Since the __CxxDetectRethrow may increment __ProcessingThrow, which then
must be decremented in __CxxUnregisterExceptionObject, and the latter does
many other funny things with exception handling CRT machinery, we cannot
use those internal functions (neither alone, nor together), or we end up
breaking runtime's expectations: the runtime code checks __ProcessingThrow
left and right, expecting its non-0 value to indicate "we are unwinding...
possibly called from a dtor()". In this case, e.g., std::current_exception
returns nullptr inside catch block.
This creates our own copy of __CxxDetectRethrow, which does not mangle the
global state, and just performs the same checks. This is a dirty hack, and
it relies on current layout of the exception description layout - so must
be synchronized in the event of changes!
Reviewed-by: Stephan Bergmann <email@example.com>