[cpp-threads] Asynchronous Execution Issues (caching_async)

[Boehm, Hans]

The more I think about these issues, the less comfortable I get with anything like caching_async.  Given the lifetime issues we've already been running into, the fact that caching_async essentially delays destruction of thread_local objects far beyond the context in which they were created seems really dangerous.

If I write a function that implements a parallel algorithm, I generally have no way of knowing whether any iterators I'm passed refer to local (automatic) objects or heap objects of limited lifetime.  But if I create a thread_local reference to them, I run the risk of accidentally accessing them long past completion of the requested task, in the destructor for the thread_local, with exceptionally disastrous consequences.  I don't know how easy it is to make such a mistake, but I can't convince myself that it would be rare.  In any case, this seems like way too dangerous a feature to introduce at this stage without any experience to back it up.

Note that Java experience doesn't apply here.  Java has some issues with persistent thread-locals.  As Doug points out, they can largely be addressed.  But it doesn't share this kind of object lifetime issue.

Hans