Simplify site-specific service overrides

1. +++ b/core/lib/Drupal/Core/DrupalKernel.php
   @@ -138,6 +131,13 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
      /**
   +   * The list of the classnames of the service providers in this kernel.
   +   *
   +   * @var array
   +   */
   +  protected $serviceProviderClasses;
   +
   

   This is no longer true as you key by app as well. Can you elobarate here about the difference between app and site, please?

2. +++ b/core/lib/Drupal/Core/DrupalKernel.php
   @@ -226,30 +228,30 @@ public function discoverServiceProviders() {
   +    if ($exists = file_exists($site_services_yml = conf_path() . '/services.yml')) {
   +      $this->serviceYamls['site'][] = $site_services_yml;
        }
   

   Do we hae a follow up to include also one for enviroment specific overrides? Can we get some documentation for the overrides into settings.php?

3. +++ b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
   @@ -0,0 +1,50 @@
   +
   +  public static function getInfo() {
   +    return array(
   ...
   +  function testServicesYml() {
   

   nitpicks: @inheritdoc + public

Yeah that patch is looking fine.

Committed/pushed to 8.x, thanks!

How about an example.services.yml that folks can use to learn syntax and hint at the possibilities?

This broke CLI use of run-tests.sh see #2266547: TestKernel is not adding its service provider correctly.

I no longer understand core process.

A) how did a new feature made into core at this stage?

B) how did this made in when these particular site overrides don't work with tests despite https://drupal.org/node/2229011#comment-8645239 predates the last patch (by a month) written by the author of this patch -- that comment says, if you override in settings.php then the overrides from site specific settings.php should persist into the tests. But, apparently not YAML ones? What?

C) How could this possibly get in before #2183323: Replace $GLOBALS['conf']['container_service_providers'] in DrupalKernel with Settings ?

Reverted.

A) how did a new feature made into core at this stage?

It's significantly harder to override implementations in settings.php than it was in 7.x, so simplifying that now isn't really a feature. Also I've been very clear on several occasions that I think 'feature freeze' is a pointless and counterproductive approach to the release cycle (although that doesn't mean trying to land massive new modules all the time either).

C) How could this possibly get in before #2183323: Replace $GLOBALS['conf']['container_service_providers'] in DrupalKernel with Settings

I didn't think there was much of a conflict. But let's postpone it.

Can't revert that when #2266547: TestKernel is not adding its service provider correctly. has been committed already. Not enough tea yet today.

:( So I guess then B) at least needs to be fixed: the override yaml needs to be copied over for testing. tstoeckler says that could be done in #2229011: Tests are no longer modifiable so let's just do that.