package main

import (
	"fmt"
	"syscall"
	_ "unsafe" // to use go:linkname
)

func main() {
	fmt.Println("start")
	pid2, err := testVfork()
	if err != syscall.Errno(0) {
		fmt.Printf("%s\n", err)
	}
	syscall.Wait4(pid2, nil, 0, nil)
	fmt.Println("finished")
}

//go:norace
func testVfork() (pid int, err syscall.Errno) {
	upid, err := testVforkChild()
	afterFork()
	if err != 0 {
		return 0, err
	}
	pid = int(upid)
	return pid, 0
}

//go:noinline
//go:norace
//go:nocheckptr
func testVforkChild() (pid uintptr, err1 syscall.Errno) {
	var flags uintptr = syscall.CLONE_VFORK | syscall.CLONE_VM | uintptr(syscall.SIGCHLD)

	beforeFork()
	pid, err1 = rawVforkSyscall(syscall.SYS_CLONE, flags, 0, 0)
	if err1 != 0 || pid != 0 {
		return
	}
	afterForkInChild()

	// child
	syscall.RawSyscall(syscall.SYS_EXIT, 0, 0, 0)

	// should not reach... however reached by parent
	for {
		syscall.RawSyscall(syscall.SYS_EXIT_GROUP, 253, 0, 0)
	}
}

//go:linkname beforeFork syscall.runtime_BeforeFork
func beforeFork()

//go:linkname afterFork syscall.runtime_AfterFork
func afterFork()

//go:linkname afterForkInChild syscall.runtime_AfterForkInChild
func afterForkInChild()

//go:linkname rawVforkSyscall syscall.rawVforkSyscall
func rawVforkSyscall(trap, a1, a2, a3 uintptr) (r1 uintptr, err syscall.Errno)