MDEV-30442: Assertion `!m_innodb' failed in ha_partition::cmp_ref ...

The failed assertion was about encountering the same rowid value in
two different partitions.
This wasn't possible with InnoDB previously: InnoDB used a global counter
to produce rowid values for hidden PK.

After the fix for MDEV-19506, it uses per-table counters so it's easily
possible to get the same hidden PK values in different tables.

mysql-test/main/partition_innodb2.result

@@ -0,0 +1,24 @@
+#
+# MDEV-30442: Assertion `!m_innodb' failed in ha_partition::cmp_ref on MULTI-DELETE
+#
+create table t1 (a int) engine=innodb;
+insert into t1 values (1),(2),(1),(2);
+create table t2 (
+a int,
+b int,
+key(a)
+) engine=innodb partition by list(a)
+(
+partition p0 values in (1),
+partition p1 values in (2),
+partition p2 values in (0,3,4,5,6,7,8,9)
+);
+insert into t2 select
+mod(seq, 10), seq from seq_1_to_50;
+explain
+delete t1, t2 from t1, t2 where t1.a=t2.a;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	SIMPLE	t1	ALL	NULL	NULL	NULL	NULL	4	Using where
+1	SIMPLE	t2	ref	a	a	5	test.t1.a	1	
+delete t1, t2 from t1, t2 where t1.a=t2.a;
+drop table t1,t2;

mysql-test/main/partition_innodb2.test

@@ -0,0 +1,30 @@
+--source include/not_embedded.inc
+--source include/have_partition.inc
+--source include/have_innodb.inc
+--source include/have_sequence.inc
+
+--echo #
+--echo # MDEV-30442: Assertion `!m_innodb' failed in ha_partition::cmp_ref on MULTI-DELETE
+--echo #
+create table t1 (a int) engine=innodb;
+insert into t1 values (1),(2),(1),(2);
+
+create table t2 (
+  a int,
+  b int,
+  key(a)
+) engine=innodb partition by list(a)
+(
+  partition p0 values in (1),
+  partition p1 values in (2),
+  partition p2 values in (0,3,4,5,6,7,8,9)
+);
+
+insert into t2 select
+  mod(seq, 10), seq from seq_1_to_50;
+
+explain
+delete t1, t2 from t1, t2 where t1.a=t2.a;
+delete t1, t2 from t1, t2 where t1.a=t2.a;
+
+drop table t1,t2;

sql/ha_partition.cc

@@ -10784,13 +10784,6 @@ int ha_partition::cmp_ref(const uchar *ref1, const uchar *ref2)
     DBUG_RETURN(0);
   }
 
-  /*
-    In Innodb we compare with either primary key value or global DB_ROW_ID so
-    it is not possible that the two references are equal and are in different
-    partitions, but in myisam it is possible since we are comparing offsets.
-    Remove this assert if DB_ROW_ID is changed to be per partition.
-  */
-  DBUG_ASSERT(!m_innodb);
   DBUG_RETURN(diff2 > diff1 ? -1 : 1);
 }