Is the Go bytes.Buffer thread-safe?

In the Go programming language, is bytes.Buffer thread-safe? AFAIK, its documentation does not mention thread safety.

No.

The Go documentation follows a simple rule: If it is not explicitly stated that concurrent access to something is safe, it is not.

No - but you can easily wrap it in a thread safe struct!

For simple things:

type Buffer struct {
	b bytes.Buffer
	m sync.Mutex
}
func (b *Buffer) Read(p []byte) (n int, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Read(p)
}
func (b *Buffer) Write(p []byte) (n int, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Write(p)
}
func (b *Buffer) String() string {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.String()
}

..and use it as usual var buf Buffer, etc.

Want more of bytes.Buffer? Feel free to cherry pick:

func (b *Buffer) Bytes() []byte {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Bytes()
}
func (b *Buffer) Cap() int {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Cap()
}
func (b *Buffer) Grow(n int) {
	b.m.Lock()
	defer b.m.Unlock()
	b.b.Grow(n)
}
func (b *Buffer) Len() int {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Len()
}
func (b *Buffer) Next(n int) []byte {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.Next(n)
}
func (b *Buffer) ReadByte() (c byte, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.ReadByte()
}
func (b *Buffer) ReadBytes(delim byte) (line []byte, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.ReadBytes(delim)
}
func (b *Buffer) ReadFrom(r io.Reader) (n int64, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.ReadFrom(r)
}
func (b *Buffer) ReadRune() (r rune, size int, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.ReadRune()
}
func (b *Buffer) ReadString(delim byte) (line string, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.ReadString(delim)
}
func (b *Buffer) Reset() {
	b.m.Lock()
	defer b.m.Unlock()
	b.b.Reset()
}
func (b *Buffer) Truncate(n int) {
	b.m.Lock()
	defer b.m.Unlock()
	b.b.Truncate(n)
}
func (b *Buffer) UnreadByte() error {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.UnreadByte()
}
func (b *Buffer) UnreadRune() error {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.UnreadRune()
}
func (b *Buffer) WriteByte(c byte) error {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.WriteByte(c)
}
func (b *Buffer) WriteRune(r rune) (n int, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.WriteRune(r)
}
func (b *Buffer) WriteString(s string) (n int, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.WriteString(s)
}
func (b *Buffer) WriteTo(w io.Writer) (n int64, err error) {
	b.m.Lock()
	defer b.m.Unlock()
	return b.b.WriteTo(w)
}

Use io.Pipe() function call which provide pair of connected objects (*PipeReader, *PipeWriter) for synchronous read/write. This could be done in parallel, and it's a thread-safe.

bytes.Buffer is not thread safe, but here's an example of writing to a Buffer (or any io.StringWriter) safely using "channels and communication".

type SyncWriter struct {
	w      io.StringWriter
	finish sync.WaitGroup
	queue  chan string
	err    error
}

func NewSyncWriter(w io.StringWriter) *SyncWriter {
	var t SyncWriter
	t.w = w
	t.queue = make(chan string, 5)
	t.finish.Add(1)
	go t.writer()
	return &t
}

func (t *SyncWriter) Close() error {
	close(t.queue)
	t.finish.Wait()
	return t.err
}

func (t *SyncWriter) WriteString(s string) (int, error) {
	t.queue <- s
	return len(s), nil
}

func (t *SyncWriter) Write(p []byte) (int, error) {
	return t.WriteString(string(p))
}

func (t *SyncWriter) writer() {
	defer t.finish.Done()
	for s := range t.queue {
		_, err := t.w.WriteString(s)
		if err != nil && t.err == nil {
			t.err = err
		}
	}
}

And here's how to use it:

func write(wg *sync.WaitGroup, w io.Writer, d int) {
	fmt.Fprintf(w, "%d", d)
	wg.Done()
}

func main() {
	var buf bytes.Buffer
	w := NewSyncWriter(&buf)
	w.WriteString("hello ")
	var wg sync.WaitGroup
	wg.Add(10)
	for i := 0; i < 10; i++ {
		go write(&wg, w, i)
	}
	wg.Wait()
	w.Close()
	fmt.Printf("%s\n", buf.String())
}