[meta] Explore using more service closures to break deep dependency chains and load fewer services

As mentioned in #3540398: Remove Url generator from router class, I wrote a little patch to write out all essentially all Container::createService() with their initiator. Initiator here means the first createService() call when we're not already within a createService() chain. And when leaving that initial createService() all, I write out that list as a single line into a txt file. The result is a list of all dependency chains. Any direct call to get() that is a new server, including such as plugin/storage/controller "DI" is seen as a standalone thing. As well as service locators and similar factories that initiate services lazily.

A separate file is written for each request with timestamp and path, which allow to compare them. It's a single line, but it's actually a tree of services, so not every -> actually represents a chained call. It would be possible to visualize this better, but it might also make it harder to diff. That wasn't a focus yet.

The idea is to track when a given service is used the first time. And the goal of this issue would be to find meaningful and worthwhile ways to push them "back", non-html (such as json api, but also things like autocomplete callbacks and assets) and cached pages are likely where we'll see the most benefit, as "back" might mean that they wouldn't be needed at all. The script can be run before and after a change to visual the change.

I excluded private services because those would differ between cache clears and likely aren't so meaningful here.

That can either be done with a regular diff, or what I prefer, within a fresh git repository with a word-based, as seen in the screenshot in that other issue.

The changes as diff:

diff --git a/core/lib/Drupal/Component/DependencyInjection/Container.php b/core/lib/Drupal/Component/DependencyInjection/Container.php
index dc68c9c8a24..5f4cc3d6521 100644
--- a/core/lib/Drupal/Component/DependencyInjection/Container.php
+++ b/core/lib/Drupal/Component/DependencyInjection/Container.php
@@ -46,6 +46,8 @@
  */
 class Container implements ContainerInterface, ResetInterface {

+  protected array $currentDependencyChain = [];
+
   /**
    * The parameters of the container.
    *
@@ -225,6 +227,15 @@ public function reset(): void {
    *   and cannot be instantiated.
    */
   protected function createService(array $definition, $id) {
+
+    $initial = FALSE;
+    if (!str_starts_with($id, 'private__')) {
+      if (empty($this->currentDependencyChain)) {
+        $initial = TRUE;
+      }
+      $this->currentDependencyChain[] = $id;
+    }
+
     if (isset($definition['synthetic']) && $definition['synthetic'] === TRUE) {
       throw new RuntimeException(sprintf('You have requested a synthetic service ("%s"). The service container does not know how to construct this service. The service will need to be set before it is first used.', $id));
     }
@@ -299,6 +310,11 @@ protected function createService(array $definition, $id) {
       call_user_func($callable, $service);
     }

+    if ($initial && defined('DEPENDENCY_DEBUG_FILE')) {
+      file_put_contents(DEPENDENCY_DEBUG_FILE, implode(' -> ', $this->currentDependencyChain) . "\n", FILE_APPEND);
+      $this->currentDependencyChain = [];
+    }
+
     return $service;
   }

diff --git a/index.php b/index.php
index 750dc282dc2..d4fe93a2b0a 100644
--- a/index.php
+++ b/index.php
@@ -16,6 +16,9 @@
 $kernel = new DrupalKernel('prod', $autoloader);

 $request = Request::createFromGlobals();
+define('DEPENDENCY_DEBUG_FILE', __DIR__ . '/dependencies/' . $request->server->get('REQUEST_TIME') . '-' . str_replace('/', '-', $request->getPathInfo()) .  '.txt');
+touch(DEPENDENCY_DEBUG_FILE);
+
 $response = $kernel->handle($request);
 $response->send();

I attached the current state on head as a text file, it's a bit much as a snippet and wrapping makes it hard to read.

In some cases, we'll want to specifically test dynamic page or even page cache hits, I'll do some testing with this in the middleware issue with those scenarios.

Added #3541649: Use service closures in options_request_listener/maintenance_mode_subscriber event subscribers

Looked into the session_manager/database topic, but it's way more complex than I thought for a real benefit on dynamic page cache hits: #3541705: Lazy inject database into varous services