Re: [PATCH] pthread_once hangs when init routine throws an exception [BZ #18435]

[Torvald Riegel <triegel at redhat dot com>]

On Mon, 2015-06-01 at 09:55 -0600, Martin Sebor wrote:
> On 06/01/2015 02:38 AM, Florian Weimer wrote:
> > It's not immediately obvious to me why this change works. :)
> 
> The pthread_cleanup_push and pthread_cleanup_pop macros are
> defined in sysdeps/nptl/pthread.h. In C code outside of
> glibc compiled with -fexceptions (with __EXCEPTIONS defined),
> they make use of GCC attribute cleanup to invoke the cleanup
> handler during stack unwinding. (In C++ they make use of
> a destructor of a class object.) These macros do the right
> thing when an exception is thrown in the code they surround.
> 
> The problem is that the nptl/pthreadP.h header redefines
> the macros for internal use by glibc without the use of
> the cleanup attribute. As a result, when an exception is
> thrown, the cleanup handler is not invoked. This is what
> happens in pthread_once.c.
> 
> By removing the macro redefinitions from nptl/pthreadP.h
> the change causes pthread_once.c to be compiled with the
> more robust macros defined in pthread.h and allows cleanup
> to take place even after an exception has been thrown so
> long as glibc has been compiled with -fexceptions.

This sounds reasonable for me, but I don't know enough about this to
know for sure :)

I agree with the motivation of the change (ie, making it possible for
libstdc++ to use pthread_once in the implementation);  I haven't looked
at this in detail, but your arguments regarding C/C++ compatibility that
you made elsewhere in the thread seemed fine for me.