func ReadAllLimitSize(r io.Reader, maxSize int64) ([]byte, error) {
	// when maxSize <= 0, read all
	if maxSize <= 0 {
		return ioutil.ReadAll(r)
	}
	const (
		B  = 1
		KB = 1024
		MB = 1024 * 1024
		GB = 1024 * 1024 * 1024
	)
	result := make([]byte, 0, 512)
	step,n :=0,0
	buf := make([]byte,512)
	var er error
	for {
		step ,er =r.Read(buf)
		n += step
		if int64(n) >= maxSize {
			return nil, errors.New(fmt.Sprintf("read  %d,but limit %d", n, maxSize))
		}

		result = append(result, buf[:step]...)
		if er!=nil && er == io.EOF{
			break
		}
		if er !=nil {
			return nil,er
		}
	}
	return result, nil
}

func TestReadAllLimitSize(t *testing.T) {
	buf,e :=ReadAllLimitSize(strings.NewReader("hello world"), 122)
	if e!=nil {
		fmt.Println(e.Error())
		t.Fail()
	}
	fmt.Println(string(buf))
}

// 5000000	       214 ns/op
func BenchmarkReadAllLimitSize(b *testing.B) {
	r := strings.NewReader("benchmark for readAllLimitSize")
	const maxSize = int64(100)
	var buf []byte
	var e error
	for i:=0;i<b.N;i++{
		buf,e =ReadAllLimitSize(r, maxSize)
		if e!=nil{
			b.Fatal(e.Error())
		}
	}
	fmt.Println(string(buf))
}