Possible race condition in WithLatestFrom when setting the latest #227

If I understand C# memory model correctly (should be very similar to Java's), there is a potential race when reading _latest in the main 'rail' and writing _latest it from the other 'rail'.

Since the value is not volatile here, if hasLatest has been set for a previous element, a partially setup/constructed object may be visible on the reader side. For example:

class MyValue {
   int field;
}

MyValue v = new MyValue()
v.field = 5;
// other.onNext(v);
_parent._latest = v; // this is plain write
// <-- writer gets interrupted here
_parent._hasLatest = true;

Even if constructing v has a full barrier, it is possible v.field = 5 gets reordered with _parent._latest = v and reading _latest.field on the other side may yield 0.

A solution would be to wrap TSecond into an object that is written via volatile write to the _latest field. Then, there is no need for hasLatest but check for null (or other special value) to see if there is one latest available or not.