[decode] regression from v4: `invalid code=d3 decoding string/bytes length` on complex nested structure

[lizthegrey]

Expected Behavior (v4)

https://go.dev/play/p/BXgPx5l-SDf

Structure roundtrips successfully.

[142 161 105 192 161 103 202 64 160 0 0 161 97 210 0 0 0 5 161 106 129 161 97 211 0 0 0 0 0 0 0 1 161 109 129 211 0 0 0 0 0 0 0 1 129 161 50 129 211 0 0 0 0 0 0 0 3 211 0 0 0 0 0 0 0 4 161 98 207 0 0 0 0 0 0 0 5 161 107 146 211 0 0 0 0 0 0 0 1 211 0 0 0 0 0 0 0 2 161 108 129 211 0 0 0 0 0 0 0 1 129 203 64 0 0 0 0 0 0 0 203 64 8 0 0 0 0 0 0 161 110 129 161 110 192 161 99 204 5 161 100 195 161 101 163 102 111 111 161 102 196 3 102 111 111 161 104 203 127 240 0 0 0 0 0 0]
<nil>
map[a:5 b:5 c:5 d:true e:foo f:[102 111 111] g:5 h:+Inf i:<nil> j:map[a:1] k:[1 2] l:map[1:map[2:3]] m:map[1:map[2:map[3:4]]] n:map[n:<nil>]]

Current Behavior (v5)

Decode error happens when roundtripping

https://go.dev/play/p/5hcEjhSWohg

[142 161 101 163 102 111 111 161 104 203 127 240 0 0 0 0 0 0 161 108 129 211 0 0 0 0 0 0 0 1 129 203 64 0 0 0 0 0 0 0 203 64 8 0 0 0 0 0 0 161 98 207 0 0 0 0 0 0 0 5 161 100 195 161 102 196 3 102 111 111 161 107 146 211 0 0 0 0 0 0 0 1 211 0 0 0 0 0 0 0 2 161 105 192 161 99 204 5 161 106 129 161 97 1 161 97 210 0 0 0 5 161 103 202 64 160 0 0 161 109 129 211 0 0 0 0 0 0 0 1 129 161 50 129 211 0 0 0 0 0 0 0 3 211 0 0 0 0 0 0 0 4 161 110 129 161 110 192]
msgpack: invalid code=d3 decoding string/bytes length

Steps to Reproduce

See above

v4 encode -> v5 decode has the error. https://go.dev/play/p/gYEuy6p42C6
v5 encode -> v4 decode does not have the error. https://go.dev/play/p/fQBCJZiW03f

[vmihailenco]

Hi,

The smaller reproducer looks like this:

package main

import (
	"bytes"
	"fmt"

	v4 "github.com/vmihailenco/msgpack/v4"
	v5 "github.com/vmihailenco/msgpack/v5"
)

func main() {
	var ret interface{}
	b, _ := v4.Marshal(map[string]interface{}{
		"2": map[int64]int64{
			3: 4,
		},
	})
	fmt.Println(b)
	decoder := v5.NewDecoder(bytes.NewReader(b))
	// decoder.SetMapDecoder(func(dec *v5.Decoder) (interface{}, error) {
	// 	return dec.DecodeTypedMap()
	// })
	decoder.UseLooseInterfaceDecoding(true)
	decoder.SetCustomStructTag("json")
	err := decoder.Decode(&ret)
	fmt.Println(err)
	fmt.Println(ret)
}

To fix it, uncomment the code that sets map decoder. This behavior is explained and documented here. I am not sure there is anything else we can do.