Keep empty map in kustomize output

[Shell32-Natsu]

#2734
kptdev/kpt#913

[k8s-ci-robot]

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[k8s-ci-robot]

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[Shell32-Natsu]

/assign @monopole

[monopole]

/ok-to-test

[monopole]

So this PR also retains scalar nulls?

[Shell32-Natsu]

This null is in input. Because {} is not considered as empty so only null can be used as empty in input.

[monopole]

looks good, but i think the API is getting odd

[monopole]

It now seems we have:

IsMissingOrNull(n) => n == nil || n.YNode() == nil || n.YNode().Tag == NullNodeTag
        IsEmpty(n) => IsMissingOrNull(n)
         IsNull(n) => n != nil && n.YNode() != nil && n.YNode().Tag == NullNodeTag
     IsEmptyMap(n) => IsEmpty(n) ||
                          (n.YNode().Kind == yaml.MappingNode && len(n.YNode().Content) == 0)

This feels random. And we have this strange contest between
Kind and Tag, which i'm not sure how to resolve yet.

We've not hit 1.0 yet, so we can change the API.

How about we try:

  IsNil(n)        => n == nil || n.YNode() == nil
  IsEmptyDoc(n)   => !IsNodeNil(n) && n.YNode().Tag == NodeTagNull
  IsEmptyMap(n)   => !IsNodeNil(n) &&
                        n.YNode().Kind == yaml.MappingNode &&
                        len(n.YNode().Content) == 0

with this, the tags are hidden, and we start a pattern where
IsEmptyFoo means the object is non-nil with a defined type -
so empty docs, lists and maps are different things.

Translation:

• IsNull becomes IsEmptyDoc
• IsEmptyand IsMissingOrNull are written as IsNil || IsEmptyDoc

The compiler can optimize away redundancy.

So L54 in kyaml/yaml/walk/associative_sequence.go which currently reads

if yaml.IsEmpty(val) || yaml.IsEmptyMap(val) {

becomes

if yaml.IsNil(val) || yaml.IsEmptyDoc(val) || yaml.IsEmptyMap(val) {

[Shell32-Natsu]

That sounds good. But because these methods are fundamental and widely used so if we change them, we need to update a lot of files.

[Shell32-Natsu]

What about create another issue about this refactoring?

[monopole]

ack

/lgtm

Fixes #2734