cancellable: use sync.Once to prevent double channel close

Signed-off-by: Alexander Morozov <lk4d4math@gmail.com>

client/transport/cancellable/cancellable.go

@@ -8,6 +8,7 @@ package cancellable
 import (
 	"io"
 	"net/http"
+	"sync"
 
 	"github.com/docker/engine-api/client/transport"
 
@@ -82,7 +83,7 @@ func Do(ctx context.Context, client transport.Sender, req *http.Request) (*http.
 			// The response's Body is closed.
 		}
 	}()
-	resp.Body = &notifyingReader{resp.Body, c}
+	resp.Body = &notifyingReader{ReadCloser: resp.Body, notify: c}
 
 	return resp, nil
 }
@@ -91,23 +92,24 @@ func Do(ctx context.Context, client transport.Sender, req *http.Request) (*http.
 // Close is called or a Read fails on the underlying ReadCloser.
 type notifyingReader struct {
 	io.ReadCloser
-	notify chan<- struct{}
+	notify     chan<- struct{}
+	notifyOnce sync.Once
 }
 
 func (r *notifyingReader) Read(p []byte) (int, error) {
 	n, err := r.ReadCloser.Read(p)
-	if err != nil && r.notify != nil {
-		close(r.notify)
-		r.notify = nil
+	if err != nil {
+		r.notifyOnce.Do(func() {
+			close(r.notify)
+		})
 	}
 	return n, err
 }
 
 func (r *notifyingReader) Close() error {
 	err := r.ReadCloser.Close()
-	if r.notify != nil {
+	r.notifyOnce.Do(func() {
 		close(r.notify)
-		r.notify = nil
-	}
+	})
 	return err
 }