[c++-pthreads] Re: thread-safety definition

[Wil Evers]

Jason Merrill wrote:

> On Sat, 10 Jan 2004 22:19:41 +0100, Wil Evers <wil at bogo.xs4all.nl> wrote:
>  
>>Richard Henderson wrote:
>>
>>>On Fri, Jan 09, 2004 at 07:01:31AM +0100, Wil Evers wrote:
>>>
>>>>A second exception escaping from a destructor called while unwinding
>>>>the stack because of some earlier exception will result in program
>>>>termination.
>>>>
>>>>If catching (and not rethrowing) this second exception breaks the
>>>>cancellation machinery, then it is the cancellation machinery - and not
>>>>the program in question - that is broken.  In other words: we need a way
>>>>to prevent this scenario.
>>>
>>> It would be trivial to adjust the c++ runtime to disable cancelation
>>>while exceptions are in flight.  That avoids that scenario entirely.
>>
>>It obviously does.  But does that imply we don't need
>>'catch-and-dont-rethrow' blocks in destructors anymore?
> 
> No, of course not; we can get double faults in non-threaded programs.  It
> does, however, mean that a cancellation exception will never be caught by
> such a block.
> 
>>[...]
>>I've noticed that some people on this list object to this design, but so
>>far, I haven't seen an alternative.  In other words, I think we should be
>>prepared for threads that discard cancellation exceptions - which is why
>>Nathan's 'sticky cancellation' makes sense to me.
>  
> I agree that cancellation should be sticky, and it is in my model as well
> as Nathan's.  But I don't think that this particular example would discard
> cancellation exceptions.

I think it will.  Even if cancellation is disabled while exceptions are 
in flight, a 'catch-everything-and-never-rethrow' block in a destructor 
will discard a cancellation exception when that destructor is triggered 
by a regular (non-exceptional, non-unwinding) exit from a block scope.

Call me paranoid, but as a C++ programmer, the thing that scares me most 
  in the discussions on this list is a certain (perceived?) pressure to 
open the door for letting exceptions escape from destructors.  Speaking 
for myself, all I can say is that such a license would break nearly 
every piece of non-trivial code I've written over the last eight years 
or so.

- Wil