On Sun, Oct 1, 2017 at 2:40 PM, roger peppe <rogpe...@gmail.com> wrote:
> One might argue that Run should respect the context deadline even in the
> presence of pipes that don't EOF.
Fair point.
Ian
> 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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