One might argue that Run should respect the context deadline even in the
presence of pipes that don't EOF.
On 29 Sep 2017 22:26, "Ian Lance Taylor" <i...@golang.org> wrote:
> On Fri, Sep 29, 2017 at 12:33 PM, Alex Buchanan <buchanae.o...@gmail.com>
> wrote:
> >
> > package main
> >
> > import (
> > "bytes"
> > "context"
> > "log"
> > "os/exec"
> > "time"
> > )
> >
> > func main() {
> > var stdout bytes.Buffer
> >
> > ctx, cancel := context.WithTimeout(context.Background(), time.Second)
> > defer cancel()
> >
> > cmd := exec.CommandContext(ctx, "/bin/sh", "-c", "sleep 10; echo foo")
> > cmd.Stdout = &stdout
> >
> > err := cmd.Run()
> > log.Printf("%s, %s, '%s'", err, ctx.Err(), stdout.String())
> > }
> >
> >
> > This runs for 10 seconds, but I was expecting 1 second. Can someone help
> me
> > understand what is happening? I think it has something to do with the
> stdout
> > bytes.Buffer?
>
> Yes. The problem is that when the context passed to CommandContext
> expires, it kills the process started by the command, but does not
> kill any subprocesses that that command may have started. So when the
> context expires the /bin/sh is killed, but the sleep subprocess is
> still running. Because you use a bytes.Buffer, the program has
> created a pipe to capture the standard output of the command. After
> the process dies, cmd.Run is waiting for that pipe to be closed to
> make sure that it gathers all the data. But the pipe is held open by
> the sleep subprocess. It is only after that subprocess exits that the
> pipe is closed and your program continues.
>
> Ian
>
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