.../modules/system/src/MessagesPostRenderCache.php | 22 +++++---------
 .../src/Plugin/Block/SystemPageMessagesBlock.php   | 34 +++++++++++++++++-----
 2 files changed, 33 insertions(+), 23 deletions(-)

diff --git a/core/modules/system/src/MessagesPostRenderCache.php b/core/modules/system/src/MessagesPostRenderCache.php
index 2cfc912..a8b1a57 100644
--- a/core/modules/system/src/MessagesPostRenderCache.php
+++ b/core/modules/system/src/MessagesPostRenderCache.php
@@ -12,19 +12,16 @@
 
 /**
  * Defines a service for messages post render cache callbacks.
+ *
+ * Note: this is designed to replace all #post_render_cache placeholders for
+ *   messages in a single #post_render_cache callback; hence all placeholders
+ *   must be identical.
+ *
+ * @see \Drupal\system\Plugin\Block\SystemPageMessagesBlock::build()
  */
 class MessagesPostRenderCache {
 
   /**
-   * The messages.
-   *
-   * They are stored in a property so they can appear in multiple blocks.
-   *
-   * @var string[]
-   */
-  protected $messages;
-
-  /**
    * The renderer.
    *
    * @var \Drupal\Core\Render\RendererInterface
@@ -53,15 +50,10 @@ public function __construct(RendererInterface $renderer) {
    *   A renderable array containing the messages.
    */
   public function renderMessages(array $element, array $context) {
-    // Collect the messages.
-    if (!isset($this->messages)) {
-      $this->messages = drupal_get_messages();
-    }
-
     // Render the messages.
     $messages = [
       '#theme' => 'status_messages',
-      '#message_list' => $this->messages,
+      '#message_list' => drupal_get_messages(),
       '#status_headings' => [
         'status' => t('Status message'),
         'error' => t('Error message'),
diff --git a/core/modules/system/src/Plugin/Block/SystemPageMessagesBlock.php b/core/modules/system/src/Plugin/Block/SystemPageMessagesBlock.php
index 7dfc02f..ed494a5 100644
--- a/core/modules/system/src/Plugin/Block/SystemPageMessagesBlock.php
+++ b/core/modules/system/src/Plugin/Block/SystemPageMessagesBlock.php
@@ -7,10 +7,12 @@
 
 namespace Drupal\system\Plugin\Block;
 
+use Drupal\Component\Utility\Crypt;
 use Drupal\Core\Block\BlockBase;
 use Drupal\Core\Config\ConfigFactoryInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Site\Settings;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
@@ -70,15 +72,27 @@ public function defaultConfiguration() {
 
   /**
    * {@inheritdoc}
+   *
+   * Ensure the same token is used for all instances, hence resulting in the
+   * same placeholder for all message blocks. This ensures we can put the
+   * rendered messages in all message blocks in one go.
+   * Also ensure the same context key is used for the #post_render_cache
+   * property, this ensures that if multiple messages blocks exists, their
+   * individual (but identical!) #post_render_cache properties are merged,
+   * ensuring the callback is only invoked once.
+   *
+   * @see \Drupal\system\MessagesPostRenderCache()
    */
   public function build() {
     $callback = 'system.messages_post_render_cache:renderMessages';
-    $context = [];
+    $context = [
+      'token' => Crypt::hmacBase64($this->getPluginId(), Settings::getHashSalt()),
+    ];
     $placeholder = drupal_render_cache_generate_placeholder($callback, $context);
     return [
       '#post_render_cache' => array(
         $callback => array(
-          $context,
+          $this->getPluginId() => $context,
         ),
       ),
       '#markup' => $placeholder,
@@ -92,9 +106,12 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
     $form = parent::buildConfigurationForm($form, $form_state);
 
     // @see ::isCacheable()
-    $form['cache']['#disabled'] = TRUE;
-    $form['cache']['#description'] = $this->t('This block is never cacheable, it is not configurable.');
-    $form['cache']['max_age']['#value'] = 0;
+    $form['cache']['#description'] = $this->t('This block is cacheable forever, it is not configurable.');
+    $form['cache']['max_age']['#value'] = -1;
+    $form['cache']['max_age']['#disabled'] = TRUE;
+    // Don't allow cache contexts to be configured, this block is globally
+    // cacheable.
+    $form['cache']['contexts']['#access'] = FALSE;
 
     return $form;
   }
@@ -103,9 +120,10 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
    * {@inheritdoc}
    */
   public function isCacheable() {
-    // The page messages block is never cacheable, because its contents are
-    // session-specific and caching is useless.
-    return FALSE;
+    // The messages are session-specific and hence aren't cacheable, but the
+    // block itself *is* cacheable because it uses a #post_render_cache callback
+    // and hence the block has a globally cacheable render array.
+    return TRUE;
   }
 
 }