Race condition in loading of client plugins

When multiple threads in the same application attempt to connect to a server using a client-side authentication plugin, the following error can be observed from time to time.

1045: Plugin client_ed25519 could not be loaded: it is already loaded

Looking at the code in ma_client_plugin.c.in:

  /* make sure the plugin wasn't loaded meanwhile */

  if (find_plugin(plugin->name, plugin->type))

  {

    my_set_error(mysql, CR_AUTH_PLUGIN_CANNOT_LOAD,

                 SQLSTATE_UNKNOWN, ER(CR_AUTH_PLUGIN_CANNOT_LOAD),

                 plugin->name, "it is already loaded");

    plugin= NULL;

  }

we see that something that can only occur with more than one thread is treated as an error. Additionally, it seems that the initial find_plugin() call is not done under the same lock under which the plugin loading is done. This would explain why more than one thread attempts to load the same plugin.

I believe that either the whole mysql_client_find_plugin should be covered by the same mutex or the already existing plugin should be returned instead of treating it as an error.