MDEV-23596: Assertion `tab->ref.use_count' failed in join_read_key_unlock_row

The issue here was that the query was using ORDER BY LIMIT optimzation where
the access method was changed from EQ_REF access to an index scan (index that would
resolve the ORDER BY clause).
But the parameter READ_RECORD::unlock_row was not reset to rr_unlock_row, which is
used when the access method is not EQ_REF access.

mysql-test/r/order_by.result

@@ -3356,3 +3356,19 @@ SET max_sort_length= @save_max_sort_length;
 SET sort_buffer_size= @save_sort_buffer_size;
 SET max_length_for_sort_data= @save_max_length_for_sort_data;
 DROP TABLE t1;
+#
+# MDEV-23596: Assertion `tab->ref.use_count' failed in join_read_key_unlock_row
+#
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, KEY(b));
+INSERT INTO t1 VALUES (0, 1),(1, 2);
+CREATE TABLE t2 SELECT * FROM t1;
+EXPLAIN SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+id	select_type	table	type	possible_keys	key	key_len	ref	rows	Extra
+1	PRIMARY	t2	ALL	NULL	NULL	NULL	NULL	2	
+2	DEPENDENT SUBQUERY	t1	index	PRIMARY	b	5	NULL	1	Using where
+SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+c
+1
+NULL
+DROP TABLE t1,t2;
+# end of 10.1 tests

mysql-test/t/order_by.test

@@ -2196,3 +2196,18 @@ SET max_sort_length= @save_max_sort_length;
 SET sort_buffer_size= @save_sort_buffer_size;
 SET max_length_for_sort_data= @save_max_length_for_sort_data;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-23596: Assertion `tab->ref.use_count' failed in join_read_key_unlock_row
+--echo #
+
+CREATE TABLE t1 (a INT PRIMARY KEY, b INT, KEY(b));
+INSERT INTO t1 VALUES (0, 1),(1, 2);
+CREATE TABLE t2 SELECT * FROM t1;
+
+EXPLAIN SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+SELECT (SELECT 1 FROM t1 WHERE t1.a=t2.b ORDER BY t1.b LIMIT 1) AS c FROM t2;
+
+DROP TABLE t1,t2;
+
+--echo # end of 10.1 tests

sql/sql_select.cc

@@ -21601,6 +21601,9 @@ test_if_skip_sort_order(JOIN_TAB *tab,ORDER *order,ha_rows select_limit,
     else if (select && select->quick)
       select->quick->need_sorted_output();
 
+    tab->read_record.unlock_row= (tab->type == JT_EQ_REF) ?
+                                 join_read_key_unlock_row : rr_unlock_row;
+
   } // QEP has been modified
 
   /*