How can I execute a long-running process in Golang and detach it from my program while controlling the user, redirecting output, and preventing it from becoming a zombie?

Running a Command in Go and Detaching It from the Process

Problem:

You want to execute a long-running process in Golang while meeting several requirements:

• Redirect the process's standard output to a file
• Control the user of the process
• Prevent the process from dying when your program exits
• Avoid the process becoming a zombie
• Obtain the PID of the running process

Attempted Solution:

You have attempted a solution using exec.Command but it doesn't meet all the requirements, particularly when your program receives SIGTERM/SIGKILL signals.

Key Point to Understand:

It's important to note that you cannot change the parent of a process once it has been started - a process's parent-child relationship is fixed.

Alternative Libraries:

Instead of reinventing the wheel, it's recommended to use an existing library that solves this problem. The following libraries are suggested:

• https://github.com/hashicorp/go-reap
• https://github.com/krallin/tini
• https://busybox.net/
• https://software.clapper.org/daemonize/
• https://wiki.gentoo.org/wiki/OpenRC
• https://www.freedesktop.org/wiki/Software/systemd/

Example with go-reap:

<code class="go">import (
    "fmt"
    "os"
    "os/exec"
    "strings"
    "sync"
    "time"

    "github.com/fatih/color"
    "github.com/hashicorp/go-reap"
)

func main() {

    if reap.IsSupported() {
        done := make(chan struct{})
        var reapLock sync.RWMutex
        pids := make(reap.PidCh, 1)

        errors := make(reap.ErrorCh, 1)
        go reap.ReapChildren(pids, errors, done, &reapLock)
        go report(pids, errors, done)

        // Here is where you would start your long-running process
        Sh()

        close(done)
    } else {
        fmt.Println("Sorry, go-reap isn't supported on your platform.")
    }
}

func report(pids reap.PidCh, errors reap.ErrorCh, done chan struct{}) {

    sprintf := color.New(color.FgWhite, color.Bold).SprintfFunc()

    for ;; {
        select {
        case pid := <-pids:
            println(sprintf(&quot;raeper pid %d&quot;, pid))
        case err := <-errors:
            println(sprintf(&quot;raeper er %s&quot;, err))
        case <-done:
            return
        }
    }
}

func Sh() {

    args := os.Args[1:]
    script := args[0:0]
    if len(args) >= 1 {
        if args[0] == &quot;-c&quot; {
            script = args[1:]
        }
    }
    if len(script) == 0 {
        fn.CyanBold(&quot;cmd: expecting sh -c 'foobar'&quot;)
        os.Exit(111)
    }

    var cmd *exec.Cmd
    parts, _ := shlex.Split(strings.Join(script, &quot; &quot;))
    if len(parts) >= 2 {
        cmd = fn.Merge(exec.Command(parts[0], parts[1:]...), nil)
    }
    if len(parts) == 1 {
        cmd = fn.Merge(exec.Command(parts[0]), nil)
    }

    // ... Here you can customize how the process is started and controlled

    if fn.IfEnv(&quot;HANG&quot;) {
        fn.CyanBold(&quot;cmd: %v\n      start&quot;, parts)
        ex := cmd.Start()
        if ex != nil {
            fn.CyanBold(&quot;cmd %v err: %s&quot;, parts, ex)
        }
        go func() {
            time.Sleep(time.Millisecond * 100)
            errw := cmd.Wait()
            if errw != nil {
                fn.CyanBold(&quot;cmd %v err: %s&quot;, parts, errw)
            } else {
                fn.CyanBold(&quot;cmd %v all done.&quot;, parts)
            }
        }()

        fn.CyanBold(&quot;cmd: %v\n      dispatched, hanging forever (i.e. to keep docker running)&quot;, parts)
        for {
            time.Sleep(time.Millisecond * time.Duration(fn.EnvInt(&quot;HANG&quot;, 2888)))
            fn.SystemCyan(&quot;/bin/ps&quot;, &quot;-e&quot;, &quot;-o&quot;, &quot;stat,comm,user,etime,pid,ppid&quot;)
        }

    } else {

        if fn.IfEnv(&quot;NOWAIT&quot;) {
            ex := cmd.Start()
            if ex != nil {
                fn.CyanBold(&quot;cmd %v start err: %s&quot;, parts, ex)
            }
        } else {

            ex := cmd.Run()
            if ex != nil {
                fn.CyanBold(&quot;cmd %v run err: %s&quot;, parts, ex)
            }
        }
        fn.CyanBold(&quot;cmd %v\n      dispatched, exit docker.&quot;, parts)
    }
}</code>

This example uses go-reap to start a shell (Sh() function) and execute a command within that shell. It sets up the reaper to handle cleaning up the child processes.

By using a reputable library, you can avoid common pitfalls and ensure that your application behaves as intended.

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