os/exec: shell command can't be killed when output redirected with cmd.exe

[math2001]

When I run a shell command with cmd.exe and redirect either the stdout or the stderr, the command cannot be killed (it says that it has been killed once the command is done, but it keeps running).

Here's the code (try to comment out cmd.Stdout =... and cmd.Stderr =...):

package main

import "os/exec"
import "fmt"
import "time"
import "bytes"

func main() {
    cmd := exec.Command("cmd.exe", "/C", "sleep 2")
    var stdout bytes.Buffer
    var stderr bytes.Buffer

    cmd.Stdout = &stdout
    cmd.Stderr = &stderr

    start := time.Now()
    fmt.Println(cmd.Start())
    timer := time.AfterFunc(1 * time.Second, func () {
        fmt.Println("killed", cmd.Process.Kill())

    })
    fmt.Println(cmd.Wait())
    timer.Stop()
    fmt.Println(time.Since(start))
}

It outputs this:

<nil>
killed <nil>
exit status 1
2.0313611s

And without the redirects, the output is:

<nil>
killed <nil>
exit status 1
1.0205677s

System details

go version go1.9.1 windows/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=".exe"
GOHOSTARCH="amd64"
GOHOSTOS="windows"
GOOS="windows"
GOPATH="C:\Users\Home\gopackages"
GORACE=""
GOROOT="C:\Go"
GOTOOLDIR="C:\Go\pkg\tool\windows_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-m64 -mthreads -fmessage-length=0"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOROOT/bin/go version: go version go1.9.1 windows/amd64
GOROOT/bin/go tool compile -V: compile version go1.9.1

[karkason]

The issue here is how Windows Named Pipes react to handles inheritence.

When you invoke cmd /c "sleep 2" you launch a cmd.exe which in turn launches sleep.exe (by the way, that's not a native Windows exectuable - it's from a MinGW and the likes).

Cmd.Start launches an I/O loop copying data from an internally-managed os.Pipe to the stdout/stderr io.Writers (the writerDescriptor function adds it to the queue).
The Named Pipe handle created by os.Pipe is inherited by the sleep.exe. When calling Process.Kill the cmd.exe is in fact killed, but the child sleep.exe isn't, and because it still has the inherited pipe it keeps on running until it's done.

So if we want it to properly function we have a couple of options:

• Add a method: Cmd.Kill which will make sure we are properly closing the Pipe handle on our side (which would render the Pipe unusable)
• Add a way to register extra handles (or fds) to close when calling Process.Kill

[ianlancetaylor]

See #23019 and the issues to which it refers. Closing as dup.