MDEV-10418 Assertion `m_extra_cache' failed in

ha_partition::late_extra_cache(uint).

        m_extra_prepare_for_update should be cleaned in
        ha_partition::reset()

mysql-test/r/partition_myisam.result

@@ -248,3 +248,19 @@ PARTITION p1 VALUES LESS THAN (100) MAX_ROWS=100,
 PARTITION pMax VALUES LESS THAN MAXVALUE);
 INSERT INTO t1 VALUES (1, "Partition p1, first row");
 DROP TABLE t1;
+#
+# MDEV-10418 Assertion `m_extra_cache' failed
+#    in ha_partition::late_extra_cache(uint)
+#
+CREATE TABLE t1 (f1 INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+CREATE TABLE t2 (f2 INT) ENGINE=MyISAM PARTITION BY RANGE(f2) (PARTITION pmax VALUES LESS THAN MAXVALUE);
+INSERT INTO t2 VALUES (8);
+CREATE ALGORITHM = MERGE VIEW v AS SELECT f2 FROM t2, t1;
+UPDATE v SET f2 = 1;
+SELECT * FROM t2;
+f2
+1
+DROP VIEW v;
+DROP TABLE t2;
+DROP TABLE t1;

mysql-test/t/partition_myisam.test

@@ -180,3 +180,24 @@ PARTITION BY RANGE (a)
  PARTITION pMax VALUES LESS THAN MAXVALUE);
 INSERT INTO t1 VALUES (1, "Partition p1, first row");
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-10418 Assertion `m_extra_cache' failed
+--echo #    in ha_partition::late_extra_cache(uint)
+--echo #
+
+CREATE TABLE t1 (f1 INT) ENGINE=MyISAM;
+INSERT INTO t1 VALUES (1),(2);
+
+CREATE TABLE t2 (f2 INT) ENGINE=MyISAM PARTITION BY RANGE(f2) (PARTITION pmax VALUES LESS THAN MAXVALUE);
+INSERT INTO t2 VALUES (8);
+
+CREATE ALGORITHM = MERGE VIEW v AS SELECT f2 FROM t2, t1;
+
+UPDATE v SET f2 = 1;
+
+SELECT * FROM t2;
+
+DROP VIEW v;
+DROP TABLE t2;
+DROP TABLE t1;

sql/ha_partition.cc

@@ -6667,6 +6667,7 @@ int ha_partition::reset(void)
   DBUG_ENTER("ha_partition::reset");
   if (m_part_info)
     bitmap_set_all(&m_part_info->used_partitions);
+  m_extra_prepare_for_update= FALSE;
   file= m_file;
   do
   {