@@ -157,7 +157,7 @@ func (d *Daemon) ContainerExecStart(name string, stdin io.ReadCloser, stdout io.

 	logrus.Debugf("starting exec command %s in container %s", ec.ID, c.ID)

 	d.LogContainerEvent(c, "exec_start: "+ec.ProcessConfig.Entrypoint+" "+strings.Join(ec.ProcessConfig.Arguments, " "))

-	if ec.OpenStdin {

+	if ec.OpenStdin && stdin != nil {

 		r, w := io.Pipe()

 		go func() {

 			defer w.Close()

@@ -122,6 +122,36 @@ func (s *DockerSuite) TestExecApiStartMultipleTimesError(c *check.C) {

 	startExec(c, execID, http.StatusConflict)

 }

+// #20638

+func (s *DockerSuite) TestExecApiStartWithDetach(c *check.C) {

+	name := "foo"

+	dockerCmd(c, "run", "-d", "-t", "--name", name, "busybox", "top")

+	data := map[string]interface{}{

+		"cmd":         []string{"true"},

+		"AttachStdin": true,

+	}

+	_, b, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), data)

+	c.Assert(err, checker.IsNil, check.Commentf(string(b)))

+

+	createResp := struct {

+		ID string `json:"Id"`

+	}{}

+	c.Assert(json.Unmarshal(b, &createResp), checker.IsNil, check.Commentf(string(b)))

+

+	_, body, err := sockRequestRaw("POST", fmt.Sprintf("/exec/%s/start", createResp.ID), strings.NewReader(`{"Detach": true}`), "application/json")

+	c.Assert(err, checker.IsNil)

+

+	b, err = readBody(body)

+	comment := check.Commentf("response body: %s", b)

+	c.Assert(err, checker.IsNil, comment)

+

+	resp, _, err := sockRequestRaw("GET", "/_ping", nil, "")

+	c.Assert(err, checker.IsNil)

+	if resp.StatusCode != http.StatusOK {

+		c.Fatal("daemon is down, it should alive")

+	}

+}

+

 func createExec(c *check.C, name string) string {

 	_, b, err := sockRequest("POST", fmt.Sprintf("/containers/%s/exec", name), map[string]interface{}{"Cmd": []string{"true"}})

 	c.Assert(err, checker.IsNil, check.Commentf(string(b)))