Issue #3037823 by alexpott, Berdir, amateescu: The system.theme.data key...

Issue #3037823 by alexpott, Berdir, amateescu: The system.theme.data key remains corrupted in state causing performance issues and is not used in >= 8.7

core/lib/Drupal/Core/Update/UpdateKernel.php

@@ -236,6 +236,8 @@ public static function fixSerializedExtensionObjects(ContainerInterface $contain
       foreach ($container->getParameter('cache_bins') as $service_id => $bin) {
         $container->get($service_id)->deleteAll();
       }
+      // The system.theme.data key is no longer used in Drupal 8.7.x.
+      $container->get('state')->delete('system.theme.data');
       // Also rebuild themes because it uses state as cache.
       $container->get('theme_handler')->refreshInfo();
     }

core/modules/system/system.install

@@ -2239,3 +2239,11 @@ function system_update_8601() {
 
   return t('The %install_profile install profile has been added to the installed module list.', ['%install_profile' => $install_profile]);
 }
+
+/**
+ * Remove the unused 'system.theme.data' from state.
+ */
+function system_update_8701() {
+  // The system.theme.data key is no longer used in Drupal 8.7.x.
+  \Drupal::state()->delete('system.theme.data');
+}

core/modules/system/tests/src/Functional/Update/WarmCacheUpdateFrom8dot6Test.php

@@ -25,19 +25,30 @@ protected function setDatabaseDumpFiles() {
   }
 
   /**
-   * Tests that the content and configuration were properly updated.
+   * Tests \Drupal\Core\Update\UpdateKernel::fixSerializedExtensionObjects().
    */
   public function testUpdatedSite() {
+    // The state API cannot be used because the value is corrupted.
+    $row_count = Database::getConnection()->select('key_value')
+      ->condition('collection', 'state')
+      ->condition('name', 'system.theme.data')
+      ->countQuery()
+      ->execute()
+      ->fetchField();
+    $this->assertSame('1', $row_count, 'The system.theme.data key exists in state.');
     $this->runUpdates();
     $this->drupalGet('');
 
-    $this->resetAll();
     // Ensure that drupal-8.test-config-init.php has run correctly.
     $this->assertSame('test_mail_collector', $this->config('system.mail')->get('interface.default'));
     $this->assertSame('verbose', $this->config('system.logging')->get('error_level'));
     $this->assertSame(FALSE, $this->config('system.performance')->get('css.preprocess'));
     $this->assertSame(FALSE, $this->config('system.performance')->get('js.preprocess'));
     $this->assertSame('Australia/Sydney', $this->config('system.date')->get('timezone.default'));
+
+    // Ensure \Drupal\Core\Update\UpdateKernel::fixSerializedExtensionObjects()
+    // has removed the corrupted state key.
+    $this->assertNull(\Drupal::state()->get('system.theme.data'), 'The system.theme.data key does not exist in state.');
   }
 
   /**