操作系统：发送syscall.SIGCONT时进程已经完成（可能的错误？）

Question

When executing a process and sending signals to it using: Process.Signal I notice that after sending the second signal syscall.SIGCONT I got a: os: process already finished but if using syscall.Kill everything works as expected.

For demonstrative purposes I have created this naive example:

package main

import (
    "fmt"
    "os"
    "os/exec"
    "syscall"
    "time"
)

func main() {
    exit := make(chan error, 1)
    go run(exit)

    for {
        select {
        case <-exit:
            println("fin, restarting")
            run(exit)
        default:
            time.Sleep(time.Second)
            println("running...")
        }
    }
}

func run(ch chan<- error) {
    cmd := exec.Command("sleep", "3")
    if err := cmd.Start(); err != nil {
        print(err.Error())
        os.Exit(1)
    }
    fmt.Printf("Pid: %d
", cmd.Process.Pid)
    go func() {
        ch <- cmd.Wait()
    }()

    time.Sleep(2 * time.Second)
    fmt.Printf("%v
", cmd.Process.Signal(syscall.SIGSTOP))

    time.Sleep(2 * time.Second)

    // Using this will return an os: process already finished  
    fmt.Printf("%v
", cmd.Process.Signal(syscall.SIGCONT)) 

    // This works as expected
    //fmt.Printf("%v
", syscall.Kill(cmd.Process.Pid, syscall.SIGCONT))
}

So basically if using:

cmd.Process.Signal(syscall.SIGCONT)

os: process already finished is returned

But when using:

syscall.Kill(cmd.Process.Pid, syscall.SIGCONT)

It works as expected.

Could this be a bug on os.exec or it the expected behavior?

UPDATE: seems to be happening only on mac os X

Answer 1

This issue seems to be only happening on Mac OS X, tested on "Sierra" and "El Capitan" https://go-review.googlesource.com/#/c/37610/

So, for now, to keep things cross-platform better user:

syscall.Kill(cmd.Process.Pid, syscall.SIGCONT))

Example code to test if your system has this issue:

package main

import (
    "fmt"
    "log"
    "os/exec"
    "syscall"
    "unsafe"
)

func main() {
    cmd := exec.Command("sleep", "10")
    if err := cmd.Start(); err != nil {
        log.Fatal(err)
    }

    // signal when wait4 will return immediately
    go func() {
        var siginfo [128]byte
        psig := &siginfo[0]
        _, _, e := syscall.Syscall6(syscall.SYS_WAITID, 1, uintptr(cmd.Process.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
        fmt.Println("WAITID RETURNED -- this shouldn't happen:", e)
    }()

    err := cmd.Process.Signal(syscall.SIGSTOP)
    if err != nil {
        log.Fatal(err)
    }
    cmd.Wait()
}