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,
	}
}