Add io.Reader/Writer utility functions

go/helper/stream.go

+package helper
+
+import (
+	"io"
+)
+
+// NewReceiveReader turns receiver into an io.Reader. Errors from the
+// receiver function are passed on unmodified. This means receiver should
+// emit io.EOF when done.
+func NewReceiveReader(receiver func() ([]byte, error)) io.Reader {
+	return &receiveReader{receiver: receiver}
+}
+
+type receiveReader struct {
+	receiver func() ([]byte, error)
+	data     []byte
+	err      error
+}
+
+func (rr *receiveReader) Read(p []byte) (int, error) {
+	if len(rr.data) == 0 {
+		rr.data, rr.err = rr.receiver()
+	}
+	n := copy(p, rr.data)
+	rr.data = rr.data[n:]
+	if len(rr.data) == 0 {
+		return n, rr.err
+	}
+	return n, nil
+}
+
+// NewSendWriter turns sender into an io.Writer. The number of 'bytes
+// written' reported back is always len(p).
+func NewSendWriter(sender func(p []byte) error) io.Writer {
+	return &sendWriter{sender: sender}
+}
+
+type sendWriter struct {
+	sender func([]byte) error
+}
+
+func (sw *sendWriter) Write(p []byte) (int, error) {
+	return len(p), sw.sender(p)
+}