[go-nuts] How to correctly use exec.Command and properly clean up all pipes

[tomas . volf]

Hello,

I need to spawn a subprocess and I'm trying to use os/exec. I need to 
capture
stdout/err so I'm using Cmd.StdoutPipe and Cmd.StderrPipe . The problem I'm
having is how to correctly clean up the pipes after I am done. If Cmd.Start
works fine, all I need to do is read everything and then call Cmd.Wait, 
which
states that it will clean up ( https://golang.org/pkg/os/exec/#Cmd.Wait ):

> Wait releases any resources associated with the Cmd.

However, what I am not sure about is how to handle cases when Cmd.Start 
returns
an error. Nowhere in its documentation it's said that it cleans up the
resources, so originally I thought I have to do the cleanup myself. However,
after checking the source code it does seem to close the pipes when it 
fails:

https://golang.org/src/os/exec/exec.go?s=11462:11489#L374

And now I'm not sure what to do. io.Closer states that double-close is 
no-no:

> The behavior of Close after the first call is undefined.

so I cannot even close them myself (since they will be already closed). It 
seems
I have to rely on undocumented behaviour, but those have tendency to change 
and
I don't want to start leaking file descriptors after patch version 
update... I
assume there must be a way to solve this using just documented behaviour. 
Can
someone point me in a right direction?



Maybe to better show my problem, here it's in pseudo code (error handling
omitted):

        cmd := exec.Command(name, args...)
        stdout, _ := cmd.StdoutPipe()
        stderr, _ := cmd.StderrPipe()
        if err := cmd.Start(); err != nil {
                // --> How should I close stdout/stderr here to 
future-proof for
                //     changes in cmd.Start behavior?
                return err
        }
        consumeAll(stdout)
        consumeAll(stderr)
        cmd.Wait()
        return nil



Thank you,
Tomas Volf


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