internal/impl: fix data race in atomicNilMessage
The problem is that atomicNilMessage.m.mi is accessed both by atomic and
non-atomic operations. (Init uses an atomic read to verify that m.mi is
non-nil, but then returns a non-atomic m.)
Race condition is demonstrated by this test with
"go test -race -count=1000":
func TestPointer(t *testing.T) {
var m atomicNilMessage
var mi MessageInfo
ch := make(chan *MessageInfo)
for i := 0; i < 20; i++ {
go func() {
r := m.Init(&mi)
if &mi != r.mi {
// This conditional exists just
// ensure r.mi is touched.
t.Error("mismatch")
}
ch <- r.mi
}()
}
for i := 0; i < 20; i++ {
<-ch
}
}
I chose not to add the test since it seems a bit overfit to the specific
situation.
Change-Id: Id4664ef3cd5b29515ed310851b9aeb7561be30d0
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/188337
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/internal/impl/pointer_unsafe.go b/internal/impl/pointer_unsafe.go
index 5cebd5c..3f53cbc 100644
--- a/internal/impl/pointer_unsafe.go
+++ b/internal/impl/pointer_unsafe.go
@@ -154,11 +154,13 @@
atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(&ms.mi)), unsafe.Pointer(mi))
}
-type atomicNilMessage struct{ m messageReflectWrapper }
+type atomicNilMessage struct{ p unsafe.Pointer } // p is a *messageReflectWrapper
func (m *atomicNilMessage) Init(mi *MessageInfo) *messageReflectWrapper {
- if (*messageReflectWrapper)(atomic.LoadPointer((*unsafe.Pointer)(unsafe.Pointer(&m.m.mi)))) == nil {
- atomic.StorePointer((*unsafe.Pointer)(unsafe.Pointer(&m.m.mi)), unsafe.Pointer(mi))
+ if p := atomic.LoadPointer(&m.p); p != nil {
+ return (*messageReflectWrapper)(p)
}
- return &m.m
+ w := &messageReflectWrapper{mi: mi}
+ atomic.CompareAndSwapPointer(&m.p, nil, (unsafe.Pointer)(w))
+ return (*messageReflectWrapper)(atomic.LoadPointer(&m.p))
}
