cgroup: do not create a sub-cgroup by default

[giuseppe]

The run.oci.systemd.subgroup annotation previously defaulted to creating a "container" sub-cgroup on cgroup v2 systems if the annotation was not specified. This change alters the behavior so that no sub-cgroup is created by default.

A sub-cgroup will only be created if the
run.oci.systemd.subgroup annotation is explicitly provided with a non-empty value.

Closes: #1766

Summary by Sourcery

Change default systemd sub-cgroup behavior to only create a sub-cgroup when the run.oci.systemd.subgroup annotation is explicitly set with a non-empty value.

Enhancements:

• Make run.oci.systemd.subgroup default to no sub-cgroup instead of "container" and return NULL in code when unset.

Documentation:

• Update man pages and Markdown docs to reflect that no sub-cgroup is created by default unless explicitly configured.

[sourcery-ai]

Reviewer's Guide

The PR changes the default behavior of the run.oci.systemd.subgroup annotation so that no sub-cgroup is created unless explicitly set, updating documentation (man page and Markdown) to reflect this and modifying the runtime logic to return NULL instead of a default “container” on cgroup v2.

File-Level Changes

Change	Details	Files
Revise documentation to describe new default of no sub-cgroup	Reword man page to explain that SUBGROUP must be set to create a cgroup Remove references to an implicit “container” default Adjust examples to show behavior with empty or missing SUBGROUP Apply identical updates in the Markdown guide	crun.1 crun.1.md
Alter cgroup-systemd logic to drop implicit “container” default	Change find_systemd_subgroup to return NULL when annotation is absent Remove hardcoded “container” return path	src/libcrun/cgroup-systemd.c

Possibly linked issues

• drop default sub-cgroup with the systemd driver #1766: PR removes default systemd sub-cgroup creation as requested.
• drop default sub-cgroup with the systemd driver #1766: The PR changes the default systemd cgroup creation behavior relevant to the issue.

---

Tips and commands

Interacting with Sourcery

• Trigger a new review: Comment @sourcery-ai review on the pull request.
• Continue discussions: Reply directly to Sourcery's review comments.
• Generate a GitHub issue from a review comment: Ask Sourcery to create an
  issue from a review comment by replying to it. You can also reply to a
  review comment with @sourcery-ai issue to create an issue from it.
• Generate a pull request title: Write @sourcery-ai anywhere in the pull
  request title to generate a title at any time. You can also comment
  @sourcery-ai title on the pull request to (re-)generate the title at any time.
• Generate a pull request summary: Write @sourcery-ai summary anywhere in
  the pull request body to generate a PR summary at any time exactly where you
  want it. You can also comment @sourcery-ai summary on the pull request to
  (re-)generate the summary at any time.
• Generate reviewer's guide: Comment @sourcery-ai guide on the pull
  request to (re-)generate the reviewer's guide at any time.
• Resolve all Sourcery comments: Comment @sourcery-ai resolve on the
  pull request to resolve all Sourcery comments. Useful if you've already
  addressed all the comments and don't want to see them anymore.
• Dismiss all Sourcery reviews: Comment @sourcery-ai dismiss on the pull
  request to dismiss all existing Sourcery reviews. Especially useful if you
  want to start fresh with a new review - don't forget to comment
  @sourcery-ai review to trigger a new review!

Customizing Your Experience

Access your dashboard to:

• Enable or disable review features such as the Sourcery-generated pull request
  summary, the reviewer's guide, and others.
• Change the review language.
• Add, remove or edit custom review instructions.
• Adjust other review settings.

Getting Help

• Contact our support team for questions or feedback.
• Visit our documentation for detailed guides and information.
• Keep in touch with the Sourcery team by following us on X/Twitter, LinkedIn or GitHub.

[giuseppe]

@haircommander FYI