StackSender.Send never returns a nil value

Fix StackSender.Send to always honor the `W` and `R` modes, by:

1) returning a valid Sender and Receiver when the respective flags are set, and
2)  returning a value which doesn't need to be Closed, when they are not.

This also moves `NopSender` and `NopReceiver` out of `utils`, to prevent
a circular dependency.

Signed-off-by: Solomon Hykes <solomon@docker.com>

inmem/inmem.go

@@ -161,8 +161,13 @@ func (r *PipeReceiver) Receive(mode int) (*beam.Message, beam.Receiver, beam.Sen
 		return nil, nil, nil, err
 	}
 	var (
-		in  beam.Receiver
-		out beam.Sender
+		// Always return NopReceiver/NopSender instead of nil values,
+		// because:
+		// - if they were requested in the mode, they can safely be used
+		// - if they were not requested, they can safely be ignored (ie no leak if they
+		// aren't closed)
+		in  beam.Receiver = beam.NopReceiver{}
+		out beam.Sender   = beam.NopSender{}
 	)
 	if pin != nil {
 		in = pin

inmem/inmem_test.go

@@ -9,6 +9,20 @@ import (
 	"testing"
 )
 
+func TestReceiveW(t *testing.T) {
+	r, w := Pipe()
+	go func() {
+		w.Send(&beam.Message{Name: "hello"}, 0)
+	}()
+	_, _, ww, err := r.Receive(beam.W)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, _, err := ww.Send(&beam.Message{Name: "this better not crash"}, 0); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func TestSimpleSend(t *testing.T) {
 	r, w := Pipe()
 	defer r.Close()
@@ -37,8 +51,8 @@ func TestSimpleSend(t *testing.T) {
 
 // assertMode verifies that the values of r and w match
 // mode.
-// If mode has the R bit set, r must be non-nil. Otherwise it must be nil.
-// If mode has the W bit set, w must be non-nil. Otherwise it must be nil.
+// If mode has the R bit set, r must be non-nil.
+// If mode has the W bit set, w must be non-nil.
 //
 // If any of these conditions are not met, t.Fatal is called and the active
 // test fails.
@@ -49,10 +63,6 @@ func assertMode(t *testing.T, r beam.Receiver, w beam.Sender, mode int) {
 			t.Fatalf("should be non-nil: %#v", r)
 		}
 		// Otherwise it must be nil.
-	} else {
-		if r != nil {
-			t.Fatalf("should be nil: %#v", r)
-		}
 	}
 	// If mode has the W bit set, w must be non-nil
 	if mode&beam.W != 0 {
@@ -60,10 +70,6 @@ func assertMode(t *testing.T, r beam.Receiver, w beam.Sender, mode int) {
 			t.Fatalf("should be non-nil: %#v", w)
 		}
 		// Otherwise it must be nil.
-	} else {
-		if w != nil {
-			t.Fatalf("should be nil: %#v", w)
-		}
 	}
 }
 

nop.go

@@ -0,0 +1,21 @@
+package beam
+
+import (
+	"io"
+)
+
+type NopSender struct{}
+
+func (s NopSender) Send(msg *Message, mode int) (Receiver, Sender, error) {
+	return NopReceiver{}, NopSender{}, nil
+}
+
+func (s NopSender) Close() error {
+	return nil
+}
+
+type NopReceiver struct{}
+
+func (r NopReceiver) Receive(mode int) (*Message, Receiver, Sender, error) {
+	return nil, nil, nil, io.EOF
+}

utils/buf.go

@@ -8,7 +8,7 @@ type Buffer []*beam.Message
 
 func (buf *Buffer) Send(msg *beam.Message, mode int) (beam.Receiver, beam.Sender, error) {
 	(*buf) = append(*buf, msg)
-	return NopReceiver{}, NopSender{}, nil
+	return beam.NopReceiver{}, beam.NopSender{}, nil
 }
 
 func (buf *Buffer) Close() error {

utils/nop.go

@@ -1,22 +0,0 @@
-package utils
-
-import (
-	"github.com/docker/beam"
-	"io"
-)
-
-type NopSender struct{}
-
-func (s NopSender) Send(msg *beam.Message, mode int) (beam.Receiver, beam.Sender, error) {
-	return NopReceiver{}, NopSender{}, nil
-}
-
-func (s NopSender) Close() error {
-	return nil
-}
-
-type NopReceiver struct{}
-
-func (r NopReceiver) Receive(mode int) (*beam.Message, beam.Receiver, beam.Sender, error) {
-	return nil, nil, nil, io.EOF
-}

utils/stack.go

@@ -38,7 +38,7 @@ func (s *StackSender) Send(msg *beam.Message, mode int) (r beam.Receiver, w beam
 		return r, w, err
 	}
 	// Silently drop messages if no valid backend is available.
-	return NopSender{}.Send(msg, mode)
+	return beam.NopSender{}.Send(msg, mode)
 }
 
 func (s *StackSender) Add(dst beam.Sender) *StackSender {