Bug #45058 init_available_charsets uses double checked locking

A client doing multiple mysql_library_init() and
mysql_library_end() calls over the lifetime of the process may
experience lost character set data, potentially even a
SIGSEGV.

This patch reinstates the reloading of character set data when
a mysql_library_init() is done after a mysql_library_end().

include/my_sys.h

@@ -950,7 +950,7 @@ extern my_bool resolve_charset(const char *cs_name,
 extern my_bool resolve_collation(const char *cl_name,
                                  CHARSET_INFO *default_cl,
                                  CHARSET_INFO **cl);
-
+extern void free_charsets(void);
 extern char *get_charsets_dir(char *buf);
 extern my_bool my_charset_same(CHARSET_INFO *cs1, CHARSET_INFO *cs2);
 extern my_bool init_compiled_charsets(myf flags);

libmysql/libmysql.c

@@ -211,6 +211,7 @@ void STDCALL mysql_server_end()
   }
   else
   {
+    free_charsets();
     mysql_thread_end();
   }
 

mysys/charset.c

@@ -427,6 +427,11 @@ static void init_available_charsets(void)
 }
 
 
+void free_charsets(void)
+{
+  charsets_initialized= MY_PTHREAD_ONCE_INIT;
+}
+
 uint get_collation_number(const char *name)
 {
   my_pthread_once(&charsets_initialized, init_available_charsets);

mysys/my_init.c

@@ -165,6 +165,7 @@ void my_end(int infoflag)
       my_print_open_files();
     }
   }
+  free_charsets();
   my_error_unregister_all();
   my_once_free();
 

sql/mysqld.cc

@@ -1287,6 +1287,7 @@ void clean_up(bool print_message)
   lex_free();				/* Free some memory */
   item_create_cleanup();
   set_var_free();
+  free_charsets();
   if (!opt_noacl)
   {
 #ifdef HAVE_DLOPEN