نحوه ی call شدن متد Error

سلام وقت بخیر

من یه سوالی داشتم در رابطه با نحوه ی call شدن متد Error

این کد را در نظیر بگیرید

package main
import (
    "errors"
    "fmt"
    "io"
    "os"
)
type IOError struct {
    FileName string
    Message  string
    Err      error
}
func (error *IOError) Unwrap() error {
    return error.Err
}
func (error IOError) Error() string {
    return fmt.Sprintf("IO error occurred: FileName: %s Message: %s Detail: %s", error.FileName, error.Message, error.Err.Error())
}
func CopyFile(srcName, dstName string) error {
    src, err := os.Open(srcName)
    if err != nil {
        return &IOError{FileName: srcName, Message: "during copy file could not open source file", Err: err}
    }
    //defer src.Close()
    dst, err := os.Create(dstName)
    if err != nil {
        return &IOError{FileName: srcName, Message: "during copy file could not create destination file", Err: err}
    }
    dst.Close()
    _, err = io.Copy(dst, src)
    fmt.Printf("**************%s***************\\n", err)
    if err != nil {
        return &IOError{FileName: srcName, Message: "during copy file could not copy", Err: err}
    }
    return nil
}
func main() {
    err := CopyFile("src.txt", "dst.txt")
    fmt.Printf("%s\\n", err)
    fmt.Printf("Unwrap Error: %s", errors.Unwrap(err))
    return
    if err != nil {
        fmt.Printf("Error: %s\\n", err)
    }
}

وقتی خطای err را در main پرینت میکنیم میره متد Error را که خودمان rewrite کردیم call میکنه اما در فانکشن CopyFile وقتی مقدار err را printf میکنم سراغ متد Error نمیره

این قسمت برام مبهم هست اگر امکانش هست نحوه ی call شدن متد Error را trace کنید تا بیشتر متوجه بشیم ؟

//output 
**************write dst.txt: file already closed***************
IO error occurred: FileName: src.txt Message: during copy file could not copy Detail: write dst.txt: file already closed
Unwrap Error: write dst.txt: file already closed

func (err IOError) Error() string { fmt.Println("Calling Error() method of IOError") return fmt.Sprintf("fileName: err input/output : %s msg: %s details: %s", err.FileName, err.Message, err.Err.Error()) }

func (p *pp) handleMethods(verb rune) (handled bool) { if p.erroring { return } if verb == 'w' { // It is invalid to use %w other than with Errorf, more than once,// or with a non-error arg. err, ok := p.arg.(error) if !ok || !p.wrapErrs || p.wrappedErr != nil { p.wrappedErr = nil p.wrapErrs = false p.badVerb(verb) return true } p.wrappedErr = err // If the arg is a Formatter, pass 'v' as the verb to it. verb = 'v' } // Is it a Formatter?if formatter, ok := p.arg.(Formatter); ok { handled = true defer p.catchPanic(p.arg, verb, "Format") formatter.Format(p, verb) return } // If we're doing Go syntax and the argument knows how to supply it, take care of it now.if p.fmt.sharpV { if stringer, ok := p.arg.(GoStringer); ok { handled = true defer p.catchPanic(p.arg, verb, "GoString") // Print the result of GoString unadorned. p.fmt.fmtS(stringer.GoString()) return } } else { // If a string is acceptable according to the format, see if// the value satisfies one of the string-valued interfaces.// Println etc. set verb to %v, which is "stringable".switch verb { case 'v', 's', 'x', 'X', 'q': // Is it an error or Stringer?// The duplication in the bodies is necessary:// setting handled and deferring catchPanic// must happen before calling the method.switch v := p.arg.(type) { case error: handled = true defer p.catchPanic(p.arg, verb, "Error") p.fmtString(v.Error(), verb) return case Stringer: handled = true defer p.catchPanic(p.arg, verb, "String") p.fmtString(v.String(), verb) return } } } return false }

switch verb { case 'v', 's', 'x', 'X', 'q': // Is it an error or Stringer?// The duplication in the bodies is necessary:// setting handled and deferring catchPanic// must happen before calling the method.switch v := p.arg.(type) { case error: handled = true defer p.catchPanic(p.arg, verb, "Error") p.fmtString(v.Error(), verb) return case Stringer: handled = true defer p.catchPanic(p.arg, verb, "String") p.fmtString(v.String(), verb) return }

type IOError struct { FileName string Message string Err error } func (error *IOError) Unwrap() error { return error.Err } func (error IOError) Error() string { return fmt.Sprintf("IO error occurred: FileName: %s Message: %s Detail: %s", error.FileName, error.Message, error.Err.Error()) }