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

[John McCall]

> On Sep 3, 2016, at 1:40 AM, Thiago Macieira <thiago at kde.org> wrote:
> Hello
> 
> Summary: abi::__foced_unwind is available when #include <cxxabi.h> from GCC 
> (libsupc++), but is not documented in https://mentorembedded.github.io/cxx-abi/abi-eh.html nor is it available in other implementations. Should it be 
> adde to the spec so we can detect forced unwinds? Or is there another way to 
> detect the forced unwinding not caused by an exception?
> 
> Background:
> 
> 30.3.1.2 [thread.thread.constr] p5 says
> 
> " If the invocation of INVOKE (DECAY_COPY ( std::forward<F>(f)), DECAY_COPY 
> (std::forward<Args>(args))...) terminates with an uncaught exception, 
> std::terminate shall be called."
> 
> When trying to implement this requirement for QThread, we've run into a snag: 
> with glibc's pthread implementation on Linux, pthread_exit() as well as all 
> POSIX asynchronous cancellations happen by way of a forced stack unwinding. 
> This means the obvious solution to implement the requirement from the standard 
> fails:
> 
> 	try {
> 		thr->run();
> 	} catch (...) {
> 		std::terminate();
> 	}
> 
> That catch (...) block is run by the forced stack unwinding started with 
> _Unwind_ForcedUnwind. Since it then calls std::terminate(), the whole 
> application terminates instead of just the thread.
> 
> Now, the libstdc++ implementation[1][2] handles this gracefully, but does so 
> by catching abi::__forced_unwind.
> 
> That brings me to my original question in the subject: how can code outside of 
> the compiler's own implementations detect a forced stack unwinding not caused 
> by a thrown exception in a portable fashion?
> 
> Option 1: make abi::__forced_unwind documented and have the other 
> implementations (besides GCC) implement it too.

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.

John.

> 
> Option 2: add a getter to either the Base ABI for unwinding (_Unwind_*) or the 
> C++ ABI to get a flag indicating whether the unwind is forced. In fact, this 
> information is already available in the actions parameter to the personality 
> routine: https://mentorembedded.github.io/cxx-abi/abi-eh.html#base-personality 
> has:
>    static const _Unwind_Action _UA_FORCE_UNWIND = 8;
> 
> Option 3: change the way that GCC and glibc force unwinding from non-C++ 
> contexts.
> 
> [1] https://github.com/gcc-mirror/gcc/blob/master/libstdc%2B%2B-v3/src/c%2B
> %2B11/thread.cc#L77
> [2] I don't see any code in libc++ for handling exceptions at all.
> 
> -- 
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
>   Software Architect - Intel Open Source Technology Center
> 
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