boost::posix_time::time_from_string not threadsafe until the get_month_map_ptr() has completed once

[ksankaran2025]

The below function which is invoked by time_from_string when a month name such as "May" is used - populates the table when empty. However when two threads make the same call at the same time - there is no synchronization as a result both end up inserting to the same table and causes a crash like this

#0 0x0000000001015f85 in std::pair<std::_Rb_tree_iterator<std::pair<std::string const, unsigned short> >, bool> std::_Rb_tree<std::string, std::pair<std::string const, unsigned short>, std::_Select1st<std::pair<std::string const, unsigned short> >, std::lessstd::string, std::allocator<std::pair<std::string const, unsigned short> > >::M_insert_unique<std::pair<std::string, unsigned short> >(std::pair<std::string, unsigned short>&&) ()
boostorg/boost#1 0x0000000001013d14 in boost::gregorian::greg_month::get_month_map_ptr() ()
boostorg/boost#2 0x00000000006fe148 in boost::date_time::month_str_to_ushortboost::gregorian::greg_month (s="May") at /opt/rh/devtoolset-10/root/usr/include/c++/10/ext/new_allocator.h:89
boostorg/boost#3 boost::date_time::parse_dateboost::gregorian::date (s=..., order_spec=) at .../boost/date_time/date_parsing.hpp:142

greg_month::month_map_ptr_type greg_month::get_month_map_ptr()
{
  static month_map_ptr_type month_map_ptr(new greg_month::month_map_type());

  if(month_map_ptr->empty()) {
    std::string s("");
    for(unsigned short i = 1; i <= 12; ++i) {
      greg_month m(static_cast<month_enum>(i));
      s = m.as_long_string();
      s = date_time::convert_to_lower(s);
      month_map_ptr->insert(std::make_pair(s, i));
      s = m.as_short_string();
      s = date_time::convert_to_lower(s);
      month_map_ptr->insert(std::make_pair(s, i));
    }
  }
  return month_map_ptr;

}

[phetdam]

Noticed the same issue when calling boost::posix_time::time_from_string from multiple threads.

Can at least the documentation be updated to indicate that the function is not thread-safe? It would be nice (but not necessary) if there was a thread-safe variant, e.g. boost::posix_time::time_from_string_reentrant.

I suppose the implementation could be as simple as using a static mutex to serialize calls to time_from_string.