[cxx-abi-dev] ABI modification for exception propagation

[Sebastian Redl]

David Vandevoorde wrote:
>
> On May 27, 2008, at 4:39 AM, Sebastian Redl wrote:
>> This might work. But *only* if all modules link to the support 
>> library dynamically, of course. If any of them links statically, it 
>> would keep the old implementation, which would wreak havoc.
>
>
> What sort of failure modes would we expect.  Could the 
> "__cxa_exception by reference" be made to look like std::bad_exception 
> to an old runtime support library?
The __cxa_exception by reference (I've called it "dependent exception" 
in my new writeup) could indeed be made to look like that, but that's 
not the real problem. The real problem is code using the old library 
catching a primary exception to which exception_ptrs refer. The old 
support library might disrespect the reference counting necessary to 
keep the object alive, and would delete it, leaving the exception_ptrs 
dangling. (I say "might", because if the library uses 
_Unwind_DeleteException to delete native exceptions, like G++ does, the 
reference counting could be put there.)
That said, this shouldn't be that much of a problem. Strictly speaking, 
G++ already doesn't allow cross-module throwing if a static support 
library is used. The reason is that in static links, there would be 
multiple emergency memory pools. An exception allocated in the emergency 
pool of one module and caught in a different module would then be 
incorrectly passed to free() for deletion, which would crash anyway. And 
that's with all code compiled with exactly the same compiler version.

> FWIW, I've contacted Peter Dimov about this issue, and he seems 
> agreeable to prohibit rethrowing an exception captured with 
> current_exception() using "throw;" (until it has been rethrown with 
> rethrow_exception, at which point it can be caught/rethrown the usual 
> way).
I don't see a particular reason to prohibit this. At least, I don't 
foresee any implementation problems that would be solved this way. (Put 
it this way: of all the crazy things you can do with exception_ptrs - 
and you can do very crazy things - this is on the mild side.)

Sebastian