MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator...

MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator from THD::drop_temporary_table (10.5.3 opt only) on ALTER

The server crashes if ALTER TABLE, which accesses physical data
placed at data nodes, is performed on a Spider table.

The cause of the bug is that spider_check_trx_and_get_conn() does
not allocate connections if sql_command == SQLCOM_ALTER_TABLE.
Some ALTER TABLE statements, like ALTER TABLE ... CHECK PARTITION,
access data nodes. So, we need to allocate a new connection before
performing ALTER TABLEs.

storage/spider/mysql-test/spider/bugfix/r/mdev_26539.result

+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3
+#
+# MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator from THD::drop_temporary_table (10.5.3 opt only) on ALTER
+#
+connection child2_1;
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+CREATE TABLE tbl_a (
+c INT
+) ENGINE=InnoDB DEFAULT CHARSET=utf8;
+connection master_1;
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+CREATE TABLE tbl_a (
+c INT
+) ENGINE=Spider DEFAULT CHARSET=utf8 COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS (c) (
+PARTITION pt1 DEFAULT COMMENT = 'srv "s_2_1"'
+);
+INSERT INTO tbl_a VALUES (1);
+ALTER TABLE tbl_a CHECK PARTITION ALL;
+Table	Op	Msg_type	Msg_text
+auto_test_local.tbl_a	check	status	OK
+DROP DATABASE auto_test_local;
+connection child2_1;
+DROP DATABASE auto_test_remote;
+for master_1
+for child2
+child2_1
+child2_2
+child2_3
+for child3

storage/spider/mysql-test/spider/bugfix/t/mdev_26539.cnf

+!include include/default_mysqld.cnf
+!include ../my_1_1.cnf
+!include ../my_2_1.cnf

storage/spider/mysql-test/spider/bugfix/t/mdev_26539.test

+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+
+--echo #
+--echo # MDEV-26539 SIGSEGV in spider_check_and_set_trx_isolation and I_P_List_iterator from THD::drop_temporary_table (10.5.3 opt only) on ALTER
+--echo #
+
+--connection child2_1
+CREATE DATABASE auto_test_remote;
+USE auto_test_remote;
+eval CREATE TABLE tbl_a (
+    c INT
+) $CHILD2_1_ENGINE $CHILD2_1_CHARSET;
+
+--connection master_1
+CREATE DATABASE auto_test_local;
+USE auto_test_local;
+
+eval CREATE TABLE tbl_a (
+    c INT
+) $MASTER_1_ENGINE $MASTER_1_CHARSET COMMENT='table "tbl_a"' PARTITION BY LIST COLUMNS (c) (
+    PARTITION pt1 DEFAULT COMMENT = 'srv "s_2_1"'
+);
+
+INSERT INTO tbl_a VALUES (1);
+ALTER TABLE tbl_a CHECK PARTITION ALL;
+
+DROP DATABASE auto_test_local;
+
+--connection child2_1
+DROP DATABASE auto_test_remote;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log

storage/spider/spd_trx.cc

@@ -3744,10 +3744,8 @@ int spider_check_trx_and_get_conn(
   }
   spider->wide_handler->trx = trx;
   spider->set_error_mode();
-  if (
-    spider->wide_handler->sql_command != SQLCOM_DROP_TABLE &&
-    spider->wide_handler->sql_command != SQLCOM_ALTER_TABLE
-  ) {
+  if (spider->wide_handler->sql_command != SQLCOM_DROP_TABLE)
+  {
     SPIDER_TRX_HA *trx_ha = spider_check_trx_ha(trx, spider);
     if (!trx_ha || trx_ha->wait_for_reusing)
       spider_trx_set_link_idx_for_all(spider);