Add ReadCloser method to jwriter and buffer

When marshalling something as body for a http.Request we can only use
BuildBytes() which always allocates a buffer for our whole body. This
pull request adds a ReadCloser() method which returns an io.ReadCloser
that can be passed as body into http.NewRequest. This ReadCloser will
read from the existing buffs and putBuf them when they are not needed
anymore.

buffer/pool.go

@@ -205,3 +205,57 @@ func (b *Buffer) BuildBytes() []byte {
 
 	return ret
 }
+
+type readCloser struct {
+	offset int
+	bufs   [][]byte
+}
+
+func (r *readCloser) Read(p []byte) (n int, err error) {
+	for _, buf := range r.bufs {
+		// Copy as much as we can.
+		x := copy(p[n:], buf[r.offset:])
+		n += x // Increment how much we filled.
+
+		// Did we empty the whole buffer?
+		if r.offset+x == len(buf) {
+			// On to the next buffer.
+			r.offset = 0
+			r.bufs = r.bufs[1:]
+
+			// We can release this buffer.
+			putBuf(buf)
+		} else {
+			r.offset += x
+		}
+
+		if n == len(p) {
+			break
+		}
+	}
+	// No buffers left or nothing read?
+	if len(r.bufs) == 0 {
+		err = io.EOF
+	}
+	return
+}
+
+func (r *readCloser) Close() error {
+	// Release all remainig buffers.
+	for _, buf := range r.bufs {
+		putBuf(buf)
+	}
+
+	return nil
+}
+
+// ReadCloser creates an io.ReadCloser with all the contents of the buffer.
+func (b *Buffer) ReadCloser() io.ReadCloser {
+	ret := &readCloser{0, append(b.bufs, b.Buf)}
+
+	b.bufs = nil
+	b.toPool = nil
+	b.Buf = nil
+
+	return ret
+}

buffer/pool_test.go

@@ -77,3 +77,31 @@ func TestDumpTo(t *testing.T) {
 		t.Errorf("DumpTo() = %v; want %v", n, len(want))
 	}
 }
+
+func TestReadCloser(t *testing.T) {
+	var b Buffer
+	var want []byte
+
+	s := "test"
+	for i := 0; i < 1000; i++ {
+		b.AppendBytes([]byte(s))
+		want = append(want, s...)
+	}
+
+	out := &bytes.Buffer{}
+	rc := b.ReadCloser()
+	n, err := out.ReadFrom(rc)
+	if err != nil {
+		t.Errorf("ReadCloser() error: %v", err)
+	}
+	rc.Close() // Will always return nil
+
+	got := out.Bytes()
+	if !bytes.Equal(got, want) {
+		t.Errorf("DumpTo(): got %v; want %v", got, want)
+	}
+
+	if n != int64(len(want)) {
+		t.Errorf("DumpTo() = %v; want %v", n, len(want))
+	}
+}

jwriter/writer.go

@@ -46,6 +46,16 @@ func (w *Writer) BuildBytes() ([]byte, error) {
 	return w.Buffer.BuildBytes(), nil
 }
 
+// ReadCloser returns an io.ReadCloser that can be used to read the data.
+// ReadCloser also resets the buffer.
+func (w *Writer) ReadCloser() (io.ReadCloser, error) {
+	if w.Error != nil {
+		return nil, w.Error
+	}
+
+	return w.Buffer.ReadCloser(), nil
+}
+
 // RawByte appends raw binary data to the buffer.
 func (w *Writer) RawByte(c byte) {
 	w.Buffer.AppendByte(c)