[cxx-abi-dev] How to detect a forced unwind in a catch block?

[Richard Smith]

On 6 September 2016 at 11:56, John McCall <rjmccall at apple.com> wrote:

> > On Sep 6, 2016, at 11:50 AM, Thiago Macieira <thiago at kde.org> wrote:
> > Em terça-feira, 6 de setembro de 2016, às 11:38:43 PDT, John McCall
> escreveu:
> >> This would be inappropriate; the appropriate solution would be to ask
> the
> >> committee for a new function in namespace std that could be implemented
> >> with abi::__forced_unwind.  I believe that's the chief purpose of that
> >> function, to allow that sort of functionality to be implemented in the
> >> future.
> >>
> >> If std::current_exception works, of course, all the better.
> >
> > I will post to std-discussion and see if there's a consensus. I'm not
> sure
> > there can be because this is outside of the C++ language: there's no such
> > thing as forced unwinds in the standard.
>
> Hmm.  I think this used to not be true; there were library features (e.g.
> longjmp)
> that were allowed, but not required, to be implemented by unwinding the
> stack.
> But now it seems that these places have all been changed to specify
> undefined
> behavior if they would bypass any non-trivial destructors.
>

That's true, but it does not place restrictions on intervening 'catch
(...)' blocks, and (since no exception has been thrown) they presumably
must /not/ be entered. (You don't get UB from a longjmp where a
corresponding throw would enter a catch(...) block -- although that may
just be a wording defect.)

The possibility of forced unwinding invoking a 'catch (...)' block with no
current exception seems broken to me. For instance, this may or may not
call terminate() if f() results in a forced unwind, depending on how
'throw;' is implemented:

  try {
    f();
  } catch (...) {
    log_error(); // might reasonably assert(std::current_exception())
    throw;
  }

Pretending that a forced unwind throws a C++ exception of some known type
seems likely to be the best approach -- and I'd expect that exception to be
capturable via std::current_exception and counted by
std::uncaught_exceptions, or exception-safe code using those facilities
will not function correctly in the presence of a forced unwind. (And
likewise for foreign exceptions; we presumably want to pretend that they
are just like C++ exceptions but are of a type that doesn't match that of
any handlers.)
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