From ca6de008bae698dd79adf795cd24668f36578d77 Mon Sep 17 00:00:00 2001
From: Roger Peppe <rogpeppe@gmail.com>
Date: Wed, 30 Nov 2011 09:29:58 -0800
Subject: [PATCH] math/big: fix destination leak into result value This code
 would panic: z := big.NewInt(1) z.SetBit(big.NewInt(0), 2, 1) if
 z.Cmp(big.NewInt(1<<2)) != 0 {         panic("fail") }

R=rsc, gri
CC=golang-dev
https://golang.org/cl/5437081
---
 src/pkg/math/big/int_test.go | 8 ++++++--
 src/pkg/math/big/nat.go      | 6 ++++--
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/src/pkg/math/big/int_test.go b/src/pkg/math/big/int_test.go
index 163c662b0bb..aa7c1949549 100644
--- a/src/pkg/math/big/int_test.go
+++ b/src/pkg/math/big/int_test.go
@@ -1242,10 +1242,14 @@ func TestBitSet(t *testing.T) {
 		x.SetString(test.x, 0)
 		b := x.Bit(test.i)
 		if b != test.b {
-
-			t.Errorf("#%d want %v got %v", i, test.b, b)
+			t.Errorf("#%d got %v want %v", i, b, test.b)
 		}
 	}
+	z := NewInt(1)
+	z.SetBit(NewInt(0), 2, 1)
+	if z.Cmp(NewInt(4)) != 0 {
+		t.Errorf("destination leaked into result; got %s want 4", z)
+	}
 }
 
 func BenchmarkBitset(b *testing.B) {
diff --git a/src/pkg/math/big/nat.go b/src/pkg/math/big/nat.go
index 9fba2d2a062..680445dc9a7 100644
--- a/src/pkg/math/big/nat.go
+++ b/src/pkg/math/big/nat.go
@@ -1065,9 +1065,11 @@ func (z nat) setBit(x nat, i uint, b uint) nat {
 		return z.norm()
 	case 1:
 		if j >= n {
-			n = j + 1
+			z = z.make(j + 1)
+			z[n:].clear()
+		} else {
+			z = z.make(n)
 		}
-		z = z.make(n)
 		copy(z, x)
 		z[j] |= m
 		// no need to normalize