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762 B

package main
import "io"
type prefixingWriter struct {
prefix []byte
writer io.Writer
func (pw *prefixingWriter) Write(p []byte) (int, error) {
if len(p) == 0 {
return 0, nil
// Perform an "atomic" write of a prefix and p to make sure that it doesn't interleave
// sub-line when used concurrently with io.PipeWrite.
n, err := pw.writer.Write(append(pw.prefix, p...))
if n > len(p) {
// To comply with the io.Writer interface requirements we must
// return a number of bytes written from p (0 <= n <= len(p)),
// so we are ignoring the length of the prefix here.
return len(p), err
return n, err
func prefixWriter(prefix string, sink io.Writer) io.Writer {
return &prefixingWriter{
prefix: []byte(prefix),
writer: sink,