[Bug]: Caching in background jobs (almost) worthless when instance uses APCu for local caching

⚠️ 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

APCu cache memory is volatile. For web servers it lives as long as the webserver process. For CLI it lives as long as the cache.

We recommend APCu for local caching because it's faster than Redis and the TCP roundtrip.

Nextcloud uses a lot of background (cron) processes. When set up for performance, background jobs are triggered by a system cron. Within this cron, the APCu cache does work. Between two cron executions all cached values are lost. This severely limits the usefulness of caching.

Steps to reproduce

1. Set up an instance with APCu as local cache
2. Cache data in a background job
3. Build on the cached value later on

Expected behavior

Cached value is there

Installation method

None

Nextcloud Server version

27

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

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

None

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

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

No source file, test, or entry point is named. Start by reproducing the APCu behavior across separate system-cron executions and trace the local-cache configuration and background-job cache access; done means values cached by one execution remain usable by a later execution.