Boost.Locale 1.84.0: boost::locale::to_title() is no longer threadsafe under Windows with ICU

[pkl97]

The program below runs stable against Boost 1.83.0 but crashes under Boost 1.84.0 when run under Windows together with the ICU library:

#include <iostream>
#include <vector>
#include <thread>
#include <boost/locale/conversion.hpp>
#include <boost/locale/generator.hpp>

int main()
{
    static const std::locale s_theLocale = boost::locale::generator().generate("");
    const auto& threadFunction = []()
    {
        for (unsigned int i = 0;i < 1'000'000;++i)
            boost::locale::to_title("TEST", s_theLocale);
    };

    std::vector<std::thread> threads;
    for (std::size_t i = 1;i <= 5;++i)
    {
        threads.emplace_back(threadFunction);
        std::cout << "Thread " << i << " has been created" << std::endl;
    }
    for (std::size_t i = 0;i < threads.size();++i)
        threads[i].join();
    std::cout << "Threads have finished" << std::endl;

    return EXIT_SUCCESS;
}

Boost is compiled with these arguments:

b2.exe --toolset=msvc boost.locale.icu=on boost.locale.iconv=off boost.locale.winapi=off boost.locale.std=off boost.locale.posix=off

We use ICU 74.2 and link it dynamically. Interestingly the problem does not occur with to_upper() and to_lower(), just with to_title().

There was a change in Boost.Locale 1.84.0 regarding to_title() (but not related to ICU according to the changelog). Maybe there is a connection.

[pkl97]

After a bit of additional testing I can confirm that the problem also occurs under Linux.

[Flamefire]

Indeed this is an issue introduced by a seemingly unrelated change: https://github.com/boostorg/locale/commit/0faa3c10cc929d0803bbac3ec9bfff9093956155#diff-534aadc69c09a9fce955212e657a8638e05aa877b8f753df917303419562f66fL141-R149

I created a fix and hope that I can still include it into the next release.

Thanks for the report!