math/rand panic:runtime error: index out of range

[rjcb99]

Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   go version go1.6.1 darwin/amd64
2. What operating system and processor architecture are you using (go env)?
   ubuntu server 12.04
3. What did you do?
   If possible, provide a recipe for reproducing the error.
   A complete runnable program is good.
   A link on play.golang.org is best.
   ps: this func run too many times by many goroutine and just panic once.
   // Random [m,n]
   func GetRandom(m, n int) int {
   if randnum == nil {
   randnum = rand.New(rand.NewSource(time.Now().UnixNano()))
   }
   if m > n {
   return 0
   }
   if m == n {
   return m
   }
   return m + randnum.Intn(n-m+1)
   }
4. What did you expect to see?
   return a expect rand int32
5. What did you see instead?
   panic: runtime error: index out of range

goroutine 1018903 [running]:
panic(0x941da0, 0xc8200100a0)
/usr/local/go/src/runtime/panic.go:464 +0x3e6
math/rand.(_rngSource).Int63(0xc82008d500, 0x0)
/usr/local/go/src/math/rand/rng.go:244 +0xc1
math/rand.(_Rand).Int63(0xc8200e5740, 0x0)
/usr/local/go/src/math/rand/rand.go:46 +0x39
math/rand.(_Rand).Int31(0xc8200e5740, 0xc82469dec8)
/usr/local/go/src/math/rand/rand.go:52 +0x21
math/rand.(_Rand).Int31n(0xc8200e5740, 0x5, 0xc82469dec0)
/usr/local/go/src/math/rand/rand.go:87 +0xc1
math/rand.(*Rand).Intn(0xc8200e5740, 0x5, 0x0)
/usr/local/go/src/math/rand/rand.go:101 +0x92
utils.GetRandom(0x3, 0x7, 0x1)

[dsnet]

Have you tried running this with the race detector using the -race flag?

The example you posted is racy:

// Random [m,n]
func GetRandom(m, n int) int {
    if randnum == nil {
        randnum = rand.New(rand.NewSource(time.Now().UnixNano())) // This line is not concurrent safe!
    }
    if m > n {
        return 0
    }
    if m == n {
        return m
    }
    return m + randnum.Intn(n-m+1)
}

I'm going to close this for now. Let us know if you discover it turns out to be a legitimate issue.