Re: [Boost-users] [Serialization] Thread-unsafe singleton

From: Sohail Somani (s.somani_at_[hidden])
Date: 2007-05-23 18:58:10

On Mon, 2007-05-21 at 20:04 -0700, Robert Ramey wrote:
> Sohail Somani wrote:
> 
> > Thanks for your quick response. Could you point me to the files that
> > have been changed to address the issues?
> 
> You'll have to browse the CVS - look into extended type info

I am looking at:

http://boost.cvs.sourceforge.net/boost/boost/libs/serialization/src/extended_type_info.cpp?r1=1.11&r2=1.12 
which I assume is the change you are suggesting.

This still doesn't address the thread-safety issues inherent in Myers
Singleton that is used here:

http://boost.cvs.sourceforge.net/boost/boost/boost/serialization/extended_type_info_typeid.hpp?revision=1.9&view=markup on line 96.

With the above code, it is possible to have double initialization of the
extended_type_info derived instances and then eventually double
initialization of detail::*map which could be a more dramatic error. So
if anyone uses BOOST_CLASS_EXPORT() you cannot reason about your
program's thread-safety.

I was sent a patch privately by someone who changed the instance in the
first link to be a class member static with a caveat that it invokes
"dynamic initialization." I am new to the whole C++ language lawyer
thing but in section 3.6.3(.3) of the standard, it says:

It is implementation-defined whether or not the dynamic initialization
(8.5, 9.4, 12.1, 12.6.1) of an object of namespace scope is done before
the first statement of main.

So I really don't think there is any way to do this lazy initialization
portably in the presence of threads (threads? what are those?) However,
the choice to delay it to a function-scope static makes it easier to
make it work.

I can develop a patch using boost once that I think would work. Locally,
I'm going to have to do this anyway. Fortunately, my test has ceased to
fail so I can't test it. To make it as "pay for what you use", one can
#ifdef it out if they have special knowledge of the platform/compiler.

Suggestions, comments or flames? All three are appreciated.

Sohail