Empty uint64 array will unmarshal to nil if struct is not initialized

[jtraglia]

Hi there. Noticed something that I believe is a bug in sszgen.

Given a structure with a uint64 array field, the field will unmarshal to a nil value if (1) the array is empty and (2) the variable that calls UnmarshalSSZ is not initialized. And if you swap uint64 for byte it works as expected.

See the following program:

package main

import "fmt"

type Foo struct {
  Bar []uint64 `json:"bar" ssz-max:"1024"`
}

func main() {
  val := Foo{Bar:[]uint64{}}
  var dec Foo // not initialized

  enc, err := val.MarshalSSZ()
  if err != nil {
    panic(err)
  }
  err = dec.UnmarshalSSZ(enc)
  if err != nil {
    panic(err)
  }

  fmt.Println("val.Bar nil?", val.Bar == nil)
  fmt.Println("dec.Bar nil?", dec.Bar == nil)
}

When executed, this will output:

$ sszgen --path main.go && go run .
val.Bar nil? false
dec.Bar nil? true

On the other hand, if dec.Bar is initialized with anything (even an empty array), it will behave as expected:

func main() {
  val := Foo{Bar:[]uint64{}}
  dec := Foo{Bar:[]uint64{}} // initialized

  enc, err := val.MarshalSSZ()
  if err != nil {
    panic(err)
  }
  err = dec.UnmarshalSSZ(enc)
  if err != nil {
    panic(err)
  }

  fmt.Println("val.Bar nil?", val.Bar == nil)
  fmt.Println("dec.Bar nil?", dec.Bar == nil)
}

When executed, this will output:

$ sszgen --path main.go && go run .
val.Bar nil? false
dec.Bar nil? false

Fixes: flashbots/go-boost-utils#23

[ferranbt]

Can you share the generated go file?

[ferranbt]

I found the issue, it is not on the generated code but on this function that extends the uint64 array. It should be easy to fix.

[ferranbt]

Could you try the fix in #97 and see if it resolves the issue?

[jtraglia]

Yes, but I won't be able to today. On a day trip away from my workstation. Thank you for the quick fix!

[jtraglia]

@ferranbt Yes, it resolves the issue. Thanks!