You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: unmarshal.go
    path of file: ./repos/encore/runtimes/go/appruntime/shared/etype
    
    the code of the file until where you have to start completion: package etype

import (
	"encoding/base64"
	stdjson "encoding/json"
    
func (u *Unmarshaller) ReadBody(body io.Reader) (payload []byte) {
	payload, err := io.ReadAll(body)
	if err == nil && len(payload) == 0 {
		u.setErr("missing request body", "request_body", fmt.Errorf("missing request body"))
	} else if err != nil {
		u.setErr("could not parse request body", "request_body", err)
	}
	return payload
}