go: os/exec: race on cmd.Wait() might lead to panic

The current implementation of cmd.Wait has a race condition: if multiple goroutines call it, it might cause a panic.

In the first part of the method, copied below, two concurrent calls might check c.finished, get false, set it to true, invoke c.Process.Wait() and close c.waitDone before any error checking is performed. c.waitDone is a chan struct{}, and a double close will cause a panic.

func (c *Cmd) Wait() error {
	if c.Process == nil {
		return errors.New("exec: not started")
	}
	if c.finished {
		return errors.New("exec: Wait was already called")
	}
	c.finished = true

	state, err := c.Process.Wait()
	if c.waitDone != nil {
		close(c.waitDone)
	}
	//[...]

Since waiting is a synchronization primitive I’d expect one of the two:

• The documentation should state that this is not safe for concurrent use (probably the best approach here)
• Some form of synchronization to prevent races. I would either atomically CAS c.finished (but I’m not a fan of atomics and would require to change type to some sort of int) or protect c with a mutex, which would be my suggested solution for this case

I would happily send in CLs in both cases.