A session is now present on every request

A session object backed by symfony MockArraySessionStorage is added unconditionally to every request in DrupalKernel::preHandle()

As a result, a session is automatically available on every request in the following environments:

1. All tests inheriting from KernelTestBase.
2. All tests using FunctionalTestSetupTrait::initKernel().
3. All drush commands (since bootstrapDrupalFull() calls DrupalKernel::preHandle().
4. All drupal console commands (since boot() calls DrupalKernel::preHandle().

Note: This change affects test code and command line utilities. Availability of the session backed by persistent storage in production code remains unchanged.

Deprecations

Tests in contrib and custom modules might start generating the following deprecation warning:

Pushing requests without a session onto the request_stack is deprecated in drupal 10.3.0 and an error will be thrown from drupal 11.0.0.

This happens when a Request object is constructed and pushed on the request_stack service during a test. The correct way to fix this deprecation is to add a mock session on the request before pushing it on the stack. The following example is from core RouteNoneTest.php:

diff --git a/core/tests/Drupal/KernelTests/Core/RouteProcessor/RouteNoneTest.php b/core/tests/Drupal/KernelTests/Core/RouteProcessor/RouteNoneTest.php
index 21e62d9680..7e35066eeb 100644
--- a/core/tests/Drupal/KernelTests/Core/RouteProcessor/RouteNoneTest.php
+++ b/core/tests/Drupal/KernelTests/Core/RouteProcessor/RouteNoneTest.php
@@ -8,6 +8,8 @@
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\Core\Routing\RouteObjectInterface;
 use Symfony\Component\HttpFoundation\Request;
+use Symfony\Component\HttpFoundation\Session\Session;
+use Symfony\Component\HttpFoundation\Session\Storage\MockArraySessionStorage;
 use Symfony\Component\Routing\Route;
 
 /**
@@ -59,6 +61,7 @@ public function testProcessOutbound() {
     $request = Request::create('/subdir', 'GET', [], [], [], $server);
     $request->attributes->set(RouteObjectInterface::ROUTE_NAME, '<front>');
     $request->attributes->set(RouteObjectInterface::ROUTE_OBJECT, new Route('/'));
+    $request->setSession(new Session(new MockArraySessionStorage()));
 
     $request_stack->push($request);
     $request_context->fromRequest($request);