Enable use of Rails' new cookie encryption

Old cookies are still valid and are automatically
upgraded by Rails

changelogs/unreleased/63942-remove-config-action_dispatch-use_authenticated_cookie_encryption-configuration.yml

+---
+title: Enable authenticated cookie encryption
+merge_request: 31463
+author:
+type: other

config/application.rb

@@ -293,10 +293,5 @@ module Gitlab
       Gitlab::Routing.add_helpers(project_url_helpers)
       Gitlab::Routing.add_helpers(MilestonesRoutingHelper)
     end
-
-    # This makes generated cookies to be compatible with Rails 5.1 and older
-    # We can remove this when we're confident that there are no issues with the Rails 5.2 upgrade
-    # and we won't need to rollback to older versions
-    config.action_dispatch.use_authenticated_cookie_encryption = false
   end
 end

doc/update/README.md

@@ -135,6 +135,30 @@ If you need to downgrade your Enterprise Edition installation back to Community
 Edition, you can follow [this guide][ee-ce] to make the process as smooth as
 possible.
 
+## Version specific upgrading instructions
+
+### 12.2.0
+
+In 12.2.0, we enabled Rails' authenticated cookie encryption. Old sessions are
+automatically upgraded.
+
+However, session cookie downgrades are not supported. So after upgrading to 12.2.0,
+any downgrades would result to all sessions being invalidated and users are logged out.
+
+### 12.0.0
+
+In 12.0.0 we made various database related changes. These changes require that
+users first upgrade to the latest 11.11 patch release. Once upgraded to 11.11.x,
+users can upgrade to 12.x. Failure to do so may result in database migrations
+not being applied, which could lead to application errors.
+
+Example 1: you are currently using GitLab 11.11.3, which is the latest patch
+release for 11.11.x. You can upgrade as usual to 12.0.0, 12.1.0, etc.
+
+Example 2: you are currently using a version of GitLab 10.x. To upgrade, first
+upgrade to 11.11.3. Once upgraded to 11.11.3 you can safely upgrade to 12.0.0
+or future versions.
+
 ## Miscellaneous
 
 - [MySQL to PostgreSQL](mysql_to_postgresql.md) guides you through migrating

doc/update/upgrading_from_source.md

@@ -378,20 +378,6 @@ Example:
 Additional instructions here.
 -->
 
-### 12.0.0
-
-In 12.0.0 we made various database related changes. These changes require that
-users first upgrade to the latest 11.11 patch release. Once upgraded to 11.11.x,
-users can upgrade to 12.x. Failure to do so may result in database migrations
-not being applied, which could lead to application errors.
-
-Example 1: you are currently using GitLab 11.11.3, which is the latest patch
-release for 11.11.x. You can upgrade as usual to 12.0.0, 12.1.0, etc.
-
-Example 2: you are currently using a version of GitLab 10.x. To upgrade, first
-upgrade to 11.11.3. Once upgraded to 11.11.3 you can safely upgrade to 12.0.0
-or future versions.
-
 ## Things went south? Revert to previous version
 
 ### 1. Revert the code to the previous version