Spider uninstall fails with "plugin busy"

install soname 'ha_spider';

create table t (c int) Engine=SPIDER;

drop table t;

uninstall soname 'ha_spider';

The uninstall statement results in warning:

Warnings:

Warning	1620	Plugin is busy and will be uninstalled on shutdown

Initial analysis shows this could be caused by a call to thd_set_ha_data(thd, spider_hton_ptr, trx) which increases the ref count to the plugin which later did not get decreased, from spider_get_trx() during the create table statement. See also kentoku's comment in MDEV-7914[1], whose commit is to blame for this call.

[1] https://jira.mariadb.org/browse/MDEV-7914?focusedCommentId=109258&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-109258

Trace:

intern_plugin_lock > plugin_lock > ha_lock_engine > thd_set_ha_data > ha_spider::create > handler::ha_create > create_table_impl > mysql_create_table_no_lock > Sql_cmd_create_table_like::execute > mysql_parse > dispatch_command > do_command > handle_one_connection > pfs_spawn_thread