go/src/testing/iotest/logger.go

type writeLogger …

func (l *writeLogger) Write(p []byte) (n int, err error) { … }

// NewWriteLogger returns a writer that behaves like w except
// that it logs (using [log.Printf]) each write to standard error,
// printing the prefix and the hexadecimal data written.
func NewWriteLogger(prefix string, w io.Writer) io.Writer { … }

type readLogger …

func (l *readLogger) Read(p []byte) (n int, err error) { … }

// NewReadLogger returns a reader that behaves like r except
// that it logs (using [log.Printf]) each read to standard error,
// printing the prefix and the hexadecimal data read.
func NewReadLogger(prefix string, r io.Reader) io.Reader { … }