Panic in other goroutine not stopping child process

Question

I need to run a long-running child process and kill it if I quit (for any reason) out of parent application.

Here is the code:

cmd := exec.Command("./long-process")

defer cmd.Process.Kill()

if err != nil {
    log.Fatal(err)
}

var fail io.ReadCloser
fail.Close()

The fail here produces obvious

panic: runtime error: invalid memory address or nil pointer dereference

It works as expected - the child process is killed.

But this happens in a goroutine:

cmd := exec.Command("./long-process")

defer cmd.Process.Kill()

if err != nil {
    log.Fatal(err)
}

go func() {
    var fail io.ReadCloser
    fail.Close()
}()

The panic still happens, but then it seems defer is not called and the child process is not killed.

Any way to go around this?

UPDATE I need a cross-platform solution (at least for Linux and FreeBSD)

Minimal example:

infinite-loop.sh

#!/bin/bash

while true; do
  sleep 1
done

Don't forget to

chmod +x infinite-loop.sh

test1.go (error checking left out for brevity):

package main

import (
    "time"
    "io"
    "os/exec"
    "runtime"
)

func main() {

    cmd := exec.Command("./infinite-loop.sh")

    cmd.Start()

    defer cmd.Process.Kill()

    go func() {
        time.Sleep(100 * time.Millisecond)
        var fail io.ReadCloser
        fail.Close()
    }()

    for {
        runtime.Gosched()
    }
}

Let's run

ps aux | grep infinite-loop.sh | grep -v grep | wc -l; \
go run test1.go; \
ps aux | grep infinite-loop.sh | grep -v grep | wc -l


     0 <--- !!


panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xb code=0x1 addr=0x20 pc=0x2130]

goroutine 5 [running]:
main.main.func1()
.../multiline/test1.go:19 +0x30
created by main.main
.../multiline/test1.go:20 +0x9a

goroutine 1 [runnable]:
runtime.Gosched()
/usr/local/Cellar/go/1.5.1/libexec/src/runtime/proc.go:166 +0x14
main.main()
.../multiline/test1.go:23 +0x9f
exit status 2

     1 <--- !!

0 processes before and 1 after exit.

If you comment out goroutine code - it works fine.

Now we can kill it:

kill $(ps aux | grep infinite-loop.sh | grep -v grep | awk {'print $2'})

you don't want to defer like that, since if the function returns before the cmd starts, Process will be nil, and you get another panic. — JimB, Dec 04 '15 at 19:19
Please add a [minimal, complete and verifiable example](http://www.stackoverflow.com/help/mcve). — Markus W Mahlberg, Dec 04 '15 at 19:20

JimB · Accepted Answer · 2015-12-04T19:42:49.240

5

There's no cross-platform solution to automatically kill a child process.

On Linux, you can use the pdeathsig functionality:

cmd := exec.Command("./long-process")

cmd.SysProcAttr = &syscall.SysProcAttr{
    Pdeathsig: syscall.SIGTERM,
}

On other platforms, the child needs to determine when to exit on its own. One way is to monitor a pipe or socket FD given to it from the parent. You could also have a process manager of some sort monitor the processes and cleanup if something goes wrong.

In general though, panics should be rare and get fixed. If you do have areas of code that are prone to panic'ing, you can recover locally and call for the cleanup of child processes before exiting.

edited Dec 04 '15 at 19:42

answered Dec 04 '15 at 19:18

JimB

104,193
13
262
255

Yes, sorry, forgot to mention - I need a cross-platform solution. But, thanks! – Slava V Dec 04 '15 at 19:19
There is no cross-platform solution. An old cross-platform trick was to have a lifeline socket fd given to the child process, and have the child exit when it closes, but that requires cooperation with the child process of course. – JimB Dec 04 '15 at 19:23
@JimB this bit of code is necessary, but not sufficient to guarantee that the child stays around. See discussion at https://github.com/golang/go/issues/27505, including a full example that guarantees the child stays alive. – Dustin Spicuzza Oct 21 '20 at 16:51

Panic in other goroutine not stopping child process

1 Answers1

Linked