func (d *Daemon) monitorExec(container *container.Container, execConfig *exec.Config, callback execdriver.DriverCallback) error {
	pipes := execdriver.NewPipes(execConfig.Stdin(), execConfig.Stdout(), execConfig.Stderr(), execConfig.OpenStdin)
	exitCode, err := d.Exec(container, execConfig, pipes, callback)
	if err != nil {
		logrus.Errorf("Error running command in existing container %s: %s", container.ID, err)
	}
	logrus.Debugf("Exec task in container %s exited with code %d", container.ID, exitCode)

	if err := execConfig.CloseStreams(); err != nil {
		logrus.Errorf("%s: %s", container.ID, err)
	}

	if execConfig.ProcessConfig.Terminal != nil {
		if err := execConfig.WaitResize(); err != nil {
			logrus.Errorf("Error waiting for resize: %v", err)
		}
		if err := execConfig.ProcessConfig.Terminal.Close(); err != nil {
			logrus.Errorf("Error closing terminal while running in container %s: %s", container.ID, err)
		}
	}
	// remove the exec command from the container's store only and not the
	// daemon's store so that the exec command can be inspected.
	container.ExecCommands.Delete(execConfig.ID)
	return err
}

// Exec calls the underlying exec driver to run
func (d *Daemon) Exec(c *container.Container, execConfig *exec.Config, pipes *execdriver.Pipes, startCallback execdriver.DriverCallback) (int, error) {
	hooks := execdriver.Hooks{
		Start: startCallback,
	}
	exitStatus, err := d.execDriver.Exec(c.Command, execConfig.ProcessConfig, pipes, hooks)

	// On err, make sure we don't leave ExitCode at zero
	if err != nil && exitStatus == 0 {
		exitStatus = 128
	}

	execConfig.ExitCode = &exitStatus
	execConfig.Running = false

	return exitStatus, err
}

func (d *Daemon) containerExec(container *container.Container, ec *exec.Config) error {
	container.Lock()
	defer container.Unlock()

	callback := func(processConfig *execdriver.ProcessConfig, pid int, chOOM <-chan struct{}) error {
		if processConfig.Tty {
			// The callback is called after the process Start()
			// so we are in the parent process. In TTY mode, stdin/out/err is the PtySlave
			// which we close here.
			if c, ok := processConfig.Stdout.(io.Closer); ok {
				c.Close()
			}
		}
		ec.Close()
		return nil
	}

	// We use a callback here instead of a goroutine and an chan for
	// synchronization purposes
	cErr := promise.Go(func() error { return d.monitorExec(container, ec, callback) })
	return ec.Wait(cErr)
}