MDEV-20515 multi-update tries to position updated table by null reference

Cause

Join tmp table inserts null row because of OUTER JOIN, that's
expected. Since `multi_update::prepare2()` converted
`Item_temptable_rowid` into `Item_field` (28dbdf3)
`multi_update::send_data()` accesses join tmp record directly and
treats it as a normal row ignoring null status of ref field. NULL ref
field is then treated as normal in `multi_update::do_updates()` which
tries to position updated table by reference 0.

Note that reference 0 may be valid reference and the first row of
table can be wrongly updated (see multi_update.test).

Fix

Do not add row into multi-update tmp table in case of null ref
field. Join tmp table does not have null_row status at this time (as
well as `STATUS_NULL_ROW`) and cannot be skipped by these properties
(see first comment in multi_update::send_data()). But it has all null
fields (including the ref field).

mysql-test/main/multi_update.result

@@ -1081,3 +1081,20 @@ b	c
 2	0
 drop view v1;
 drop table t0, t1,t2;
+#
+# MDEV-20515 multi-update tries to position updated table by null reference
+#
+create or replace table t1 (a int);
+insert into t1 values (0), (2);
+create or replace table t2 (b int);
+insert into t2 values (1), (2);
+select * from t1 left join t2 on a = b order by b;
+a	b
+0	NULL
+2	2
+update t1 left join t2 on a = b set b= 3 order by b;
+select * from t2;
+b
+1
+3
+drop tables t1, t2;

mysql-test/main/multi_update.test

@@ -1019,3 +1019,18 @@ update v1,t0 set c=1 where b<3 and x=c order by x,b limit 1;
 select * from v1;
 drop view v1;
 drop table t0, t1,t2;
+
+--echo #
+--echo # MDEV-20515 multi-update tries to position updated table by null reference
+--echo #
+create or replace table t1 (a int);
+insert into t1 values (0), (2);
+
+create or replace table t2 (b int);
+insert into t2 values (1), (2);
+
+select * from t1 left join t2 on a = b order by b;
+update t1 left join t2 on a = b set b= 3 order by b;
+select * from t2;
+
+drop tables t1, t2;

mysql-test/suite/versioning/r/partition.result

@@ -615,6 +615,17 @@ insert into t2 values (1),(2);
 update t1, t2 set a = 1;
 drop table t1, t2;
 #
+# MDEV-20515 multi-update tries to position updated table by null reference
+#
+create or replace table t1 (a int);
+insert into t1 values (0), (1);
+create or replace table t2 (b int) with system versioning
+partition by system_time
+(partition p1 history, partition pn current);
+insert into t2 values (0), (2);
+update t1 left join t2 on a > b set b= 2 order by b;
+drop table t1, t2;
+#
 # MDEV-17091 Assertion `old_part_id == m_last_part' failed in
 # ha_partition::update_row or `part_id == m_last_part' in
 # ha_partition::delete_row upon UPDATE/DELETE after dropping versioning

mysql-test/suite/versioning/t/partition.test

@@ -567,6 +567,22 @@ update t1, t2 set a = 1;
 # cleanup
 drop table t1, t2;
 
+--echo #
+--echo # MDEV-20515 multi-update tries to position updated table by null reference
+--echo #
+create or replace table t1 (a int);
+insert into t1 values (0), (1);
+
+create or replace table t2 (b int) with system versioning
+partition by system_time
+(partition p1 history, partition pn current);
+
+insert into t2 values (0), (2);
+update t1 left join t2 on a > b set b= 2 order by b;
+
+# cleanup
+drop table t1, t2;
+
 --source suite/versioning/common_finish.inc
 --echo #
 --echo # MDEV-17091 Assertion `old_part_id == m_last_part' failed in

sql/sql_update.cc

@@ -2467,6 +2467,9 @@ int multi_update::send_data(List<Item> &not_used_values)
       TABLE *tmp_table= tmp_tables[offset];
       if (copy_funcs(tmp_table_param[offset].items_to_copy, thd))
         DBUG_RETURN(1);
+      /* rowid field is NULL if join tmp table has null row from outer join */
+      if (tmp_table->field[0]->is_null())
+        continue;
       /* Store regular updated fields in the row. */
       DBUG_ASSERT(1 + unupdated_check_opt_tables.elements ==
                   tmp_table_param[offset].func_count);
@@ -2671,6 +2674,7 @@ int multi_update::do_updates()
       uint field_num= 0;
       do
       {
+        DBUG_ASSERT(!tmp_table->field[field_num]->is_null());
         if (unlikely((local_error=
                       tbl->file->ha_rnd_pos(tbl->record[0],
                                             (uchar *) tmp_table->