How to implement fork syscall in Golang?

ref: https://github.com/moby/moby/tree/master/pkg/reexec

We don’t have a fork syscall in Golang, we have:

All those three functions is like a combination of fork + exec, but there has no pure fork syscall just like in C programming language(after syscall, which will return pid in caller). Reasons can be found in here:

It mainly says：

fork() has been invented at the time when no threads were used at all, and a process had always had just a single thread of execution in it, and hence forking it was safe.
In C, you control all the threads by your hand, but in Go, you cannot, so threads will be out of control after call fork without exec, so Go provides fork + exec only.

Let’s have a look at how we use pure fork in C:

#include <sys/types.h>
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>

void child() {
    printf("child process\n");
}

int main() {
    printf("main process\n");
    pid_t pid = fork();
    int wstatus;

    if (pid == 0) {
        child();
    } else {
        printf("main exit\n");
        waitpid(pid, &wstatus, 0);
    }
}

Run it：

$ gcc main.c && ./a.out
main process
main exit
child process

Let’s look how can we implements this in Go:

package main

import (
	"log"
	"os"

	"github.com/docker/docker/pkg/reexec"
)

func init() {
	log.Printf("init start, os.Args = %+v\n", os.Args)
	reexec.Register("childProcess", childProcess)
	if reexec.Init() {
		os.Exit(0)
	}
}

func childProcess() {
	log.Println("childProcess")
}

func main() {
	log.Printf("main start, os.Args = %+v\n", os.Args)
	cmd := reexec.Command("childProcess")
	cmd.Stdin = os.Stdin
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	if err := cmd.Start(); err != nil {
		log.Panicf("failed to run command: %s", err)
	}
	if err := cmd.Wait(); err != nil {
		log.Panicf("failed to wait command: %s", err)
	}
	log.Println("main exit")
}

Run it：

$ go run main.go
2018/03/08 19:52:39 init start, os.Args = [/tmp/go-build209640177/b001/exe/main]
2018/03/08 19:52:39 main start, os.Args = [/tmp/go-build209640177/b001/exe/main]
2018/03/08 19:52:39 init start, os.Args = [childProcess]
2018/03/08 19:52:39 childProcess
2018/03/08 19:52:39 main exit

Explanation:

init will be execute before main function. When you execute the binary executable file from command line, os.Args[0] will be the name of binary executable file, but, reexec.Command will change os.Args[0], so child process will find function registed by reexec.Register, and execute it, return true, then call os.Exit(0).

How to implement fork syscall in Golang?

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