart:
-   *   @code
-   *     list($form, $form_state, $form_id, $form_build_id) = $this->getForm();
-   *   @endcode
+   * @return \Drupal\system\FileAjaxForm
+   *   A wrapper object containing the $form, $form_state, $form_id,
+   *   $form_build_id and an initial list of Ajax $commands.
    *
-   * @throws Symfony\Component\HttpKernel\Exception\HttpExceptionInterface
+   * @throws \Symfony\Component\HttpKernel\Exception\HttpExceptionInterface
    */
   protected function getForm(Request $request) {
     $form_state = new FormState();
@@ -118,6 +127,17 @@ protected function getForm(Request $request) {
       throw new BadRequestHttpException();
     }
 
+    // When a page level cache is enabled, the form-build id might have been
+    // replaced from within form_get_cache. If this is the case, it is also
+    // necessary to update it in the browser by issuing an appropriate Ajax
+    // command.
+    $commands = [];
+    if (isset($form['#build_id_old']) && $form['#build_id_old'] != $form['#build_id']) {
+      // If the form build ID has changed, issue an Ajax command to update it.
+      $commands[] = new UpdateBuildIdCommand($form['#build_id_old'], $form['#build_id']);
+      $form_build_id = $form['#build_id'];
+    }
+
     // Since some of the submit handlers are run, redirects need to be disabled.
     $form_state->disableRedirect();
 
@@ -129,12 +149,12 @@ protected function getForm(Request $request) {
       '#action' => TRUE,
     ]);
 
-    // The form needs to be processed; prepare for that by setting a few internal
-    // variables.
+    // The form needs to be processed; prepare for that by setting a few
+    // internal variables.
     $form_state->setUserInput($request->request->all());
     $form_id = $form['#form_id'];
 
-    return array($form, $form_state, $form_id, $form_build_id);
+    return new FileAjaxForm($form, $form_state, $form_id, $form_build_id, $commands);
   }
 
 }
diff --git a/core/modules/system/src/FileAjaxForm.php b/core/modules/system/src/FileAjaxForm.php
new file mode 100644
index 0000000000000000000000000000000000000000..1be14b4e5188eafe46c9ba9c0b0c305a958c786c
--- /dev/null
+++ b/core/modules/system/src/FileAjaxForm.php
@@ -0,0 +1,122 @@
+<?php
+
+/**
+ * @file
+ * Contains Drupal\system\FileAjaxForm.
+ */
+
+namespace Drupal\system;
+
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Wrapper for Ajax forms data and commands, avoiding a multi-return-value tuple.
+ *
+ * @ingroup ajax
+ */
+class FileAjaxForm {
+
+  /**
+   * The form to cache.
+   *
+   * @var array
+   */
+  protected $form;
+
+  /**
+   * The form state.
+   *
+   * @var \Drupal\Core\Form\FormStateInterface
+   */
+  protected $formState;
+
+  /**
+   * The unique form ID.
+   *
+   * @var string
+   */
+  protected $formId;
+
+  /**
+   * The unique form build ID.
+   *
+   * @var string
+   */
+  protected $formBuildId;
+
+  /**
+   * The array of ajax commands.
+   *
+   * @var \Drupal\Core\Ajax\CommandInterface[]
+   */
+  protected $commands;
+
+  /**
+   * Constructs a FileAjaxForm object.
+   *
+   * @param array $form
+   *   The form definition.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state.
+   * @param \Drupal\Core\Ajax\string|string $form_id
+   *   The unique form ID.
+   * @param \Drupal\Core\Ajax\string|string $form_build_id
+   *   The unique form build ID.
+   * @param \Drupal\Core\Ajax\CommandInterface[] $commands
+   *   The ajax commands.
+   */
+  public function __construct(array $form, FormStateInterface $form_state, $form_id, $form_build_id, array $commands) {
+    $this->form = $form;
+    $this->formState = $form_state;
+    $this->formId = $form_id;
+    $this->formBuildId = $form_build_id;
+    $this->commands = $commands;
+  }
+
+  /**
+   * Gets all AJAX commands.
+   *
+   * @return \Drupal\Core\Ajax\CommandInterface[]
+   *   Returns all previously added AJAX commands.
+   */
+  public function getCommands() {
+    return $this->commands;
+  }
+
+  /**
+   * Gets the form definition.
+   *
+   * @return array
+   */
+  public function getForm() {
+    return $this->form;
+  }
+
+  /**
+   * Gets the unique form build ID.
+   *
+   * @return string
+   */
+  public function getFormBuildId() {
+    return $this->formBuildId;
+  }
+
+  /**
+   * Gets the unique form ID.
+   *
+   * @return string
+   */
+  public function getFormId() {
+    return $this->formId;
+  }
+
+  /**
+   * Gets the form state.
+   *
+   * @return \Drupal\Core\Form\FormStateInterface
+   */
+  public function getFormState() {
+    return $this->formState;
+  }
+
+}
diff --git a/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..3d9ffa252047667cd27ab3df307c251563dcce60
--- /dev/null
+++ b/core/modules/system/src/Tests/Ajax/AjaxFormPageCacheTest.php
@@ -0,0 +1,86 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Ajax\AjaxFormPageCacheTest.
+ */
+
+namespace Drupal\system\Tests\Ajax;
+
+/**
+ * Performs tests on AJAX forms in cached pages.
+ *
+ * @group Ajax
+ */
+class AjaxFormPageCacheTest extends AjaxTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setUp() {
+    parent::setUp();
+
+    $config = \Drupal::config('system.performance');
+    $config->set('cache.page.use_internal', 1);
+    $config->set('cache.page.max_age', 300);
+    $config->save();
+  }
+
+  /**
+   * Return the build id of the current form.
+   */
+  protected function getFormBuildId() {
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    return (string) $build_id_fields[0]['value'];
+  }
+
+  /**
+   * Create a simple form, then POST to system/ajax to change to it.
+   */
+  public function testSimpleAJAXFormValue() {
+   $this->drupalGet('ajax_forms_test_get_form');
+   $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+   $build_id_initial = $this->getFormBuildId();
+
+   $edit = ['select' => 'green'];
+   $commands = $this->drupalPostAjaxForm(NULL, $edit, 'select');
+   $build_id_first_ajax = $this->getFormBuildId();
+   $this->assertNotEqual($build_id_initial, $build_id_first_ajax, 'Build id is changed in the simpletest-DOM on first AJAX submission');
+   $expected = [
+     'command' => 'update_build_id',
+     'old' => $build_id_initial,
+     'new' => $build_id_first_ajax,
+   ];
+   $this->assertCommand($commands, $expected, 'Build id change command issued on first AJAX submission');
+
+   $edit = ['select' => 'red'];
+   $this->drupalPostAjaxForm(NULL, $edit, 'select');
+   $build_id_second_ajax = $this->getFormBuildId();
+   $this->assertEqual($build_id_first_ajax, $build_id_second_ajax, 'Build id remains the same on subsequent AJAX submissions');
+
+   // Repeat the test sequence but this time with a page loaded from the cache.
+   $this->drupalGet('ajax_forms_test_get_form');
+   $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
+   $build_id_from_cache_initial = $this->getFormBuildId();
+   $this->assertEqual($build_id_initial, $build_id_from_cache_initial, 'Build id is the same as on the first request');
+
+   $edit = ['select' => 'green'];
+   $commands = $this->drupalPostAjaxForm(NULL, $edit, 'select');
+   $build_id_from_cache_first_ajax = $this->getFormBuildId();
+   $this->assertNotEqual($build_id_from_cache_initial, $build_id_from_cache_first_ajax, 'Build id is changed in the simpletest-DOM on first AJAX submission');
+   $this->assertNotEqual($build_id_first_ajax, $build_id_from_cache_first_ajax, 'Build id from first user is not reused');
+   $expected = [
+     'command' => 'update_build_id',
+     'old' => $build_id_from_cache_initial,
+     'new' => $build_id_from_cache_first_ajax,
+   ];
+   $this->assertCommand($commands, $expected, 'Build id change command issued on first AJAX submission');
+
+   $edit = ['select' => 'red'];
+   $this->drupalPostAjaxForm(NULL, $edit, 'select');
+   $build_id_from_cache_second_ajax = $this->getFormBuildId();
+   $this->assertEqual($build_id_from_cache_first_ajax, $build_id_from_cache_second_ajax, 'Build id remains the same on subsequent AJAX submissions');
+ }
+
+}
diff --git a/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php b/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..18389569ae18cdefc44ed26d72c7cdd0957f90d9
--- /dev/null
+++ b/core/modules/system/src/Tests/Form/FormStoragePageCacheTest.php
@@ -0,0 +1,115 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Tests\Form\FormStoragePageCacheTest.
+ */
+
+namespace Drupal\system\Tests\Form;
+
+use Drupal\simpletest\WebTestBase;
+
+/**
+ * Tests form storage from cached pages.
+ *
+ * @group Form
+ */
+class FormStoragePageCacheTest extends WebTestBase {
+
+  /**
+   * @var array
+   */
+  public static $modules = array('form_test');
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+
+    $config = \Drupal::config('system.performance');
+    $config->set('cache.page.use_internal', 1);
+    $config->set('cache.page.max_age', 300);
+    $config->save();
+  }
+
+  /**
+   * Return the build id of the current form.
+   */
+  protected function getFormBuildId() {
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    return (string) $build_id_fields[0]['value'];
+  }
+
+  /**
+   * Build-id is regenerated when validating cached form.
+   */
+  public function testValidateFormStorageOnCachedPage() {
+    $this->drupalGet('form-test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_initial = $this->getFormBuildId();
+
+    // Trigger validation error by submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertText($build_id_initial, 'Old build id on the page');
+    $build_id_first_validation = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_first_validation, 'Build id changes when form validation fails');
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_second_validation = $this->getFormBuildId();
+    $this->assertEqual($build_id_first_validation, $build_id_second_validation, 'Build id remains the same when form validation fails subsequently');
+
+    // Repeat the test sequence but this time with a page loaded from the cache.
+    $this->drupalGet('form-test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'HIT', 'Page was cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_from_cache_initial = $this->getFormBuildId();
+    $this->assertEqual($build_id_initial, $build_id_from_cache_initial, 'Build id is the same as on the first request');
+
+    // Trigger validation error by submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertText($build_id_initial, 'Old build id is initial build id');
+    $build_id_from_cache_first_validation = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_from_cache_first_validation, 'Build id changes when form validation fails');
+    $this->assertNotEqual($build_id_first_validation, $build_id_from_cache_first_validation, 'Build id from first user is not reused');
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Save');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_from_cache_second_validation = $this->getFormBuildId();
+    $this->assertEqual($build_id_from_cache_first_validation, $build_id_from_cache_second_validation, 'Build id remains the same when form validation fails subsequently');
+  }
+
+  /**
+   * Build-id is regenerated when rebuilding cached form.
+   */
+  public function testRebuildFormStorageOnCachedPage() {
+    $this->drupalGet('form-test/form-storage-page-cache');
+    $this->assertEqual($this->drupalGetHeader('X-Drupal-Cache'), 'MISS', 'Page was not cached.');
+    $this->assertText('No old build id', 'No old build id on the page');
+    $build_id_initial = $this->getFormBuildId();
+
+    // Trigger rebuild, should regenerate build id.
+    $edit = ['title' => 'something'];
+    $this->drupalPostForm(NULL, $edit, 'Rebuild');
+    $this->assertText($build_id_initial, 'Initial build id as old build id on the page');
+    $build_id_first_rebuild = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_initial, $build_id_first_rebuild, 'Build id changes on first rebuild.');
+
+    // Trigger subsequent rebuild, should regenerate the build id again.
+    $edit = ['title' => 'something'];
+    $this->drupalPostForm(NULL, $edit, 'Rebuild');
+    $this->assertText($build_id_first_rebuild, 'First build id as old build id on the page');
+    $build_id_second_rebuild = $this->getFormBuildId();
+    $this->assertNotEqual($build_id_first_rebuild, $build_id_second_rebuild, 'Build id changes on second rebuild.');
+  }
+
+}
diff --git a/core/modules/system/src/Tests/Form/StorageTest.php b/core/modules/system/src/Tests/Form/StorageTest.php
index 01189d1803d49e64b3111a5b6d9b6c39e3c6dd13..ca690a6f6787c3f5913274baec4329776952716d 100644
--- a/core/modules/system/src/Tests/Form/StorageTest.php
+++ b/core/modules/system/src/Tests/Form/StorageTest.php
@@ -15,9 +15,9 @@
  *
  * The tested form puts data into the storage during the initial form
  * construction. These tests verify that there are no duplicate form
- * constructions, with and without manual form caching activiated. Furthermore
+ * constructions, with and without manual form caching activated. Furthermore
  * when a validation error occurs, it makes sure that changed form element
- * values aren't lost due to a wrong form rebuild.
+ * values are not lost due to a wrong form rebuild.
  *
  * @group Form
  */
@@ -28,7 +28,7 @@ class StorageTest extends WebTestBase {
    *
    * @var array
    */
-  public static $modules = array('form_test');
+  public static $modules = array('form_test', 'dblog');
 
   protected function setUp() {
     parent::setUp();
@@ -159,4 +159,81 @@ function testFormStatePersist() {
       $this->assertText('State persisted.');
     }
   }
+
+  /**
+   * Verify that the form build-id remains the same when validation errors
+   * occur on a mutable form.
+   */
+  public function testMutableForm() {
+    // Request the form with 'cache' query parameter to enable form caching.
+    $this->drupalGet('form_test/form-storage', ['query' => ['cache' => 1]]);
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id did not change.
+    $this->assertFieldByName('form_build_id', $buildId, 'Build id remains the same when form validation fails');
+  }
+
+  /**
+   * Verifies that form build-id is regenerated when loading an immutable form
+   * from the cache.
+   */
+  public function testImmutableForm() {
+    // Request the form with 'cache' query parameter to enable form caching.
+    $this->drupalGet('form_test/form-storage', ['query' => ['cache' => 1, 'immutable' => 1]]);
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id did change.
+    $this->assertNoFieldByName('form_build_id', $buildId, 'Build id changes when form validation fails');
+
+    // Retrieve the new build-id.
+    $buildIdFields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($buildIdFields), 1, 'One form build id field on the page');
+    $buildId = (string) $buildIdFields[0]['value'];
+
+    // Trigger validation error by again submitting an empty title.
+    $edit = ['title' => ''];
+    $this->drupalPostForm(NULL, $edit, 'Continue submit');
+
+    // Verify that the build-id does not change the second time.
+    $this->assertFieldByName('form_build_id', $buildId, 'Build id remains the same when form validation fails subsequently');
+  }
+
+  /**
+   * Verify that existing contrib code cannot overwrite immutable form state.
+   */
+  public function testImmutableFormLegacyProtection() {
+    $this->drupalGet('form_test/form-storage', ['query' => ['cache' => 1, 'immutable' => 1]]);
+    $build_id_fields = $this->xpath('//input[@name="form_build_id"]');
+    $this->assertEqual(count($build_id_fields), 1, 'One form build id field on the page');
+    $build_id = (string) $build_id_fields[0]['value'];
+
+    // Try to poison the form cache.
+    $original = $this->drupalGetAJAX('form-test/form-storage-legacy/' . $build_id);
+    $this->assertEqual($original['form']['#build_id_old'], $build_id, 'Original build_id was recorded');
+    $this->assertNotEqual($original['form']['#build_id'], $build_id, 'New build_id was generated');
+
+    // Assert that a watchdog message was logged by form_set_cache.
+    $status = (bool) db_query_range('SELECT 1 FROM {watchdog} WHERE message = :message', 0, 1, [':message' => 'Form build-id mismatch detected while attempting to store a form in the cache.']);
+    $this->assert($status, 'A watchdog message was logged by form_set_cache');
+
+    // Ensure that the form state was not poisoned by the preceding call.
+    $original = $this->drupalGetAJAX('form-test/form-storage-legacy/' . $build_id);
+    $this->assertEqual($original['form']['#build_id_old'], $build_id, 'Original build_id was recorded');
+    $this->assertNotEqual($original['form']['#build_id'], $build_id, 'New build_id was generated');
+    $this->assert(empty($original['form']['#poisoned']), 'Original form structure was preserved');
+    $this->assert(empty($original['form_state']['poisoned']), 'Original form state was preserved');
+  }
+
 }
diff --git a/core/modules/system/tests/modules/form_test/form_test.routing.yml b/core/modules/system/tests/modules/form_test/form_test.routing.yml
index 109d8f37c3ea654b988340fc44f5830db112fe9e..bebb71c034d0d5965ac6438d546cdb5ad6f74516 100644
--- a/core/modules/system/tests/modules/form_test/form_test.routing.yml
+++ b/core/modules/system/tests/modules/form_test/form_test.routing.yml
@@ -433,3 +433,19 @@ form_test.two_instances:
   requirements:
     _module_dependencies: 'node'
     _permission: 'create page content'
+
+form_test.storage_legacy_handler:
+  path: '/form-test/form-storage-legacy/{form_build_id}'
+  defaults:
+    _controller: '\Drupal\form_test\Controller\FormTestController::storageLegacyHandler'
+    form_build_id: NULL
+  requirements:
+    _access: 'TRUE'
+
+form_test.form_storage_page_cache:
+  path: '/form-test/form-storage-page-cache'
+  defaults:
+    _form: '\Drupal\form_test\Form\FormTestStoragePageCacheForm'
+    _title: 'Form storage with page cache test'
+  requirements:
+    _access: 'TRUE'
diff --git a/core/modules/system/tests/modules/form_test/src/Controller/FormTestController.php b/core/modules/system/tests/modules/form_test/src/Controller/FormTestController.php
index 600b1d9572694e4b2b95112bb9a4a45933788f66..1950076b751fecfbe5d02e2e5a5d277f7e71709a 100644
--- a/core/modules/system/tests/modules/form_test/src/Controller/FormTestController.php
+++ b/core/modules/system/tests/modules/form_test/src/Controller/FormTestController.php
@@ -7,7 +7,9 @@
 namespace Drupal\form_test\Controller;
 
 use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Form\FormState;
 use Drupal\Core\Language\LanguageInterface;
+use Symfony\Component\HttpFoundation\JsonResponse;
 
 /**
  * Controller routines for form_test routes.
@@ -35,4 +37,25 @@ public function twoFormInstances() {
     return $return;
   }
 
+  /**
+   * Emulate legacy AHAH-style ajax callback.
+   *
+   * Drupal 6 AHAH callbacks used to operate directly on forms retrieved using
+   * form_get_cache and stored using form_set_cache after manipulation. This
+   * callback helps testing whether \Drupal::formBuilder()->setCache() prevents
+   * resaving of immutable forms.
+   */
+  public function storageLegacyHandler($form_build_id) {
+    $form_state = new FormState();
+    $form = $this->formBuilder()->getCache($form_build_id, $form_state);
+    $result = [
+      'form' => $form,
+      'form_state' => $form_state,
+    ];
+    $form['#poisoned'] = TRUE;
+    $form_state->set('poisoned', TRUE);
+    $this->formBuilder()->setCache($form_build_id, $form, $form_state);
+    return new JsonResponse($result);
+  }
+
 }
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
index c5f166b997733aaf21c9a761791a9057c96a1058..60e0cdb082e707a3715fff39e9cc7bd6d55c0835 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestStorageForm.php
@@ -88,6 +88,10 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       $form_state->setCached();
     }
 
+    if ($this->getRequest()->get('immutable')) {
+      $form_state->addBuildInfo('immutable', TRUE);
+    }
+
     return $form;
   }
 
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestStoragePageCacheForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestStoragePageCacheForm.php
new file mode 100644
index 0000000000000000000000000000000000000000..223ed808dbe005995e76334a3a0da98c8b48b53d
--- /dev/null
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestStoragePageCacheForm.php
@@ -0,0 +1,80 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\form_test\Form\FormTestStorageForm.
+ */
+
+namespace Drupal\form_test\Form;
+
+use Drupal\Component\Utility\String;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+class FormTestStoragePageCacheForm extends FormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'form_test_storage_page_cache';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $form['title'] = array(
+      '#type' => 'textfield',
+      '#title' => 'Title',
+      '#required' => TRUE,
+    );
+
+    $form['test_build_id_old'] = array(
+      '#type' => 'item',
+      '#title' => 'Old build id',
+      '#markup' => 'No old build id',
+    );
+
+    $form['submit'] = array(
+      '#type' => 'submit',
+      '#value' => 'Save',
+    );
+
+    $form['rebuild'] = array(
+      '#type' => 'submit',
+      '#value' => 'Rebuild',
+      '#submit' => array(array($this, 'form_test_storage_page_cache_rebuild')),
+    );
+
+    $form['#after_build'] = array(array($this, 'form_test_storage_page_cache_old_build_id'));
+    $form_state->setCached();
+
+    return $form;
+  }
+
+  /**
+   * Form element #after_build callback: output the old form build-id.
+   */
+  function form_test_storage_page_cache_old_build_id($form) {
+    if (isset($form['#build_id_old'])) {
+      $form['test_build_id_old']['#markup'] = String::checkPlain($form['#build_id_old']);
+    }
+    return $form;
+  }
+
+  /**
+   * Form submit callback: Rebuild the form and continue.
+   */
+  function form_test_storage_page_cache_rebuild($form, FormStateInterface $form_state) {
+    $form_state->setRebuild();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // Nothing must happen.
+  }
+
+}
diff --git a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
index d08557ac8d7f1896956572c73077701d47006a3e..41d76dfa981487858d97d47d5678f7d9d6dccced 100644
--- a/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
+++ b/core/tests/Drupal/Tests/Core/Form/FormCacheTest.php
@@ -10,7 +10,6 @@
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Form\FormCache;
 use Drupal\Core\Form\FormState;
-use Drupal\Core\Session\AccountInterface;
 use Drupal\Tests\UnitTestCase;
 
 /**
@@ -68,6 +67,34 @@ class FormCacheTest extends UnitTestCase {
    */
   protected $formStateCacheStore;
 
+  /**
+   * The logger channel.
+   *
+   * @var \Psr\Log\LoggerInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $logger;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $configFactory;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $requestStack;
+
+  /**
+   * A policy rule determining the cacheability of a request.
+   *
+   * @var \Drupal\Core\PageCache\RequestPolicyInterface|\PHPUnit_Framework_MockObject_MockObject
+   */
+  protected $requestPolicy;
+
   /**
    * {@inheritdoc}
    */
@@ -92,7 +119,14 @@ protected function setUp() {
       ->disableOriginalConstructor()
       ->getMock();
     $this->account = $this->getMock('Drupal\Core\Session\AccountInterface');
-    $this->formCache = new FormCache($this->keyValueExpirableFactory, $this->moduleHandler, $this->account, $this->csrfToken);
+
+    $this->logger = $this->getMock('Psr\Log\LoggerInterface');
+    $this->configFactory = $this->getConfigFactoryStub(['system.performance' => ['cache.page.use_internal' => FALSE]]);
+    $this->requestStack = $this->getMock('\Symfony\Component\HttpFoundation\RequestStack');
+    $this->requestPolicy = $this->getMock('\Drupal\Core\PageCache\RequestPolicyInterface');
+
+
+    $this->formCache = new FormCache($this->keyValueExpirableFactory, $this->moduleHandler, $this->account, $this->csrfToken, $this->logger, $this->configFactory, $this->requestStack, $this->requestPolicy);
   }
 
   /**
@@ -212,6 +246,32 @@ public function testGetCacheNoForm() {
     $this->assertNull($form);
   }
 
+  /**
+   * @covers ::getCache
+   */
+  public function testGetCacheImmutableForm() {
+    $form_build_id = 'the_form_build_id';
+    $form_state = (new FormState())
+      ->addBuildInfo('immutable', TRUE);
+    $cached_form = [
+      '#build_id' => 'the_old_build_form_id',
+    ];
+
+    $this->account->expects($this->once())
+      ->method('isAnonymous')
+      ->willReturn(TRUE);
+    $this->formCacheStore->expects($this->once())
+      ->method('get')
+      ->with($form_build_id)
+      ->willReturn($cached_form);
+
+    $form = $this->formCache->getCache($form_build_id, $form_state);
+    $this->assertSame($cached_form['#build_id'], $form['#build_id_old']);
+    $this->assertNotSame($cached_form['#build_id'], $form['#build_id']);
+    $this->assertSame($form['#build_id'], $form['form_build_id']['#value']);
+    $this->assertSame($form['#build_id'], $form['form_build_id']['#id']);
+  }
+
   /**
    * @covers ::loadCachedFormState
    */
@@ -406,6 +466,63 @@ public function testSetCacheWithSafeStrings() {
     $this->formCache->setCache($form_build_id, $form, $form_state);
   }
 
+  /**
+   * @covers ::setCache
+   */
+  public function testSetCacheBuildIdMismatch() {
+    $form_build_id = 'the_form_build_id';
+    $form = [
+      '#form_id' => 'the_form_id',
+      '#build_id' => 'stale_form_build_id',
+    ];
+    $form_state = new FormState();
+
+    $this->formCacheStore->expects($this->never())
+      ->method('setWithExpire');
+    $this->formStateCacheStore->expects($this->never())
+      ->method('setWithExpire');
+    $this->logger->expects($this->once())
+      ->method('error')
+      ->with('Form build-id mismatch detected while attempting to store a form in the cache.');
+    $this->formCache->setCache($form_build_id, $form, $form_state);
+  }
+
+  /**
+   * @covers ::setCache
+   */
+  public function testSetCacheImmutableForm() {
+    $form_build_id = 'the_form_build_id';
+    $form = [
+      '#form_id' => 'the_form_id',
+    ];
+    $form_state = new FormState();
+
+    $this->formCacheStore->expects($this->once())
+      ->method('setWithExpire')
+      ->with($form_build_id, $form, $this->isType('int'));
+    $form_state_data = $form_state->getCacheableArray();
+    $form_state_data['build_info']['safe_strings'] = [];
+    // Ensure that the form is marked immutable.
+    $form_state_data['build_info']['immutable'] = TRUE;
+    $this->formStateCacheStore->expects($this->once())
+      ->method('setWithExpire')
+      ->with($form_build_id, $form_state_data, $this->isType('int'));
+
+    // Rebuild the FormCache with a config factory that will return a config
+    // object with the internal page cache enabled.
+    $this->configFactory = $this->getConfigFactoryStub(['system.performance' => ['cache.page.use_internal' => TRUE]]);
+    $this->formCache = $this->getMockBuilder('Drupal\Core\Form\FormCache')
+      ->setConstructorArgs([$this->keyValueExpirableFactory, $this->moduleHandler, $this->account, $this->csrfToken, $this->logger, $this->configFactory, $this->requestStack, $this->requestPolicy])
+      ->setMethods(['isPageCacheable'])
+      ->getMock();
+
+    $this->formCache->expects($this->once())
+      ->method('isPageCacheable')
+      ->willReturn(TRUE);
+
+    $this->formCache->setCache($form_build_id, $form, $form_state);
+  }
+
   /**
    * Ensures SafeMarkup does not bleed from one test to another.
    */