Write stream implementation (#19)

Defines a WriteStream structure for buffering writes, similar to the existing ReadStream. WriteStreams can be created on OutEndpoints by using ep.NewStream().

endpoint_stream.go

@@ -14,7 +14,7 @@
 
 package gousb
 
-func (e *endpoint) newStream(size, count int, submit bool) (*stream, error) {
+func (e *endpoint) newStream(size, count int) (*stream, error) {
 	var ts []transferIntf
 	for i := 0; i < count; i++ {
 		t, err := newUSBTransfer(e.ctx, e.h, &e.Desc, size, e.Timeout)
@@ -26,7 +26,7 @@ func (e *endpoint) newStream(size, count int, submit bool) (*stream, error) {
 		}
 		ts = append(ts, t)
 	}
-	return newStream(ts, submit), nil
+	return newStream(ts), nil
 }
 
 // NewStream prepares a new read stream that will keep reading data from the
@@ -36,9 +36,23 @@ func (e *endpoint) newStream(size, count int, submit bool) (*stream, error) {
 // By keeping multiple transfers active at the same time, a Stream reduces
 // the latency between subsequent transfers and increases reading throughput.
 func (e *InEndpoint) NewStream(size, count int) (*ReadStream, error) {
-	s, err := e.newStream(size, count, true)
+	s, err := e.newStream(size, count)
 	if err != nil {
 		return nil, err
 	}
-	return &ReadStream{s}, nil
+	s.submitAll()
+	return &ReadStream{s: s}, nil
+}
+
+// NewStream prepares a new write stream that will write data in the background.
+// Size defines a buffer size for a single write transaction and count
+// defines how many transactions may be active at any time.
+// By buffering the writes, a Stream reduces the latency between subsequent
+// transfers and increases writing throughput.
+func (e *OutEndpoint) NewStream(size, count int) (*WriteStream, error) {
+	s, err := e.newStream(size, count)
+	if err != nil {
+		return nil, err
+	}
+	return &WriteStream{s: s}, nil
 }