cgo race condition example

README.md

Ran on a laptop with 4 cores.

$ go version
go version go1.10.1 linux/amd64
$ go run cgo.go 
panic: Iteration 39068207 - receive: "y" != "\x00"

goroutine 5 [running]:
main.main.func1()
        /home/bob/stuff/go/playground/cgo.go:35 +0x156
created by main.main
        /home/bob/stuff/go/playground/cgo.go:28 +0x47
exit status 2
$ go run cgo.go 
panic: Iteration 44789915 - execute: "y" != "\xd0"

goroutine 1 [running]:
main.main()
        /home/bob/stuff/go/playground/cgo.go:46 +0x16e
exit status 2
$ go run cgo.go 
panic: Iteration 1963975 - receive: "y" != "\x00"

goroutine 5 [running]:
main.main.func1()
        /home/bob/stuff/go/playground/cgo.go:35 +0x156
created by main.main
        /home/bob/stuff/go/playground/cgo.go:28 +0x47
exit status 2
$ go run cgo.go 
panic: Iteration 56646123 - execute: "y" != "\xd0"

goroutine 1 [running]:
main.main()
        /home/bob/stuff/go/playground/cgo.go:46 +0x16e
exit status 2
$

cgo.go

package main

import "fmt"

// #include <stdlib.h>
// #include <assert.h>
// static __thread char *buf = NULL;
// char * free_and_allocate() {
//   if (buf) {
//     buf[0] = 'x';
//     free(buf);
//   }
//   buf = malloc(sizeof(char) * 2);
//   assert(buf != NULL);
//   buf[0] = 'y';
//   buf[1] = 0;
//   return buf;
// }
// char * receive() {
//   return free_and_allocate();
// }
// char * execute() {
//   return free_and_allocate();
// }
import "C"

func main() {
  go func () {
    i := 0
    for {
      i++
      result := C.receive()
      s := C.GoString(result)
      if s != "y" {
        panic(fmt.Sprintf("Iteration %v - receive: \"y\" != %#v", i, s))
      }
    }
  }()

  i := 0
  for {
    i++
    result := C.execute();
    s := C.GoString(result)
    if s != "y" {
      panic(fmt.Sprintf("Iteration %v - execute: \"y\" != %#v", i, s))
    }
  }
}