CSRF token seed and possibly other session data lost when set after a session regenerate

This issue will be fixed by the patch at #2228393-62: Decouple session from cookie based user authentication

Re-open to relieve #2228393: Decouple session from cookie based user authentication. Patch there corrects to much issues at once. Let's solve this here.

Fantastic work so far!

+++ b/core/modules/system/src/Tests/Session/SessionTest.php
@@ -100,6 +100,10 @@ function testDataPersistence() {
+    // Test property added to session object form hook_user_login().
+    $this->drupalGet('session-test/get-from-session-object');
+    $this->assertText('foobar', 'Session data is saved in Session() object.', 'Session');
+

I think testDataPersistence() is not quite the right place for this assertions. One option is to find a place in testSessionSaveRegenerate(), thought that is already quite convoluted. Probably a cleaner and lower risk option would be to add a specific test method which simply performs a login and then checks whether the value added from within hook_user_login is present.

Perhaps something like this:

/**
 * Tests \Drupal\Core\Session\SessionManager::regenerate().
 */
public function testSessionPersistenceOnLogin() {
  [...]
}

Changed as per #5

Great. This is RTBC if testbot if testbot permits it.

This issue addresses a critical bug and is allowed per https://www.drupal.org/core/beta-changes. Committed abf2467 and pushed to 8.0.x. Thanks!

diff --git a/core/modules/system/tests/modules/session_test/src/Controller/SessionTestController.php b/core/modules/system/tests/modules/session_test/src/Controller/SessionTestController.php
index 2db6f21..4437743 100644
--- a/core/modules/system/tests/modules/session_test/src/Controller/SessionTestController.php
+++ b/core/modules/system/tests/modules/session_test/src/Controller/SessionTestController.php
@@ -39,8 +39,8 @@ public function get() {
   public function getFromSessionObject() {
     $value = \Drupal::request()->getSession()->get("session_test_key");
     return empty($value)
-    ? []
-    : ['#markup' => $this->t('The current value of the stored session variable is: %val', array('%val' => $value))];
+      ? []
+      : ['#markup' => $this->t('The current value of the stored session variable is: %val', array('%val' => $value))];
   }
 
   /**

Fixed indentation on commit.