math/big: Rand shouldn't hang if argument is also receiver.

Fixes #2607.

R=rsc
CC=golang-dev
https://golang.org/cl/5489109

src/pkg/math/big/int_test.go

@@ -9,6 +9,7 @@ import (
 	"encoding/gob"
 	"encoding/hex"
 	"fmt"
+	"math/rand"
 	"testing"
 	"testing/quick"
 )
@@ -1405,3 +1406,9 @@ func TestIntGobEncoding(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue2607(t *testing.T) {
+	// This code sequence used to hang.
+	n := NewInt(10)
+	n.Rand(rand.New(rand.NewSource(9)), n)
+}

src/pkg/math/big/nat.go

@@ -1196,12 +1196,16 @@ func (x nat) powersOfTwoDecompose() (q nat, k int) {
 // random creates a random integer in [0..limit), using the space in z if
 // possible. n is the bit length of limit.
 func (z nat) random(rand *rand.Rand, limit nat, n int) nat {
+	if alias(z, limit) {
+		z = nil // z is an alias for limit - cannot reuse
+	}
+	z = z.make(len(limit))
+
 	bitLengthOfMSW := uint(n % _W)
 	if bitLengthOfMSW == 0 {
 		bitLengthOfMSW = _W
 	}
 	mask := Word((1 << bitLengthOfMSW) - 1)
-	z = z.make(len(limit))
 
 	for {
 		for i := range z {