Packing value fails unless struct was unpacked before

[zonque]

I've bumped into this weird issue and I still can't make sense of it.

Consider the following example:

package main

import (
	"bytes"
	"fmt"

	"github.com/lunixbochs/struc"
)

type example struct {
	A int `struc:"little"`
}

func read(buf *bytes.Buffer) {
	o := example{}
	err := struc.Unpack(buf, &o)
	if err != nil {
		panic(err)
	}
}

func write(buf *bytes.Buffer, data interface{}) {
	err := struc.Pack(buf, data)
	if err != nil {
		panic(err)
	}
}

func main() {
	o := example{123}

	buf := bytes.NewBuffer([]byte{})

	write(buf, o)
	fmt.Println(buf.Bytes())
}

The above doesn't work, the buffer remains empty. It can be fixed by passing o as a pointer:

	write(buf, &o)
	fmt.Println(buf.Bytes())

Now, the weird thing is: if the example struct is once unpacked before the packing, it works regardless of whether the struct is passed by value or by pointer.

func main() {
	o := example{123}

	buf := bytes.NewBuffer([]byte{0, 0, 0, 0})

	read(buf)
	fmt.Println(buf.Bytes())

	write(buf, o)
	fmt.Println(buf.Bytes())
}

This is not specific to the instance of the struct but a global effect. As long as the unpacking happened once in the running task, packing works fine.

Can anyone make sense of that? I'm not even sure if this is a bug.

[zonque]

FWIW, this is

go version go1.14.3 linux/amd64

and

github.com/lunixbochs/struc v0.0.0-20200521075829-a4cb8d33dbbe

[lunixbochs]

maybe put some basic prints in the parseFields call tree and see when it's being called

struc/parse.go

Line 200 in a4cb8d3

func parseFields(v reflect.Value) (Fields, error) {