local_microsec_clock::local_time(time_zone_ptr) occasionally returns an incorrect time

[boimart1]

Extremely rarely, a call to local_microsec_clock::local_time(time_zone_ptr) will return a result about 1 second in the past.

On my local machine, running the following code:

#include <boost/date_time/local_time/local_time.hpp>
#include <iostream>

int main() {
    boost::local_time::time_zone_ptr zone(new boost::local_time::posix_time_zone("EST-5"));

    for (int i = 0; i < 100'000'000; ++i)
    {
        boost::local_time::local_date_time t0 = boost::local_time::local_microsec_clock::local_time(zone);
        boost::local_time::local_date_time t1 = boost::local_time::local_microsec_clock::local_time(zone);
        if (t1 < t0)
        {
            std::cout << i << ": time went from " << t0 << " to " << t1 << '\n';
        }
    }

    return 0;
}

produces ~10-15 incorrect results for 100M calls, or one incorrect result per ~7-10M calls.

The output suggests that this always occurs when the second ticks. The fractional part rolls over, but the seconds do not change.

81899680: time went from 2023-Aug-21 09:17:12.999739 EST to 2023-Aug-21 09:17:12.000740 EST
91444271: time went from 2023-Aug-21 09:17:27.999755 EST to 2023-Aug-21 09:17:27.000752 EST
92719444: time went from 2023-Aug-21 09:17:29.999467 EST to 2023-Aug-21 09:17:29.000465 EST

It looks like the issue is caused by the second ticking between the calls to second_clock::universal_time() and second_clock::local_time(), which causes utc_offset to be off by 1 second. https://github.com/boostorg/date_time/blob/39714907b7d32ed8f005b5a01d1c2b885b5717b3/include/boost/date_time/microsec_time_clock.hpp#L59-L66

I intend to submit a PR with a solution soon.