tar: fix Go 1.10 breakage

[Kubuxu]

Resolves #4781 #4785

Thanks to @schomatis for tracing it.

[Kubuxu]

Best would be to merge it before 0.4.14 is released as Go 1.10 fixes performance issues on MacOS.

[whyrusleeping]

I wonder how many other places we're making this mistake.

[whyrusleeping]

I don't know if i'm comfortable using go1.10 until we test on it a bit to make sure nothing else weird is happening.

[Kubuxu]

We have problem with Go 1.10 because they refactored tar reader. I don't think Go 1.10 has any more failures of this kind.

[kevina]

This seams to change the behavior so they will always be a single Read now, instead of reading contentiously until there is an EOF. Also from https://golang.org/pkg/io/#Reader

Callers should treat a return of 0 and nil as indicating that nothing happened; in particular it does not indicate EOF

Shouldn't this be:

switch err {
case nil:
  // continue
case io.EOF:
  return nil
default:
  return err
}

[kevina]

I would actually write this as

if err != nil {
  if err == EOF {
    return nil
  }
  return err
}

I think it makes it clearer what is going on, but either way is fine.

[Kubuxu]

Yes, that was my mistake. For some reason it sill works correctly :P

[kevina]

@Kubuxu do you plan on also updating circleci and travis to test with go 1.10? For now I would recommend we test with both go 1.9 and 1.10 but I can understand if that will take too long.

[Kubuxu]

In my opinion we can leave them be for now on Go 1.9.
We might just want to set up Jenkins for Go 1.9 and 1.10 for the time we are supporting both versions.

[whyrusleeping]

Alright, since the change that broke this was specific to the tar code, i'm okay with us shipping an rc2 with this fix, and building the rc2 release with go1.10.