[Bug libstdc++/78015] New: pthread_cancel while some exception is pending results in std::terminate ()

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78015

            Bug ID: 78015
           Summary: pthread_cancel while some exception is pending results
                    in std::terminate ()
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

#include <pthread.h>
#include <unistd.h>

namespace __cxxabiv1
{  
  class __forced_unwind
  {
    virtual ~__forced_unwind() throw();
    virtual void __pure_dummy() = 0; 
  };
}

int a;

extern "C" void *
fun (void *)
{
#ifdef WORKAROUND
  try
  {
    throw 1;
  }
  catch (int &)
#endif
  {
    try
    {
      char buf[10];
      for (;;)
        read (4, buf, 0);
    }
    catch (__cxxabiv1::__forced_unwind &)
    {
      a = 5;
      throw;
    }
  }

  return NULL;
}

int
main ()
{
  pthread_t thread;
  pthread_create (&thread, NULL, fun, NULL);
  pthread_cancel (thread);
  pthread_join (thread, NULL);
}

fails with std::terminate (), while works with -DWORKAROUND.
The problem is in __cxa_begin_catch:
51        // Note that this use of "header" is a lie.  It's fine so long as we
only
52        // examine header->unwindHeader though.
53        if (!__is_gxx_exception_class(header->unwindHeader.exception_class))
54          {
55            if (prev != 0)
56              std::terminate ();

With -DWORKAROUND, prev is NULL and thus we handle it fine as forced unwinding
exception.
But without that, prev is non-NULL here.  I don't know what exactly should we
do instead if prev is non-NULL,
whether to somehow destruct the previous exception, or just continue normally
and destruct it when the forced unwinding finishes,
or when.