Littleroot Blog

Validate Writes and Reads to underlying io.Writers and io.Readers

In Go it’s common to have an io.Writer wrapping another io.Writer. The gzip.Writer type in the standard library is a good example. Writes to the gzip.Writer eventually result in Writes to the wrapped io.Writer (the one given to gzip.NewWriter).

package gzip // import "compress/gzip"

type Writer struct { ... }

func NewWriter(w io.Writer) *Writer
func (z *Writer) Write(p []byte) (int, error)

Misbehaving Writers

If you implement an io.Writer that wraps another io.Writer it helps to check for misbehaving Writers. An incorrect underlying io.Writer, for example, is one that return a nil error along with n < len(p)1 either accidentally or because of a true bug in its implementation.

If you propagate n, err return values from an incorrect underlying Writer without checking, as in:

package me

type Writer struct{ w io.Writer }

func (w *Writer) Write(p []byte) (int, error) {
    return w.w.Write(p)

it makes it harder for users to detect and debug the underlying misbehaving Writer, because the bug is propagated instead of being detected at the earliest. Additionally your Write method too now violates the io.Writer interface.

So it helps to validate the return values of Write calls to the underlying Writer.

Validating a Write

Validating an Write involves checking two requirements from the io.Writer interface.

Write returns the number of bytes written from p (0 <= n <= len(p)) and any error encountered that caused the write to stop early.


Write must return a non-nil error if it returns n < len(p).

func (w *Writer) Write(p []byte) (int, error) {
    return validatedWrite(p, w.w)
func validatedWrite(p []byte, w io.Writer) (int, error) {
    m, err := w.Write(p)
    if m < 0 || m > len(p) {
        panic("invalid Write count")
    if m < len(p) && err == nil {
        return m, io.ErrShortWrite
    return m, err


Same goes for io.Readers and Read calls too!

func validatedRead(p []byte, r io.Reader) (int, error) {
    n, err := r.Read(p)
    if n < 0 || n > len(p) {
        panic("invalid Read count")
    return n, err
  1. This violates the io.Writer interface, whose documentation says: “Write must return a non-nil error if it returns n < len(p).”