package main

import (
    "errors"
    "fmt"
    "io"
    "os"
    "strconv"
)

type User struct {
    Name    string
    PostNum int
    Role    int
}

func (u *User) Write(buf []byte) (int, error) {
    var seps []int
    fmt.Println("writing") // for simple debug, just be sure that Write is calling, which is not in (1) case (see below)
    for i := range buf {
        if buf[i] == 44 {
            seps = append(seps, i)
        }
    }
    if len(seps) != 2 {
        return 0, fmt.Errorf("user writing error, got %d seps with buf: %v(%s)", len(seps), buf, buf)
    }
    u.Name = string(buf[:seps[0]])
    posts, err := strconv.Atoi(string(buf[seps[0]+1 : seps[1]]))
    if err != nil {
        return seps[0], fmt.Errorf("user writing error in parsing post num: %w", err)
    }
    u.PostNum = posts
    role, err := strconv.Atoi(string(buf[seps[1]+1 : len(buf)-2]))
    if err != nil {
        return seps[1], fmt.Errorf("user writing error in parsing role: %w", err)
    }
    u.Role = role
    return len(buf), nil
}
func (u User) Read(buf []byte) (int, error) {
    s := fmt.Sprintf("%s,%d,%d\r\n", u.Name, u.PostNum, u.Role)
    n := min(len(s), len(buf))
    for i := range n {
        buf[i] = s[i]
    }
    return n, io.EOF
}

func main() {
    originalUser := &User{"username", 100, 1}
    f, err := os.Create("user.txt")
    if err != nil {
        fmt.Println(err)
        return
    }
    n, err := io.Copy(f, originalUser)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(n, "bytes was writed to file") // file is writed correct
    /*
        (1) now if same file instance will be used in reading with io.Copy(userFromFile, f) - userFromFile will be with zero values
        (2) and if first close that file anf reopen it - userFromFile will be correctly read
    */
    // comment next 5 lines for case (1) and uncomment for case (2)
    f.Close()
    f, err = os.Open("user.txt")
    if err != nil {
        fmt.Println(err)
    }
    userFromFile := &User{}
    n, err = io.Copy(userFromFile, f)
    if err != nil {
        fmt.Println(err)
    }
    fmt.Println(n, userFromFile)
}