MDEV-18371 Server crashes in ha_innobase::cmp_ref upon UPDATE with PARTITION clause.

m_file[0] not always is a good sample.

mysql-test/main/partition_explicit_prune.result

@@ -1888,4 +1888,22 @@ ALTER TABLE t1 EXCHANGE PARTITION p0 WITH TABLE t2;
 SELECT * FROM t1 PARTITION (p0);
 i
 UNLOCK TABLES;
+DROP TABLE t1;
+#
+# MDEV-18371 Server crashes in ha_innobase::cmp_ref upon UPDATE with PARTITION clause.
+#
+CREATE TABLE t1 (a INT, b INT, KEY (a)) ENGINE=InnoDB PARTITION BY KEY(b) PARTITIONS 4;
+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;
+a	b
+2	0
+7	8
+2	4
+2	4
+0	7
+4	3
+8	2
+2	6
 DROP TABLE t1, t2;

mysql-test/main/partition_explicit_prune.test

@@ -874,6 +874,18 @@ SELECT * FROM t1 PARTITION (p0);
 ALTER TABLE t1 EXCHANGE PARTITION p0 WITH TABLE t2;
 SELECT * FROM t1 PARTITION (p0);
 UNLOCK TABLES;
+DROP TABLE t1;
+
+--echo #
+--echo # MDEV-18371 Server crashes in ha_innobase::cmp_ref upon UPDATE with PARTITION clause.
+--echo #
+
+CREATE TABLE t1 (a INT, b INT, KEY (a)) ENGINE=InnoDB PARTITION BY KEY(b) PARTITIONS 4;
+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;
+
 
 # Cleanup
 DROP TABLE t1, t2;

sql/ha_partition.cc

@@ -5580,8 +5580,9 @@ extern "C" int cmp_key_rowid_part_id(void *ptr, uchar *ref1, uchar *ref2)
   {
     return res;
   }
-  if ((res= file->m_file[0]->cmp_ref(ref1 + PARTITION_BYTES_IN_POS + file->m_rec_length,
-                                     ref2 + PARTITION_BYTES_IN_POS + file->m_rec_length)))
+  if ((res= file->get_open_file_sample()->cmp_ref(ref1 +
+          PARTITION_BYTES_IN_POS + file->m_rec_length,
+          ref2 + PARTITION_BYTES_IN_POS + file->m_rec_length)))
   {
     return res;
   }
@@ -9577,7 +9578,7 @@ uint8 ha_partition::table_cache_type()
 {
   DBUG_ENTER("ha_partition::table_cache_type");
 
-  DBUG_RETURN(m_file[0]->table_cache_type());
+  DBUG_RETURN(get_open_file_sample()->table_cache_type());
 }