bytes: don't grow Buffer if capacity is available

Also added a new benchmark from the same test:

benchmark                           old ns/op    new ns/op    delta
BenchmarkBufferNotEmptyWriteRead      2643698       709189  -73.17%

Fixes #5154

R=golang-dev, r, gri
CC=golang-dev
https://golang.org/cl/8164043

src/pkg/bytes/buffer.go

@@ -87,6 +87,11 @@ func (b *Buffer) grow(n int) int {
 		var buf []byte
 		if b.buf == nil && n <= len(b.bootstrap) {
 			buf = b.bootstrap[0:]
+		} else if m+n <= cap(b.buf) {
+			// We can slide things down instead of
+			// allocating a new slice.
+			copy(b.buf[:], b.buf[b.off:])
+			buf = b.buf[:m]
 		} else {
 			// not enough space anywhere
 			buf = makeSlice(2*cap(b.buf) + n)

src/pkg/bytes/buffer_test.go

@@ -475,3 +475,35 @@ func TestUnreadByte(t *testing.T) {
 		t.Errorf("ReadByte = %q; want %q", c, 'm')
 	}
 }
+
+// Tests that we occasionally compact. Issue 5154.
+func TestBufferGrowth(t *testing.T) {
+	var b Buffer
+	buf := make([]byte, 1024)
+	b.Write(buf[0:1])
+	var cap0 int
+	for i := 0; i < 5<<10; i++ {
+		b.Write(buf)
+		b.Read(buf)
+		if i == 0 {
+			cap0 = b.Cap()
+		}
+	}
+	cap1 := b.Cap()
+	if cap1 > cap0 {
+		t.Errorf("buffer cap = %d; too big", cap1)
+	}
+}
+
+// From Issue 5154.
+func BenchmarkBufferNotEmptyWriteRead(b *testing.B) {
+	buf := make([]byte, 1024)
+	for i := 0; i < b.N; i++ {
+		var b Buffer
+		b.Write(buf[0:1])
+		for i := 0; i < 5<<10; i++ {
+			b.Write(buf)
+			b.Read(buf)
+		}
+	}
+}

src/pkg/bytes/export_test.go

@@ -7,3 +7,7 @@ package bytes
 // Export func for testing
 var IndexBytePortable = indexBytePortable
 var EqualPortable = equalPortable
+
+func (b *Buffer) Cap() int {
+	return cap(b.buf)
+}