pthread_cancel and EH: let's try this again

[Alexander Terekhov]

Peter Dimov wrote:
[...]
> > With cancel request pending, mandatory cancel points are required to
> > throw within a cleanup handler invoked by pthread_cleanup_pop() ["(if
> > /execute/ is non-zero)"] when cancel state is PTHREAD_CANCEL_ENABLE.
> 
> An odd requirement that could've been given the appropriate attention had
> something like the above been proposed. ;-)
> 
> The specification of the hypothetical pthread_cancelenabled can be "fixed"
> to match this behavior, but I'm not sure that this is desirable. If you have
> two calls to 'close' in the cleanup handler and the first is canceled, the
> second will be skipped.

Yep. So you either disable cancel in such a cleanup handler that can 
be invoked by pthread_cleanup_pop(), or move the second cancel point 
to another pthread_cleanup_pop(1) handler inside the first one.

regards,
alexander.