MDEV-21134 Crash with partitioned table, PARTITION syntax, and index_merge.

When the partition table is cloned, the handlers for the partitions that were not opened
should anyway be created (but not opened).

mysql-test/main/partition_explicit_prune.result

@@ -1888,7 +1888,7 @@ ALTER TABLE t1 EXCHANGE PARTITION p0 WITH TABLE t2;
 SELECT * FROM t1 PARTITION (p0);
 i
 UNLOCK TABLES;
-DROP TABLE t1;
+DROP TABLE t1, t2;
 #
 # MDEV-18371 Server crashes in ha_innobase::cmp_ref upon UPDATE with PARTITION clause.
 #
@@ -1906,4 +1906,82 @@ a	b
 4	3
 8	2
 2	6
-DROP TABLE t1, t2;
+DROP TABLE t1;
+#
+# MDEV-21134 Crash with partitioned table, PARTITION syntax, and index_merge.
+#
+create table t1 (
+pk int primary key,
+a int,
+b int,
+filler char(32),
+key (a),
+key (b)
+) engine=myisam  partition by range(pk) (
+partition p0 values less than (10),
+partition p1 values less than MAXVALUE
+) ;
+insert into t1 select 
+seq,
+MOD(seq, 100),
+MOD(seq, 100),
+'filler-data-filler-data'
+  from
+seq_1_to_5000;
+explain select * from t1 partition (p1) where a=10 and b=10;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	index_merge	a,b	a,b	5,5	NULL	1	Using intersect(a,b); Using where
+flush tables;
+select * from t1 partition (p1)where a=10 and b=10;
+pk	a	b	filler
+10	10	10	filler-data-filler-data
+110	10	10	filler-data-filler-data
+210	10	10	filler-data-filler-data
+310	10	10	filler-data-filler-data
+410	10	10	filler-data-filler-data
+510	10	10	filler-data-filler-data
+610	10	10	filler-data-filler-data
+710	10	10	filler-data-filler-data
+810	10	10	filler-data-filler-data
+910	10	10	filler-data-filler-data
+1010	10	10	filler-data-filler-data
+1110	10	10	filler-data-filler-data
+1210	10	10	filler-data-filler-data
+1310	10	10	filler-data-filler-data
+1410	10	10	filler-data-filler-data
+1510	10	10	filler-data-filler-data
+1610	10	10	filler-data-filler-data
+1710	10	10	filler-data-filler-data
+1810	10	10	filler-data-filler-data
+1910	10	10	filler-data-filler-data
+2010	10	10	filler-data-filler-data
+2110	10	10	filler-data-filler-data
+2210	10	10	filler-data-filler-data
+2310	10	10	filler-data-filler-data
+2410	10	10	filler-data-filler-data
+2510	10	10	filler-data-filler-data
+2610	10	10	filler-data-filler-data
+2710	10	10	filler-data-filler-data
+2810	10	10	filler-data-filler-data
+2910	10	10	filler-data-filler-data
+3010	10	10	filler-data-filler-data
+3110	10	10	filler-data-filler-data
+3210	10	10	filler-data-filler-data
+3310	10	10	filler-data-filler-data
+3410	10	10	filler-data-filler-data
+3510	10	10	filler-data-filler-data
+3610	10	10	filler-data-filler-data
+3710	10	10	filler-data-filler-data
+3810	10	10	filler-data-filler-data
+3910	10	10	filler-data-filler-data
+4010	10	10	filler-data-filler-data
+4110	10	10	filler-data-filler-data
+4210	10	10	filler-data-filler-data
+4310	10	10	filler-data-filler-data
+4410	10	10	filler-data-filler-data
+4510	10	10	filler-data-filler-data
+4610	10	10	filler-data-filler-data
+4710	10	10	filler-data-filler-data
+4810	10	10	filler-data-filler-data
+4910	10	10	filler-data-filler-data
+DROP TABLE t1;

mysql-test/main/partition_explicit_prune.test

@@ -1,5 +1,6 @@
 --source include/have_innodb.inc
 --source include/have_partition.inc
+--source include/have_sequence.inc
 
 # Helper statement
 let $get_handler_status_counts= SELECT * FROM INFORMATION_SCHEMA.SESSION_STATUS
@@ -874,7 +875,7 @@ SELECT * FROM t1 PARTITION (p0);
 ALTER TABLE t1 EXCHANGE PARTITION p0 WITH TABLE t2;
 SELECT * FROM t1 PARTITION (p0);
 UNLOCK TABLES;
-DROP TABLE t1;
+DROP TABLE t1, t2;
 
 --echo #
 --echo # MDEV-18371 Server crashes in ha_innobase::cmp_ref upon UPDATE with PARTITION clause.
@@ -885,7 +886,34 @@ INSERT INTO t1 VALUES (3,0),(8,2),(7,8),(3,4),(2,4),(0,7),(4,3),(3,6);
 FLUSH TABLES;
 UPDATE t1 PARTITION (p3,p1) SET a = 2 WHERE a = 3;
 SELECT * FROM t1;
+DROP TABLE t1;
 
+--echo #
+--echo # MDEV-21134 Crash with partitioned table, PARTITION syntax, and index_merge.
+--echo #
 
-# Cleanup
+create table t1 (
-DROP TABLE t1, t2;
+    pk int primary key,
+    a int,
+    b int,
+    filler char(32),
+    key (a),
+    key (b)
+    ) engine=myisam  partition by range(pk) (
+      partition p0 values less than (10),
+      partition p1 values less than MAXVALUE
+      ) ;
+
+insert into t1 select 
+seq,
+  MOD(seq, 100),
+  MOD(seq, 100),
+  'filler-data-filler-data'
+  from
+  seq_1_to_5000;
+
+explain select * from t1 partition (p1) where a=10 and b=10;
+flush tables;
+select * from t1 partition (p1)where a=10 and b=10;
+
+DROP TABLE t1;

sql/ha_partition.cc

@@ -3637,7 +3637,22 @@ int ha_partition::open(const char *name, int mode, uint test_if_locked)
     for (i= 0; i < m_tot_parts; i++)
     {
       if (!bitmap_is_set(&m_is_clone_of->m_opened_partitions, i))
+      {
+        /* Here we should just create the handler instance, not open it. */
+        if (!(m_file[i]= get_new_handler(table->s, m_clone_mem_root,
+                                         file[i]->ht)))
+        {
+          error= HA_ERR_INITIALIZATION;
+          file= &m_file[i];
+          goto err_handler;
+        }
+        if (m_file[i]->set_ha_share_ref(file[i]->ha_share))
+        {
+          error= HA_ERR_INITIALIZATION;
+          goto err_handler;
+        }
         continue;
+      }
 
       if (unlikely((error= create_partition_name(name_buff, sizeof(name_buff),
                                                  name, name_buffer_ptr,