dial-stdio: handle connections which lack CloseRead method.

This happens on Windows when dialing a named pipe (a path which is used by CLI
plugins), in that case some debugging shows:

    DEBU[0000] conn is a *winio.win32MessageBytePipe
    DEBU[0000] conn is a halfReadCloser: false
    DEBU[0000] conn is a halfWriteCloser: true
    the raw stream connection does not implement halfCloser
In such cases we can simply wrap with a nop function since closing for read
isn't too critical.

Signed-off-by: Ian Campbell <ijc@docker.com>

cli/command/system/dial_stdio.go

@@ -34,10 +34,17 @@ func runDialStdio(dockerCli command.Cli) error {
 	if err != nil {
 		return errors.Wrap(err, "failed to open the raw stream connection")
 	}
-	connHalfCloser, ok := conn.(halfCloser)
-	if !ok {
+
+	var connHalfCloser halfCloser
+	switch t := conn.(type) {
+	case halfCloser:
+		connHalfCloser = t
+	case halfReadWriteCloser:
+		connHalfCloser = &nopCloseReader{t}
+	default:
 		return errors.New("the raw stream connection does not implement halfCloser")
 	}
+
 	stdin2conn := make(chan error)
 	conn2stdout := make(chan error)
 	go func() {
@@ -90,6 +97,19 @@ type halfCloser interface {
 	halfWriteCloser
 }
 
+type halfReadWriteCloser interface {
+	io.Reader
+	halfWriteCloser
+}
+
+type nopCloseReader struct {
+	halfReadWriteCloser
+}
+
+func (x *nopCloseReader) CloseRead() error {
+	return nil
+}
+
 type halfReadCloserWrapper struct {
 	io.ReadCloser
 }