Execute on first instance:

INSTALL PLUGIN spider SONAME 'ha_spider.so';

CREATE TABLE t (c INT) ENGINE=SPIDER COMMENT='WRAPPER "mariadb",  TABLE "t", SOCKET "/your_path/socket.sock"';

INSERT INTO t VALUES (1);

Then, if the table t on the secondary instance is SPIDER, the result will be:

10.8.0-opt>INSERT INTO t3 VALUES (1);

ERROR 1429 (HY000): Unable to connect to foreign data source: localhost

Note that it is trying to make a TCP/IP connection to localhost rather than using the socket.

The same happens when using CREATE SERVER with SOCKET specified.

10.2, 10.3 Presumed affected. Tested 10.4 and 10.8, same result.