Introduce 'update' environment for DrupalKernel

+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -856,6 +891,11 @@ protected function getKernelParameters() {
   protected function initializeContainer() {
+    if ($this->environment === 'update') {
+      // Always force a container rebuild, in order to be able to override some
+      // services, see \Drupal\Core\Update\UpdateServiceProvider.
+      $this->containerNeedsRebuild = TRUE;
+    }
     $this->containerNeedsDumping = FALSE;
     $session_started = FALSE;
     $all_messages = [];
@@ -1338,6 +1378,11 @@ protected function getContainerBuilder() {

@@ -1338,6 +1378,11 @@ protected function getContainerBuilder() {
    *   TRUE if the container was successfully cached.
    */
   protected function cacheDrupalContainer(array $container_definition) {
+    if ($this->environment === 'update') {
+      // Don't save this particular container to cache, so it does not leak into
+      // the main site at all.
+      return FALSE;
+    }
     $saved = TRUE;
     try {

I'm not sure this sort of change makes code more readable or more maintainable.

Currently, UpdateKernel inherits from DrupalKernel:

In cacheDrupalContainer(), UpdateKernel overrides the method completely.

In initializeContainer(), UpdateKernel sets the property before calling the parent.

To me, these seems like things that OO inheritance is good for.

The Needs Review Queue Bot tested this issue. It either no longer applies to Drupal core, or fails the Drupal core commit checks. Therefore, this issue status is now "Needs work".

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