[Bug]: `$this->session->get('user_id')` returns `null` since PR 62241 (in dev environment)

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I'm using https://github.com/nextcloud/nextcloud-docker-dev for local development of a Nextcloud app. Using the up to date nextcloud/server master branch within the dev repo, a call to $this->session->get('user_id') returns null. I couldn't find the exact change that causes this but I narrowed it down to the PR https://github.com/nextcloud/server/pull/62241. Before that PR was merged into master, $this->session->get('user_id') would return the session's user ID.

Steps to reproduce

1. Setup https://github.com/nextcloud/nextcloud-docker-dev
2. Configure a debugger
3. Set a breakpoint in a file using $this->session->get('user_id') (I used workspace/server/lib/private/DateTimeZone.php line 30 for this)
4. Start the dev environment
5. Open the local Nextcloud instance with your browser
6. Login as admin
7. Enable debugging in the docker container
8. Start debugging
9. Refresh the browser page
10. Observe $this->session->get('user_id') returning null

Expected behavior

$this->session->get('user_id') should return the session's user ID, like it did before PR https://github.com/nextcloud/server/pull/62241

Nextcloud Server version

35 (master)

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.3

Web server

Apache (supported)

Database engine version

MySQL

Is this bug present after an update or on a fresh install?

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

Start by reviewing PR 62241 and the session access observed at workspace/server/lib/private/DateTimeZone.php line 30. Reproduce the issue with nextcloud-docker-dev, a debugger, PHP 8.3, and a logged-in admin, then trace why the session user ID is null. Done means $this->session->get('user_id') returns the logged-in user's ID again in the development environment.