Race condition with Exec

[charmes....@gmail.com]

Hello,

I am trying to monitor a process using os/exec.Cmd but when I try to Wait(), I always face a race.

I have a very basic code that works fine:

http://play.golang.org/p/_8XoRoimOI

When I quit cat with ctrl-d, the program exits as expected.

However, when I try to close manually, the race detector is not happy:

http://play.golang.org/p/zDihMzWwUj

This is exactly the same thing except that after 5 seconds I close stdin.

Then I tried using the StdinPipe() from Cmd, but still not luck:

http://play.golang.org/p/UfaX8dQbdR

Is there a way to do Cmd.Wait() and close Stdin without a race?

Thanks in advance.

Regards,

-- 

Guillaume J. Charmes

[Dmitry Vyukov]

I think you need to close the file after starting the process.

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[Guillaume J. Charmes]

I am trying to monitor the process. I need to close stdin in a different goroutine, so I need to Cmd.Wait() on the process.

-- 
Guillaume J. Charmes

[Dmitry Vyukov]

I mean:
cmd.Start()
time.AfterFunc(5*time.Second, func() { os.Stdin.Close() })
cmd.Wait()

Otherwise you are closing the pipe before starting the process, and
cmd.Start can not even reliably read fd that you are passing in.

[Guillaume J. Charmes]

Indeed, my bad on this example.

However, in the last example, the close is done after the start.

http://play.golang.org/p/UfaX8dQbdR

-- 
Guillaume J. Charmes

[Dmitry Vyukov]

On Thu, Aug 1, 2013 at 1:56 AM, Guillaume J. Charmes
<charmes....@gmail.com> wrote:
> Indeed, my bad on this example.
>
> However, in the last example, the close is done after the start.
>
> http://play.golang.org/p/UfaX8dQbdR

The following works:

cmd := exec.Command("zcat", "-f")
cmd.Stdout = os.Stdout
cmd.Stdin = os.Stdin
cmd.Start()
cmd.Wait()

There is not much you can do with stdin/stdout while the command runs,
because it accesses them concurrently. OR your stdin/stdout must be
completely thread-safe, e.g. net.Conn.

[Dmitry Vyukov]

I've filed https://code.google.com/p/go/issues/detail?id=6008
to clarify the documentation. I do not understand it all as well.

[roger peppe]

See issue 4290.
I agree entirely - this is highly unintuitive behaviour
but we can't change it now. Suggestions for
documentation re-wording would be good.