Kustomize edit panics when Kustomization file begins with document separator

[avisiedo]

Describe the bug

I get a panic when running kustomize edit set namespace mynamespace.

Files that can reproduce the issue

kustomization.yaml

---
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: freeipa

Expected output

No panic message and the namespace field updated to mynamespace.

Actual output

panic: runtime error: index out of range [-1]

goroutine 1 [running]:
sigs.k8s.io/kustomize/kustomize/v4/commands/internal/kustfile.(*kustomizationFile).parseCommentedFields(0xc0002e9140, 0xc000030c60, 0x5a, 0x60, 0x0, 0x0)
        /workspace/myClone/kustomize/commands/internal/kustfile/kustomizationfile.go:216 +0x7c5
sigs.k8s.io/kustomize/kustomize/v4/commands/internal/kustfile.(*kustomizationFile).Read(0xc0002e9140, 0xf66538, 0xc0002e9140, 0x0)
        /workspace/myClone/kustomize/commands/internal/kustfile/kustomizationfile.go:174 +0x14a
sigs.k8s.io/kustomize/kustomize/v4/commands/edit/set.(*setNamespaceOptions).RunSetNamespace(0xc000275940, 0xbcb760, 0xf66538, 0x1, 0x0)
        /workspace/myClone/kustomize/commands/edit/set/setnamespace.go:66 +0x65
sigs.k8s.io/kustomize/kustomize/v4/commands/edit/set.newCmdSetNamespace.func1(0xc000315080, 0xc000320420, 0x1, 0x1, 0x0, 0x0)
        /workspace/myClone/kustomize/commands/edit/set/setnamespace.go:41 +0xd0
github.com/spf13/cobra.(*Command).execute(0xc000315080, 0xc0003203e0, 0x1, 0x1, 0xc000315080, 0xc0003203e0)
        /go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:842 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0xc000308000, 0x0, 0xa12400, 0xc000030118)
        /go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
        /go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887
main.main()
        /workspace/myClone/kustomize/main.go:14 +0x2a

Kustomize version

kustomize version
{Version:kustomize/v4.2.0 GitCommit:d53a2ad45d04b0264bcee9e19879437d851cb778 BuildDate:2021-06-30T22:49:26Z GoOs:linux GoArch:amd64}

Platform

Linux

Additional context

Installed by:

curl -s "https://raw.githubusercontent.com/kubernetes-sigs/kustomize/master/hack/install_kustomize.sh"  | bash

[avisiedo]

I have just realized I had a blank line at the beginning of the file. Removing that blank line into the kustomization.yaml file, make the command run as expected.

[KnVerey]

/assign @KnVerey
/triage accepted
/retitle Kustomize panics when Kustomization file begins with document separator

[KnVerey]

/assign @m-Bilal

[k8s-ci-robot]

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In response to this:

/assign @m-Bilal

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[m-Bilal]

/assign

[m-Bilal]

@KnVerey the panic happens because in this line

kustomize/kustomize/commands/internal/kustfile/kustomizationfile.go

Line 216 in 402f6ca

mf.originalFields[len(mf.originalFields)-1].appendComment(squash(comments))

len(mf.originalFields) returns 0.
The reason for this is when the file begins with a blank line (or a comment) and a document separator immediately follows it, len(comments) is > 0 but len(mf.originalFields) is still 0, and since document separator does not match any field, it goes into the following else if condition

kustomize/kustomize/commands/internal/kustfile/kustomizationfile.go

Line 215 in 402f6ca

} else if len(comments) > 0 {

So, the kustomization files

---
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: freeipa

and

# This is a comment
---
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: freeipa

result in exactly same errors.

I was able to fix this by changing

kustomize/kustomize/commands/internal/kustfile/kustomizationfile.go

Line 215 in 402f6ca

} else if len(comments) > 0 {

to else if len(comments) > 0 && len(mf.originalFields) > 0. Then kustomize edit set namespace mynamespace simply ignores the document separator and results are as follows:

---
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: freeipa

becomes:

apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: mynamespace

(the blank line is still there), and

# This is a comment
---
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: freeipa

becomes

# This is a comment
apiVersion: kustomize.config.k8s.io/v1beta1
kind: Kustomization

namespace: mynamespace

I'll raise a PR with this fix, if there's anything I missed or if it should be a different fix, please let me know.