MDEV-28998 remove a known reference to a SPIDER_CONN when it is freed

storage/spider/mysql-test/spider/bugfix/r/mdev_28998.result

+#
+# MDEV-28998 ASAN errors in spider_fields::free_conn_holder or spider_create_group_by_handler
+#
+for master_1
+for child2
+for child3
+CREATE SERVER s FOREIGN DATA WRAPPER MYSQL OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t1_SPIDER (a INT) ENGINE=SPIDER COMMENT = "wrapper 'mysql', srv 's', table 't1'";
+CREATE TABLE t2 AS SELECT a FROM t1_SPIDER;
+SELECT * FROM t2;
+a
+1
+2
+DROP TABLE t2, t1_SPIDER, t1;
+drop server s;
+for master_1
+for child2
+for child3
+#
+# end of test mdev_28998
+#

storage/spider/mysql-test/spider/bugfix/t/mdev_28998.test

+--echo #
+--echo # MDEV-28998 ASAN errors in spider_fields::free_conn_holder or spider_create_group_by_handler
+--echo #
+
+if (`select not(count(*)) from information_schema.system_variables where variable_name='have_sanitizer' and global_value like "%ASAN%"`)
+{
+--skip test needs to be run with ASAN
+}
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_init.inc
+--enable_result_log
+--enable_query_log
+evalp CREATE SERVER s FOREIGN DATA WRAPPER MYSQL OPTIONS (SOCKET "$MASTER_1_MYSOCK", DATABASE 'test',user 'root');
+
+CREATE TABLE t1 (a INT);
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t1_SPIDER (a INT) ENGINE=SPIDER COMMENT = "wrapper 'mysql', srv 's', table 't1'";
+
+CREATE TABLE t2 AS SELECT a FROM t1_SPIDER;
+SELECT * FROM t2;
+
+# Cleanup
+DROP TABLE t2, t1_SPIDER, t1;
+drop server s;
+
+--disable_query_log
+--disable_result_log
+--source ../../t/test_deinit.inc
+--enable_result_log
+--enable_query_log
+--echo #
+--echo # end of test mdev_28998
+--echo #

storage/spider/spd_conn.cc

@@ -1255,6 +1255,8 @@ int spider_free_conn(
       ip_port_conn->ip_port_count--;
     pthread_mutex_unlock(&ip_port_conn->mutex);
   }
+  if (conn->conn_holder_for_direct_join)
+    conn->conn_holder_for_direct_join->conn= NULL;
   spider_free_conn_alloc(conn);
   spider_free(spider_current_trx, conn, MYF(0));
   DBUG_RETURN(0);

storage/spider/spd_group_by_handler.cc

@@ -859,7 +859,8 @@ void spider_fields::free_conn_holder(
       }
     }
   }
-  conn_holder_arg->conn->conn_holder_for_direct_join = NULL;
+  if (conn_holder_arg->conn)
+    conn_holder_arg->conn->conn_holder_for_direct_join = NULL;
   DBUG_PRINT("info",("spider free conn_holder=%p", conn_holder_arg));
   spider_free(spider_current_trx, conn_holder_arg, MYF(0));
   DBUG_VOID_RETURN;