fix: handle extra colons in /proc/self/cgroup entries

[kacpersaw]

Fixes #34

Problem

On some Kubernetes nodes, /proc/self/cgroup contains entries with extra colons in the path:

0::/system.slice/kubepods-burstable-pod72e25f20.slice:cri-containerd:d24f9cc...

currentProcCgroup() used strings.Split(entry, ":") which expected exactly 3 parts. Extra colons produced 4+ parts, causing a parse error that crashed the agent.

Additionally, the error message wrapped a nil err variable, producing %!w(<nil>) in logs.

Fix

• Use strings.SplitN(..., 3) to limit splitting to 3 parts, preserving extra colons in the path field.
• Fix nil error wrapping in the parse error message to show the actual malformed entry.

Tests

Added 3 test cases:

• Extra colons in path
• Extra colons with mixed cgroup hierarchy
• Malformed entry with too few fields

[DanielleMaywood]

Have we managed to create a reproduction of this issue? My understanding of the source is that the path we extract isn't the full path of the cgroup (although I could be completely misunderstanding, which is why it'd be nice to get a reproduction).

https://github.com/containerd/containerd/blob/cf600abecc27200a3c0e1415cd1f6c325eb05edf/pkg/cri/server/helpers_linux.go#L67-L75

// getCgroupsPath generates container cgroups path.
func getCgroupsPath(cgroupsParent, id string) string {
	base := path.Base(cgroupsParent)
	if strings.HasSuffix(base, ".slice") {
		// For a.slice/b.slice/c.slice, base is c.slice.
		// runc systemd cgroup path format is "slice:prefix:name".
		return strings.Join([]string{base, "cri-containerd", id}, ":")
	}
	return filepath.Join(cgroupsParent, id)
}

[DanielleMaywood]

Known misconfiguration, we may as well handle it cleanly, approving containerd/containerd#4900

[DanielleMaywood]

Once merged, feel free to create a patch release, and then bump the version used by coder/coder