Bug#53251 mysql_library_init fails on second execution with embedded library

thread-specific variables weren't set when we load error message files.

per-file comments:
  libmysqld/lib_sql.cc
Bug#53251      mysql_library_init fails on second execution with embedded library
      we need to call my_thread_init() once more. Normally it's called at the my_init()
      stage but that doesn't happen on the second my_init() call.

  sql/derror.cc
Bug#53251      mysql_library_init fails on second execution with embedded library
     use default errors for the embedded server.

  sql/mysqld.cc
Bug#53251      mysql_library_init fails on second execution with embedded library
        unregister server errors in clean_up(). Without it the error list contains
        that on the second mysql_server_init() which is not good.

  sql/set_var.cc
Bug#53251      mysql_library_init fails on second execution with embedded library
        sys_var::cleanup() call instead of the destructor

  sql/set_var.h
Bug#53251      mysql_library_init fails on second execution with embedded library
        sys_var::cleanup() introduced instead of the destructor
        
  sql/sys_vars.h
Bug#53251      mysql_library_init fails on second execution with embedded library
        Sys_var_charptr::cleanup() implemented

libmysqld/lib_sql.cc

@@ -481,6 +481,10 @@ int init_embedded_server(int argc, char **argv, char **groups)
   char *fake_argv[] = { (char *)"", 0 };
   const char *fake_groups[] = { "server", "embedded", 0 };
   my_bool acl_error;
+
+  if (my_thread_init())
+    return 1;
+
   if (argc)
   {
     argcp= &argc;

sql/derror.cc

@@ -35,6 +35,8 @@ static void init_myfunc_errs(void);
 C_MODE_START
 static const char **get_server_errmsgs()
 {
+  if (!current_thd)
+    return DEFAULT_ERRMSGS;
   return CURRENT_THD_ERRMSGS;
 }
 C_MODE_END

sql/mysqld.cc

@@ -1486,6 +1486,7 @@ void clean_up(bool print_message)
   cleanup_errmsgs();
   MYSQL_CALLBACK(thread_scheduler, end, ());
   finish_client_errs();
+  (void) my_error_unregister(ER_ERROR_FIRST, ER_ERROR_LAST); // finish server errs
   DBUG_PRINT("quit", ("Error messages freed"));
   /* Tell main we are ready */
   logger.cleanup_end();

sql/set_var.cc

@@ -108,7 +108,7 @@ void sys_var_end()
   my_hash_free(&system_variable_hash);
 
   for (sys_var *var=all_sys_vars.first; var; var= var->next)
-    var->~sys_var();
+    var->cleanup();
 
   DBUG_VOID_RETURN;
 }

sql/set_var.h

@@ -92,10 +92,9 @@ public:
           on_check_function on_check_func, on_update_function on_update_func,
           uint deprecated_version, const char *substitute, int parse_flag);
   /**
-    The instance should only be destroyed on shutdown, as it doesn't unlink
-    itself from the chain.
+    All the cleanup procedures should be performed here
   */
-  virtual ~sys_var() {}
+  virtual void cleanup() {}
   /**
     downcast for sys_var_pluginvar. Returns this if it's an instance
     of sys_var_pluginvar, and 0 otherwise.

sql/sys_vars.h

@@ -385,7 +385,7 @@ public:
     DBUG_ASSERT(scope() == GLOBAL);
     DBUG_ASSERT(size == sizeof(char *));
   }
-  ~Sys_var_charptr()
+  void cleanup()
   {
     if (flags & ALLOCATED)
       my_free(global_var(char*));