Server crashes in mysql_ha_flush_tables upon ALTER SERVER

install soname 'ha_federatedx';

--eval create server s foreign data wrapper 'mysql' options (HOST '127.0.0.1', DATABASE 'test', USER 'root', PASSWORD '', PORT $MASTER_MYPORT, SOCKET '', OWNER 'root');

create database federated;

create table test.t1 (a int);

create table federated.t1 (a int) engine=FEDERATED CONNECTION='s';

insert into federated.t1 values (1);

alter server s options (DATABASE 'x');

#3  <signal handler called>

#4  0x000055a7482b0b91 in mysql_ha_flush_tables (thd=0x7f6bc4000d78, all_tables=0x7f6bc4013c90) at /data/src/10.5/sql/sql_handler.cc:1132

#5  0x000055a74825791e in close_cached_tables (thd=0x7f6bc4000d78, tables=0x7f6bc4013c90, wait_for_refresh=true, timeout=86400) at /data/src/10.5/sql/sql_base.cc:429

#6  0x000055a748495135 in close_cached_connection_tables (thd=0x7f6bc4000d78, connection=0x7f6bc4006398) at /data/src/10.5/sql/sql_servers.cc:194

#7  0x000055a748497603 in alter_server (thd=0x7f6bc4000d78, server_options=0x7f6bc4006390) at /data/src/10.5/sql/sql_servers.cc:1170

#8  0x000055a74830b3a3 in mysql_execute_command (thd=0x7f6bc4000d78) at /data/src/10.5/sql/sql_parse.cc:5852

#9  0x000055a748311bd0 in mysql_parse (thd=0x7f6bc4000d78, rawbuf=0x7f6bc4013b90 "alter server s options (DATABASE 'x')", length=37, parser_state=0x7f6be0442510, is_com_multi=false, is_next_command=false) at /data/src/10.5/sql/sql_parse.cc:7953

#10 0x000055a7482fd564 in dispatch_command (command=COM_QUERY, thd=0x7f6bc4000d78, packet=0x7f6bc4008d09 "alter server s options (DATABASE 'x')", packet_length=37, is_com_multi=false, is_next_command=false) at /data/src/10.5/sql/sql_parse.cc:1839

#11 0x000055a7482fbca2 in do_command (thd=0x7f6bc4000d78) at /data/src/10.5/sql/sql_parse.cc:1358

#12 0x000055a74849cde0 in do_handle_one_connection (connect=0x55a74b1bc1f8, put_in_cache=true) at /data/src/10.5/sql/sql_connect.cc:1422

#13 0x000055a74849cb10 in handle_one_connection (arg=0x55a74b12dfd8) at /data/src/10.5/sql/sql_connect.cc:1319

#14 0x000055a7489cb9f3 in pfs_spawn_thread (arg=0x55a74b1bbe38) at /data/src/10.5/storage/perfschema/pfs.cc:2201

#15 0x00007f6be5af7fa3 in start_thread (arg=<optimized out>) at pthread_create.c:486

#16 0x00007f6be53464cf in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

commit bfdd30d3e92d3bf5570cc050967098c6b5a2d73a

Author: Sergey Vojtovich

Date:   Sat Dec 21 01:22:09 2019 +0400

    Fixed close_cached_connection_tables() flushing

    Let DROP SERVER and ALTER SERVER perform fair affected tables flushing.

    That is acquire MDL_EXCLUSIVE and do tdc_remove_table(TDC_RT_REMOVE_ALL).

    Aim of this patch is elimination of another inconsistent use of

    TDC_RT_REMOVE_UNUSED. It fixes (to some extent) a problem described in the

    beginning of sql_server.cc, when close_cached_connection_tables()

    interferes with concurrent transaction.

    A better fix should probably introduce proper MDL locks for server

    objects?

    Part of MDEV-17882 - Cleanup refresh version