In my post on persistent handles, I mentioned that it is good to keep a map between native C++ objects and their JavaScript wrappers, so that you can reuse the wrappers.

I also mentioned that I clear V8′s reference to the native object when C++ “forces” a destroy.

Everything worked, except one thing: when I cleared the reference to the native object, I did not clear the reference from the map. Lo and behold, at some later point in time, when such an object is created again, the pointer to the original is still in my map. So, when I ask the map if a wrapper exists, it does. And its reference is NULL. So, I just passed a non-NULL object to JavaScript, and JavaScript got a NULL value.

All I needed to do to fix it was add a std::set of things to alert to the clearing of the reference, add the map-containing object to that set, and call a function which erases the entry in the map-containing object when the reference is cleared.

It is quite lucky for me that this bug occurred. It occurred due to an unintended consequence of normal object destruction. Accidentally, when the object was destroyed even through a process initiated from JavaScript, the “force destruction” message was sent out. Had this unintended side-effect to destruction not been there, I would not have noticed anything, and some wacky consequences of reusing wrappers for unrelated objects could have been extremely difficult to debug.

A bug a day keeps the doctor away?