[c++-pthreads] Re: pthread_cancel and EH: let's try this again

[Dave Butenhof]

Jason Merrill wrote:

>>It is true that many codes have been written with POSIX C binding, more
>>has to come as C++ codes.  I'm not convinced that we should compromise
>>the future.
>>    
>>
>The future can use a different interface.  Or extend pthread_setcanceltype.
>PTHREAD_CANCEL_DEFERRED_EXCEPTION, anyone?
>  
>
That would be a serious mistake. C and C++ are so interoperable that 
interleaved frames are common. Bad current implementations (that don't 
have a common exception model between POSIX cleanup and 
C++/Ada/Modula/Java exceptions) are broken when two C frames with 
cancellation cleanup have intervening C++ frames -- destructors in those 
frames will be skipped. But it'd be no less bad for a "C++ cancel" to 
skip interleaved C frame cleanup handlers.

You can try to say that C frames using POSIX cleanup cannot be 
interleaved with C++ frames; and that C++ frames using C++ cancel cannot 
be interleaved with frames using POSIX cleanup. But that makes no 
conceivable sense, and in practice is an unreasonable and infeasible 
restriction. They have to interoperate. They have to be the same.

There has to be ONE cancellation mechanism. It has to propagate and 
behave as an exception because there's a vast body of existing code in 
many languages that already provides cleanup mechanisms based on 
exceptions, and POSIX cancel cleanup can also be implemented as an 
exception.
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