Fixups to scheduler/priority settings

[kolyshkin]

These commits amend commit bfbd030 (#3783) and 770728e (#4025). Should not result in any visible change, except:

• invalid IOPriority.Class value will result in an error much earlier;
• runc exec changes IO priority later, just before exec;
• configs.IOPrioClassMapping map is removed from the public API.

Should be relatively easy to review.

[kolyshkin]

@opencontainers/runc-maintainers @AkihiroSuda @cyphar @thaJeztah @rata @lifubang can we please review/merge this? This will unblock my further work (adding CPU affinity).

[lifubang]

LGTM
Left two unimportant comments.

[lifubang]

How about change the param type to *configs.Config like in setupScheduler?

[kolyshkin]

Makes sense (although in case of setupScheduler we pass the whole config because we also need to access config.Cgroups).

I've also changed its name, as other similar functions have setup not set prefix.

The change:

 
-func setIOPriority(ioprio *configs.IOPriority) error {
+func setupIOPriority(config *configs.Config) error {
        const ioprioWhoPgrp = 1
 
+       ioprio := config.IOPriority
        if ioprio == nil {
                return nil
        }

[lifubang]

I don't like this comment, but this is my personal preference. Maybe should change to // should return nil or move to an new function:

func checkIOPriorityClass(specs.IOPriorityClass) bool {
    switch config.IOPriority.Class {
	case specs.IOPRIO_CLASS_RT, specs.IOPRIO_CLASS_BE, specs.IOPRIO_CLASS_IDLE:
            return true
    }
    return false
}

[kolyshkin]

Changed the comment to // Valid class, do nothing., and the whole switch to

          switch class := config.IOPriority.Class; class {                    
          case specs.IOPRIO_CLASS_RT, specs.IOPRIO_CLASS_BE, specs.IOPRIO_CLASS_IDLE:
                  // Valid class, do nothing.
          default:
                  return fmt.Errorf("invalid ioPriority.Class: %q", class)                                             
          }