apimachinery's unstructured converter panics if the destination struct contains private fields

[hugoShaka]

What happened?

Using the DefaultUnstructuredConverter with a destination struct containing a non-exported field throws a panic.

panic: reflect: reflect.Value.Set using value obtained using unexported field

goroutine 1 [running]:
reflect.flag.mustBeAssignableSlow(0x140000337a8?)
        /usr/local/go/src/reflect/value.go:269 +0xb4
reflect.flag.mustBeAssignable(...)
        /usr/local/go/src/reflect/value.go:259
reflect.Value.Set({0x100f4c3e0?, 0x14000169800?, 0x14000011168?}, {0x100f4c3e0?, 0x101232e40?, 0x98?})
        /usr/local/go/src/reflect/value.go:2319 +0x58
k8s.io/apimachinery/pkg/runtime.structFromUnstructured({0x100f63c60?, 0x140001697d0?, 0x1012f0248?}, {0x100f7e000?, 0x14000169800?, 0x100c2d5f8?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:556 +0x490
k8s.io/apimachinery/pkg/runtime.fromUnstructured({0x100f63c60?, 0x140001697d0?, 0x1400001115c?}, {0x100f7e000?, 0x14000169800?, 0x98?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:359 +0x2c4
k8s.io/apimachinery/pkg/runtime.structFromUnstructured({0x100f63c60?, 0x140001697a0?, 0x14000033c48?}, {0x100f71840?, 0x14000169800?, 0x100cabbf4?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:550 +0x64c
k8s.io/apimachinery/pkg/runtime.fromUnstructured({0x100f63c60?, 0x140001697a0?, 0x10?}, {0x100f71840?, 0x14000169800?, 0x100c35cec?}, 0x14000033e60)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:359 +0x2c4
k8s.io/apimachinery/pkg/runtime.(*unstructuredConverter).FromUnstructuredWithValidation(0x1011c8df0, 0x140001697a0, {0x100f44b20, 0x14000169800}, 0x1)
        /Users/shaka/go/pkg/mod/k8s.io/apimachinery@v0.29.3/pkg/runtime/converter.go:247 +0x20c
main.main()
        /Users/shaka/playground/repro-runtime-panic/main.go:30 +0x110

I might be in a specific edge case, but we use protobuf to generate gRPC structs. Those structs contain private fields holding the grpc state and cause the converter to panic, even if the unstructured object only contains the public user-facing fields.

What did you expect to happen?

I would expect the converter to ignore private fields and not panic.

How can we reproduce it (as minimally and precisely as possible)?

package main

import (
	"k8s.io/apimachinery/pkg/apis/meta/v1/unstructured"
	"k8s.io/apimachinery/pkg/runtime"
	"log"
)

type StructuredObject struct {
	Spec StructuredObjectSpec
}

type StructuredObjectSpec struct {
	bar []byte
	Foo string
}

func main() {
	u := &unstructured.Unstructured{
		Object: map[string]interface{}{
			"spec": map[string]interface{}{
				"foo": "value",
			},
		},
	}

	structured := StructuredObject{}
	err := runtime.DefaultUnstructuredConverter.FromUnstructuredWithValidation(
		u.Object,
		&structured,
		true, /* returnUnknownFields */
	)
	if err != nil {
		log.Fatal(err)
	}
}

Anything else we need to know?

I can send a PR if you confirm this should be fixed and that the fix is to skip the private fields.

Kubernetes version

The go module looks like

go 1.22

require k8s.io/apimachinery v0.29.3

Cloud provider

No cloud provider.

OS version

No response

Install tools

Go 1.22

Container runtime (CRI) and version (if applicable)

No response

Related plugins (CNI, CSI, ...) and versions (if applicable)

No response

[hugoShaka]

/sig api-machinery

[alexzielenski]

/cc @sttts
/triage accepted