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

Only create a systemd sub-cgroup when the run.oci.systemd.subgroup annotation is explicitly set, removing the previous default of “container” on cgroup v2.

Bug Fixes:

• Address issue where a default sub-cgroup was created unexpectedly on cgroup v2.

Enhancements:

• Remove automatic creation of a “container” sub-cgroup on cgroup v2 by returning NULL when the annotation is unset.
• Update documentation and man pages to clarify that no sub-cgroup is created unless the SUBGROUP value is explicitly provided.

[sourcery-ai]

Reviewer's Guide

Change default behavior to no longer create a systemd sub-cgroup unless the run.oci.systemd.subgroup annotation is explicitly set, updating both code and documentation accordingly.

Class diagram for find_systemd_subgroup function change

classDiagram
    class find_systemd_subgroup {
        +string_map *annotations
        +returns: const char *
        - Old: returns "container" if annotation not set
        - New: returns NULL if annotation not set
    }

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Flow diagram for systemd sub-cgroup creation logic

flowchart TD
    A[Container start] --> B{Is run.oci.systemd.subgroup annotation set?}
    B -- No --> C[No sub-cgroup is created]
    B -- Yes, non-empty --> D[Create sub-cgroup with specified name]
    B -- Yes, empty --> C

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File-Level Changes

Change	Details	Files
Return NULL instead of "container" when no explicit subgroup is configured	Modify find_systemd_subgroup to return NULL when annotation is unset	src/libcrun/cgroup-systemd.c
Update man page and Markdown docs to reflect new default behavior	Reword option description to describe no default sub-cgroup Remove mentions of defaulting to "container" Explain behavior when annotation is empty or unset	crun.1 crun.1.md

Possibly linked issues

• drop default sub-cgroup with the systemd driver #1766: The PR changes the default systemd sub-cgroup behavior to not create one, directly addressing the issue's request.

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[giuseppe]

@haircommander FYI

[sourcery-ai]

Hey @giuseppe - I've reviewed your changes and they look great!

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