Add support for just joining in apply using cgroup paths

[mrunalp]

This is for supporting joining cgroups by container id in docker.
See moby/moby#19244

Signed-off-by: Mrunal Patel mrunalp@gmail.com

[mrunalp]

@LK4D4 @crosbymichael PTAL

[LK4D4]

LGTM

[hqhq]

Should we validate these paths before assigning to m.Paths? I know they must be valid if they're from Docker, but we might need to support this for runc config file.

[mrunalp]

EnterPid enters the cgroups only if the path exists and that is current behavior for exec. I think it makes sense to have the same behavior here as well.

[hqhq]

Sorry didn't make it clearer, I meant maybe we should do something like (This is what we did in Apply()):

for name, path := range c.Paths {
        p, err := d.path(name)
        if err != nil {
                if cgroups.IsNotFound(err) {
                        continue
                }
                return err
        }
        paths[name] = path
}
m.Paths = paths

return cgroups.EnterPid(m.Paths, pid)

Doing this because m.Path will be used by GetPaths, GetStats, etc. It'll be easier for them if it holds only valid paths.

[mrunalp]

@hqhq Updated.

[hqhq]

s/represen/represents

[hqhq]

A minor tip, otherwise LGTM, @LK4D4 needs a second look?

[mrunalp]

@hqhq Updated.

[hqhq]

LGTM