Sporadic hang on shutdown after using [SHOW] CREATE SERVER syntax

USE test;

INSTALL PLUGIN Spider SONAME 'ha_spider.so';

CREATE USER Spider@localhost IDENTIFIED BY '';

SHOW CREATE SERVER srv;

CREATE SERVER s3 FOREIGN DATA WRAPPER mysql OPTIONS (HOST 'LOCALHOST', PORT 3307);

SHOW CREATE SERVER srv;

SELECT * FROM mysql.servers;

ALTER SERVER s3 OPTIONS (HOST '192.168.0.3', PORT 3306);

CREATE OR REPLACE SERVER srv FOREIGN DATA WRAPPER foo OPTIONS (HOST "LOCALHOST", PORT "12345");

DROP SERVER s5000;

CREATE TABLE t1_s3 (c INT) ENGINE=Spider COMMENT='WRAPPER "mysql", SRV "s3", TABLE "t2"';

SHOW CREATE SERVER srv;

Will sporadically (about 1 in 100) hang mariadb-admin shutdown.

The issue was observed only a single time in 20k trials using dense feature-specific SQL.
It is thus not deemed to be a blocker to pushing MDEV-15696/MDEV-34716. Both are linked as "is caused by" as it is not clear which one is triggering the bug.

The issue somewhat readily (i.e. quite sporadically) reproduces in reducer (starting 27 subreducers and thus generates significant sever load, hich may be a contributing factor i.e. timing/OS slicing) using no additional options and CLI replay (mariadb < input.sql) , but not when looping the testcase at the Bash/CLI command lines, inc surrounding server start/stops.