SmartCache++

smartcache-plus-poc.diff

diff --git a/core/lib/Drupal/Core/Render/Renderer.php b/core/lib/Drupal/Core/Render/Renderer.php
index 69e99a0..8bdf86a 100644
--- a/core/lib/Drupal/Core/Render/Renderer.php
+++ b/core/lib/Drupal/Core/Render/Renderer.php
@@ -184,6 +184,11 @@ protected function doRender(&$elements, $is_root_call = FALSE) {
         $this->bubbleStack();
         return $elements['#markup'];
       }
+
+      // Store the cache ID for later comparison.
+      if ($this->requestStack->getCurrentRequest()->isMethodSafe() && $cid = $this->createCacheID($elements)) {
+        $elements['#cache_old_id'] = $cid;
+      }
     }
 
     // If the default values for this element have not been loaded yet, populate
@@ -493,6 +498,9 @@ protected function cacheGet(array $elements) {
 
     if (!empty($cid) && $cache = $this->cacheFactory->get($bin)->get($cid)) {
       $cached_element = $cache->data;
+      if (isset($cached_element['#cache_try_again'])) {
+        return $this->cacheGet($cached_element);
+      }
       // Return the cached element.
       return $cached_element;
     }
@@ -530,7 +538,17 @@ protected function cacheSet(array &$elements) {
 
     $bin = isset($elements['#cache']['bin']) ? $elements['#cache']['bin'] : 'render';
     $expire = isset($elements['#cache']['expire']) ? $elements['#cache']['expire'] : Cache::PERMANENT;
-    $this->cacheFactory->get($bin)->set($cid, $data, $expire, $data['#cache']['tags']);
+    $cache = $this->cacheFactory->get($bin);
+
+    // This assumes bubbling worked correctly and createCacheID() working on contexts instead of cache keys.
+    if (isset($elements['#cache_old_id']) && $cid != $elements['#cache_old_id']) {
+      $redir_data = [
+        '#cache_try_again' => TRUE,
+        '#cache' => $elements['#cache'],
+      ];
+      $cache->set($elements['#cache_old_id'], $redir_data, $expire, $data['#cache']['tags']);
+    }
+    $cache->set($cid, $data, $expire, $data['#cache']['tags']);
   }
 
   /**