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

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 >, bool> std::_Rb_tree, std::_Select1st >, std::less, std::allocator > >::_M_insert_unique >(std::pair&&) ()
boostorg/boost#1 0x0000000001013d14 in boost::gregorian::greg_month::get_month_map_ptr() ()
boostorg/boost#2 0x00000000006fe148 in boost::date_time::month_str_to_ushort (s="May") at /opt/rh/devtoolset-10/root/usr/include/c++/10/ext/new_allocator.h:89
boostorg/boost#3 boost::date_time::parse_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(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;
}