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math/big: make Rat accessors safe for concurrent use

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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/+/201205
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Robert Griesemer 2019-10-15 13:44:22 -07:00
parent 52e5987f5d
commit 898f9db81f
2 changed files with 30 additions and 4 deletions
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8
src/math/big/rat.go
View File

@ -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
}

26
src/math/big/rat_test.go
View File

@ -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)
}
}
}