Restore Redis DSN support for session storage

[melroy89]

Summary

Reuse Symfony's existing DSN-backed Redis provider for session storage instead of creating a second connection from mandatory REDIS_HOST and REDIS_PORT variables.

This restores support for all existing REDIS_DNS connection forms, including Unix sockets, while keeping TCP, authentication, and Docker configurations on the same canonical setting.

Additional information

PR #2127 added Redis-backed session storage but created a separate Redis service that requires REDIS_HOST and REDIS_PORT. That made previously valid Unix-socket configurations fail during environment-variable resolution, even though REDIS_DNS remained configured as Symfony's default Redis provider.

RedisSessionHandler now receives cache.default_redis_provider, which Symfony builds through its Redis DSN connection factory. The redundant host and port variables are removed from the environment templates.

Related issues

Follow-up to #2127.

[BentiGorlich]

If it works, this is awesome. I didn't find a solution to stay on the dsn variable...

[melroy89]

If it works, this is awesome. I didn't find a solution to stay on the dsn variable...

Yes indeed. So we basically reuse the default redis provider variables documented here as well https://symfony.com/doc/current/cache.html#configuring-cache-with-frameworkbundle

[melroy89]

And yes this should work again for both TCP DNS and socket DNS. Since the default_redis_provider resolves from %env(REDIS_DNS)%. I tested it already.

see:

mbin/config/packages/cache.yaml

Line 12 in 037b9b2

default_redis_provider: '%env(REDIS_DNS)%'

Symfony’s framework config then uses that provider here: vendor/symfony/framework-bundle/Resources/config/cache.php:139 (see locally on disk if you have executed composer install):

  'provider' => 'cache.default_redis_provider',

Please approve.