Expose StreamerWriter interface

Streamer() now returns a StreamWriter interface (not io.ReadWriteCloser)
so that the CloseWithError(error) method may be called without casting.

The idiomatic name for this interface and function should be Streamer
and NewStreamer(). However, the function is already named Streamer()
and a package can't contain a function and interface with the same name,
so we need to call it something else. I decided to name the interface
StreamWriter, as writing is the primary use case for library callers.

Author: rdingwall

buffer.go

@@ -14,6 +14,11 @@ func (b *bufCloser) Close() error {
 	return nil // No-op
 }
 
+type StreamerWriter interface {
+	io.ReadWriteCloser
+	CloseWithError(error) error
+}
+
 type streamer struct {
 	pipeR *io.PipeReader
 	pipeW *io.PipeWriter
@@ -35,7 +40,7 @@ type streamer struct {
 //
 // Note that a Streamer may not perform any internal buffering, so callers should take care not to depend on writes
 // being non-blocking. If buffering is needed, Streamer can be wrapped in a bufio.Writer.
-func Streamer() io.ReadWriteCloser {
+func Streamer() StreamerWriter {
 	pipeR, pipeW := io.Pipe()
 	return &streamer{
 		pipeR: pipeR,