bufio: reuse Writer buffers after Flush

A bufio.Writer.Flush marks the usual end of a Writer's
life. Recycle its internal buffer on those explicit flushes,
but not on normal, as-needed internal flushes.

benchmark               old ns/op    new ns/op    delta
BenchmarkWriterEmpty         1959          727  -62.89%

benchmark              old allocs   new allocs    delta
BenchmarkWriterEmpty            2            1  -50.00%

benchmark               old bytes    new bytes    delta
BenchmarkWriterEmpty         4215           83  -98.03%

R=gri, iant
CC=gobot, golang-dev, voidlogic7
https://golang.org/cl/9459044

src/pkg/bufio/bufio.go

@@ -29,7 +29,7 @@ var (
 
 // Reader implements buffering for an io.Reader object.
 type Reader struct {
-	buf          []byte // either nil or []byte of size bufSize
+	buf          []byte // either nil or []byte of length bufSize
 	bufSize      int
 	rd           io.Reader
 	r, w         int
@@ -314,7 +314,7 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
 		}
 
 		// Buffer is full?
-		if b.Buffered() >= len(b.buf) {
+		if b.Buffered() >= b.bufSize {
 			b.r = b.w
 			return b.buf, ErrBufferFull
 		}
@@ -474,7 +474,8 @@ func (b *Reader) writeBuf(w io.Writer) (int64, error) {
 // accepted and all subsequent writes will return the error.
 type Writer struct {
 	err     error
-	buf []byte
+	buf     []byte // either nil or []byte of length bufSize
+	bufSize int
 	n       int
 	wr      io.Writer
 }
@@ -485,16 +486,20 @@ type Writer struct {
 func NewWriterSize(wr io.Writer, size int) *Writer {
 	// Is it already a Writer?
 	b, ok := wr.(*Writer)
-	if ok && len(b.buf) >= size {
+	if ok && b.bufSize >= size {
 		return b
 	}
 	if size <= 0 {
 		size = defaultBufSize
 	}
-	b = new(Writer)
-	// TODO(bradfitz): make Writer buffers lazy too, like Reader's
-	b.buf = make([]byte, size)
-	b.wr = wr
+	b = &Writer{
+		wr:      wr,
+		bufSize: size,
+	}
+	if size > defaultBufSize {
+		// TODO(bradfitz): make all buffer sizes recycle
+		b.buf = make([]byte, b.bufSize)
+	}
 	return b
 }
 
@@ -503,8 +508,38 @@ func NewWriter(wr io.Writer) *Writer {
 	return NewWriterSize(wr, defaultBufSize)
 }
 
+// allocBuf makes b.buf non-nil.
+func (b *Writer) allocBuf() {
+	if b.buf != nil {
+		return
+	}
+	select {
+	case b.buf = <-bufCache:
+		b.buf = b.buf[:b.bufSize]
+	default:
+		b.buf = make([]byte, b.bufSize, defaultBufSize)
+	}
+}
+
+// putBuf returns b.buf if it's unused.
+func (b *Writer) putBuf() {
+	if b.n == 0 && cap(b.buf) == defaultBufSize {
+		select {
+		case bufCache <- b.buf:
+			b.buf = nil
+		default:
+		}
+	}
+}
+
 // Flush writes any buffered data to the underlying io.Writer.
 func (b *Writer) Flush() error {
+	err := b.flush()
+	b.putBuf()
+	return err
+}
+
+func (b *Writer) flush() error {
 	if b.err != nil {
 		return b.err
 	}
@@ -528,7 +563,7 @@ func (b *Writer) Flush() error {
 }
 
 // Available returns how many bytes are unused in the buffer.
-func (b *Writer) Available() int { return len(b.buf) - b.n }
+func (b *Writer) Available() int { return b.bufSize - b.n }
 
 // Buffered returns the number of bytes that have been written into the current buffer.
 func (b *Writer) Buffered() int { return b.n }
@@ -538,6 +573,7 @@ func (b *Writer) Buffered() int { return b.n }
 // If nn < len(p), it also returns an error explaining
 // why the write is short.
 func (b *Writer) Write(p []byte) (nn int, err error) {
+	b.allocBuf()
 	for len(p) > b.Available() && b.err == nil {
 		var n int
 		if b.Buffered() == 0 {
@@ -547,7 +583,7 @@ func (b *Writer) Write(p []byte) (nn int, err error) {
 		} else {
 			n = copy(b.buf[b.n:], p)
 			b.n += n
-			b.Flush()
+			b.flush()
 		}
 		nn += n
 		p = p[n:]
@@ -566,9 +602,12 @@ func (b *Writer) WriteByte(c byte) error {
 	if b.err != nil {
 		return b.err
 	}
-	if b.Available() <= 0 && b.Flush() != nil {
+	if b.Available() <= 0 && b.flush() != nil {
 		return b.err
 	}
+	if b.buf == nil {
+		b.allocBuf()
+	}
 	b.buf[b.n] = c
 	b.n++
 	return nil
@@ -577,6 +616,9 @@ func (b *Writer) WriteByte(c byte) error {
 // WriteRune writes a single Unicode code point, returning
 // the number of bytes written and any error.
 func (b *Writer) WriteRune(r rune) (size int, err error) {
+	if b.buf == nil {
+		b.allocBuf()
+	}
 	if r < utf8.RuneSelf {
 		err = b.WriteByte(byte(r))
 		if err != nil {
@@ -589,7 +631,7 @@ func (b *Writer) WriteRune(r rune) (size int, err error) {
 	}
 	n := b.Available()
 	if n < utf8.UTFMax {
-		if b.Flush(); b.err != nil {
+		if b.flush(); b.err != nil {
 			return 0, b.err
 		}
 		n = b.Available()
@@ -608,13 +650,14 @@ func (b *Writer) WriteRune(r rune) (size int, err error) {
 // If the count is less than len(s), it also returns an error explaining
 // why the write is short.
 func (b *Writer) WriteString(s string) (int, error) {
+	b.allocBuf()
 	nn := 0
 	for len(s) > b.Available() && b.err == nil {
 		n := copy(b.buf[b.n:], s)
 		b.n += n
 		nn += n
 		s = s[n:]
-		b.Flush()
+		b.flush()
 	}
 	if b.err != nil {
 		return nn, b.err
@@ -627,6 +670,7 @@ func (b *Writer) WriteString(s string) (int, error) {
 
 // ReadFrom implements io.ReaderFrom.
 func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
+	b.allocBuf()
 	if b.Buffered() == 0 {
 		if w, ok := b.wr.(io.ReaderFrom); ok {
 			return w.ReadFrom(r)
@@ -641,7 +685,7 @@ func (b *Writer) ReadFrom(r io.Reader) (n int64, err error) {
 		b.n += m
 		n += int64(m)
 		if b.Available() == 0 {
-			if err1 := b.Flush(); err1 != nil {
+			if err1 := b.flush(); err1 != nil {
 				return n, err1
 			}
 		}

src/pkg/bufio/bufio_test.go

@@ -1098,3 +1098,31 @@ func BenchmarkReaderEmpty(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkWriterEmpty(b *testing.B) {
+	b.ReportAllocs()
+	str := strings.Repeat("x", 1<<10)
+	bs := []byte(str)
+	for i := 0; i < b.N; i++ {
+		bw := NewWriter(ioutil.Discard)
+		bw.Flush()
+		bw.WriteByte('a')
+		bw.Flush()
+		bw.WriteRune('B')
+		bw.Flush()
+		bw.Write(bs)
+		bw.Flush()
+		bw.WriteString(str)
+		bw.Flush()
+	}
+}
+
+func BenchmarkWriterFlush(b *testing.B) {
+	b.ReportAllocs()
+	bw := NewWriter(ioutil.Discard)
+	str := strings.Repeat("x", 50)
+	for i := 0; i < b.N; i++ {
+		bw.WriteString(str)
+		bw.Flush()
+	}
+}