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

[Martin Sebor <msebor@redhat.com>]

On 06/01/2015 02:38 AM, Florian Weimer wrote:
> On 05/31/2015 11:37 PM, Martin Sebor wrote:
>> The C++ 2011 std::call_once function is specified to allow
>> the initialization routine to exit by throwing an exception.
>> Such an execution, termed exceptional, requires call_once to
>> propagate the exception to its caller. A program may contain
>> any number of exceptional executions but only one returning
>> execution (which, if it exists, must be the last execution
>> with the same once flag).
>
> What do you propose as the commit message?

I usually repeat the ChangeLog entry in my commit messages
(included in the patch) but I'd be happy to add more detail
or change the format if there's a preference one way or the
other.

>
> 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.

Martin