[c++-pthreads] Re: pthread_cancel and EH: let's try this again

[Peter Dimov]

Dave Butenhof wrote:

> What was important, and relevant, was to make the cleanup handlers
> (and TSD destructors) reliable by specifying the state of
> cancellation. The original INTENT of the working group had been that
> cancellation was disabled implicitly by delivery, and most code is
> written presuming that state. Making all cleanup handlers and
> destructors that don't explicitly disable cancel suddenly
> "non-conforming" wouldn't have been a particularly useful amendment.

Nobody's arguing against that. The point is that the clarification made it 
mandatory to implement these semantics via the equivalent of issuing a 
pthread_setcancelstate call at the cancellation point.

Before that, an implementation that used a separate "cleanup mode" flag 
(which has the advantage of not interfering with the client's cancelstate 
calls) was conforming. This was good, because - in my current opinion - this 
model makes more sense if the cleanup is allowed to not complete, as is the 
case in C++.

It's certainly questionable whether a cleanup handler (TSD destructor) that 
explicitly enables cancellation and calls a cancellation point should 
re-cancel itself... and if it does, whether it should re-invoke itself, so 
it might be argued that this clarification introduced a new problem instead 
of fixing the old.