MDEV-37183 Scrubbing empty record breaks recovery

page_delete_rec_list_end(): Do not attempt to scrub the data of
an empty record.

The test case would reproduce a debug assertion failure in branches
where commit 358921ce32 (MDEV-26938)
is present. MariaDB Server 10.6 only supports ascending indexes,
and in those, the empty string would always be sorted first, never
last in a page.

Nevertheless, we fix the bug also in 10.6, in case it would be
reproducible in a slightly different scenario.

Reviewed by: Thirunarayanan Balathandayuthapani

mysql-test/suite/innodb/r/scrub_debug.result

@@ -15,5 +15,21 @@ FLUSH TABLE t1 FOR EXPORT;
 NOT FOUND /repairman/ in t1.ibd
 UNLOCK TABLES;
 DROP TABLE t1;
+#
+# MDEV-37183 innodb_immediate_scrub_data_uncompressed=ON may break
+# crash recovery
+#
+SET GLOBAL innodb_limit_optimistic_insert_debug=0;
+CREATE TABLE t(a VARCHAR(1) PRIMARY KEY,INDEX(a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES('2'),('1'),(''),('6'),('4'),('3');
+SET GLOBAL innodb_limit_optimistic_insert_debug=3;
+INSERT INTO t VALUES('8');
+CHECK TABLE t;
+Table	Op	Msg_type	Msg_text
+test.t	check	status	OK
+SELECT COUNT(*) FROM t;
+COUNT(*)
+7
+DROP TABLE t;
 SET GLOBAL INNODB_LIMIT_OPTIMISTIC_INSERT_DEBUG=@save_debug;
 SET GLOBAL INNODB_IMMEDIATE_SCRUB_DATA_UNCOMPRESSED=@save_scrub;

mysql-test/suite/innodb/t/scrub_debug.test

@@ -24,5 +24,20 @@ FLUSH TABLE t1 FOR EXPORT;
 -- source include/search_pattern_in_file.inc
 UNLOCK TABLES;
 DROP TABLE t1;
+
+--echo #
+--echo # MDEV-37183 innodb_immediate_scrub_data_uncompressed=ON may break
+--echo # crash recovery
+--echo #
+SET GLOBAL innodb_limit_optimistic_insert_debug=0;
+# Note: MariaDB 10.6 fails to reproduce the crash; it maps DESC to ASC.
+CREATE TABLE t(a VARCHAR(1) PRIMARY KEY,INDEX(a DESC)) ENGINE=InnoDB;
+INSERT INTO t VALUES('2'),('1'),(''),('6'),('4'),('3');
+SET GLOBAL innodb_limit_optimistic_insert_debug=3;
+INSERT INTO t VALUES('8');
+CHECK TABLE t;
+SELECT COUNT(*) FROM t;
+DROP TABLE t;
+
 SET GLOBAL INNODB_LIMIT_OPTIMISTIC_INSERT_DEBUG=@save_debug;
 SET GLOBAL INNODB_IMMEDIATE_SCRUB_DATA_UNCOMPRESSED=@save_scrub;

storage/innobase/page/page0page.cc

@@ -977,8 +977,9 @@ page_delete_rec_list_end(
       size+= s;
       n_recs++;
 
-      if (scrub)
-        mtr->memset(block, rec2 - page, rec_offs_data_size(offsets), 0);
+      if (UNIV_LIKELY(!scrub));
+      else if (size_t size= rec_offs_data_size(offsets))
+        mtr->memset(block, rec2 - page, size, 0);
 
       rec2= page_rec_get_next(rec2);
     }