MDEV-25856 : SIGSEGV in ha_myisammrg::append_create_info

For MERGE-tables we need to init children list before calling
show_create_table and then detach children before we continue
normal mysql_create_like_table execution.

mysql-test/suite/galera/r/galera_create_table_like.result

@@ -47,3 +47,8 @@ DROP TABLE schema2.real_table2;
 DROP TABLE schema2.real_table3;
 DROP SCHEMA schema1;
 DROP SCHEMA schema2;
+use test;
+CREATE TEMPORARY TABLE t (c INT) ENGINE=mrg_myisam UNION=(t,t2) insert_method=FIRST;
+CREATE TABLE t2 LIKE t;
+ERROR HY000: Table 't' is differently defined or of non-MyISAM type or doesn't exist
+DROP TABLE t;

mysql-test/suite/galera/t/galera_create_table_like.test

@@ -48,3 +48,12 @@ DROP TABLE schema2.real_table3;
 
 DROP SCHEMA schema1;
 DROP SCHEMA schema2;
+
+#
+# MDEV-25856: SIGSEGV in ha_myisammrg::append_create_info
+#
+use test;
+CREATE TEMPORARY TABLE t (c INT) ENGINE=mrg_myisam UNION=(t,t2) insert_method=FIRST;
+--error 1472
+CREATE TABLE t2 LIKE t;
+DROP TABLE t;

sql/wsrep_mysqld.cc

@@ -2886,21 +2886,17 @@ bool wsrep_create_like_table(THD* thd, TABLE_LIST* table,
   }
   else
   {
-    /* here we have CREATE TABLE LIKE <temporary table> 
-       the temporary table definition will be needed in slaves to
-       enable the create to succeed
-     */
-    TABLE_LIST tbl;
-    bzero((void*) &tbl, sizeof(tbl));
-    tbl.db= src_table->db;
-    tbl.table_name= tbl.alias= src_table->table_name;
-    tbl.table= src_table->table;
+    /* Non-MERGE tables ignore this call. */
+    if (src_table->table->file->extra(HA_EXTRA_ADD_CHILDREN_LIST))
+      return (true);
+
     char buf[2048];
     String query(buf, sizeof(buf), system_charset_info);
     query.length(0);  // Have to zero it since constructor doesn't
 
-    (void)  show_create_table(thd, &tbl, &query, NULL, WITH_DB_NAME);
-    WSREP_DEBUG("TMP TABLE: %s", query.ptr());
+    int result __attribute__((unused))=
+      show_create_table(thd, src_table, &query, NULL, WITH_DB_NAME);
+    WSREP_DEBUG("TMP TABLE: %s ret_code %d", query.ptr(), result);
 
     thd->wsrep_TOI_pre_query=     query.ptr();
     thd->wsrep_TOI_pre_query_len= query.length();
@@ -2909,11 +2905,14 @@ bool wsrep_create_like_table(THD* thd, TABLE_LIST* table,
 
     thd->wsrep_TOI_pre_query=      NULL;
     thd->wsrep_TOI_pre_query_len= 0;
+
+    /* Non-MERGE tables ignore this call. */
+    src_table->table->file->extra(HA_EXTRA_DETACH_CHILDREN);
   }
 
   return(false);
 
-WSREP_ERROR_LABEL:
+wsrep_error_label:
   thd->wsrep_TOI_pre_query= NULL;
   return (true);
 }