math/big: make Rat accessors safe for concurrent use

Do not modify the underlying Rat denominator when calling
one of the accessors Float32, Float64; verify that we don't
modify the Rat denominator when calling Inv, Sign, IsInt, Num.

Fixes #34919.
Reopens #33792.

Change-Id: Ife6d1252373f493a597398ee51e7b5695b708df5
Reviewed-on: https://go-review.googlesource.com/c/go/+/201205Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/big/rat.go

@@ -271,7 +271,7 @@ func quotToFloat64(a, b nat) (f float64, exact bool) {
 func (x *Rat) Float32() (f float32, exact bool) {
 	b := x.b.abs
 	if len(b) == 0 {
-		b = b.set(natOne) // materialize denominator
+		b = natOne
 	}
 	f, exact = quotToFloat32(x.a.abs, b)
 	if x.a.neg {
@@ -287,7 +287,7 @@ func (x *Rat) Float32() (f float32, exact bool) {
 func (x *Rat) Float64() (f float64, exact bool) {
 	b := x.b.abs
 	if len(b) == 0 {
-		b = b.set(natOne) // materialize denominator
+		b = natOne
 	}
 	f, exact = quotToFloat64(x.a.abs, b)
 	if x.a.neg {
@@ -377,7 +377,7 @@ func (z *Rat) Inv(x *Rat) *Rat {
 	z.Set(x)
 	a := z.b.abs
 	if len(a) == 0 {
-		a = a.set(natOne) // materialize numerator
+		a = a.set(natOne) // materialize numerator (a is part of z!)
 	}
 	b := z.a.abs
 	if b.cmp(natOne) == 0 {
@@ -418,7 +418,7 @@ func (x *Rat) Num() *Int {
 func (x *Rat) Denom() *Int {
 	x.b.neg = false // the result is always >= 0
 	if len(x.b.abs) == 0 {
-		x.b.abs = x.b.abs.set(natOne) // materialize denominator
+		x.b.abs = x.b.abs.set(natOne) // materialize denominator (see issue #33792)
 	}
 	return &x.b
 }

src/math/big/rat_test.go

@@ -678,3 +678,29 @@ func BenchmarkRatCmp(b *testing.B) {
 		x.Cmp(y)
 	}
 }
+
+// TestIssue34919 verifies that a Rat's denominator is not modified
+// when simply accessing the Rat value.
+func TestIssue34919(t *testing.T) {
+	for _, acc := range []struct {
+		name string
+		f    func(*Rat)
+	}{
+		{"Float32", func(x *Rat) { x.Float32() }},
+		{"Float64", func(x *Rat) { x.Float64() }},
+		{"Inv", func(x *Rat) { new(Rat).Inv(x) }},
+		{"Sign", func(x *Rat) { x.Sign() }},
+		{"IsInt", func(x *Rat) { x.IsInt() }},
+		{"Num", func(x *Rat) { x.Num() }},
+		// {"Denom", func(x *Rat) { x.Denom() }}, TODO(gri) should we change the API? See issue #33792.
+	} {
+		// A denominator of length 0 is interpreted as 1. Make sure that
+		// "materialization" of the denominator doesn't lead to setting
+		// the underlying array element 0 to 1.
+		r := &Rat{Int{abs: nat{991}}, Int{abs: make(nat, 0, 1)}}
+		acc.f(r)
+		if d := r.b.abs[:1][0]; d != 0 {
+			t.Errorf("%s modified denominator: got %d, want 0", acc.name, d)
+		}
+	}
+}