MDEV-24848 Assertion rlen<llen failed when applying MEMSET

btr_cur_upd_rec_in_place(): Prefer WRITE to MEMSET for a single-byte
operation.

log_phys_t::apply(): Relax the assertion to allow a single-byte MEMSET,
even though it is 1 byte longer than a WRITE record.

mysql-test/suite/mariabackup/row_format_redundant.result

+CREATE TABLE t1 (pk INT PRIMARY KEY, a YEAR UNSIGNED) ENGINE=InnoDB
+ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 VALUES (1,2021),(2,21),(3,0);
+UPDATE t1 SET a = NULL;
+# shutdown server
+# remove datadir
+# xtrabackup move back
+# restart
+SELECT * FROM t1;
+pk	a
+1	NULL
+2	NULL
+3	NULL
+DROP TABLE t1;

mysql-test/suite/mariabackup/row_format_redundant.test

+--source include/have_innodb.inc
+
+--let $targetdir=$MYSQLTEST_VARDIR/tmp/backup
+
+CREATE TABLE t1 (pk INT PRIMARY KEY, a YEAR UNSIGNED) ENGINE=InnoDB
+ROW_FORMAT=REDUNDANT;
+INSERT INTO t1 VALUES (1,2021),(2,21),(3,0);
+UPDATE t1 SET a = NULL;
+
+exec $XTRABACKUP --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --target-dir=$targetdir;
+exec $XTRABACKUP --prepare --verbose --target-dir=$targetdir;
+--source include/restart_and_restore.inc
+
+rmdir $targetdir;
+
+SELECT * FROM t1;
+DROP TABLE t1;

storage/innobase/btr/btr0cur.cc

@@ -3,7 +3,7 @@
 Copyright (c) 1994, 2019, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2008, Google Inc.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2015, 2020, MariaDB Corporation.
+Copyright (c) 2015, 2021, MariaDB Corporation.
 
 Portions of this file contain modifications contributed and copyrighted by
 Google, Inc. Those modifications are gratefully acknowledged and are described
@@ -4149,7 +4149,16 @@ void btr_cur_upd_rec_in_place(rec_t *rec, const dict_index_t *index,
 			}
 
 			ut_ad(!index->table->not_redundant());
-			if (ulint size = rec_get_nth_field_size(rec, n)) {
+			switch (ulint size = rec_get_nth_field_size(rec, n)) {
+			case 0:
+				break;
+			case 1:
+				mtr->write<1,mtr_t::MAYBE_NOP>(
+					*block,
+					rec_get_field_start_offs(rec, n) + rec,
+					0U);
+				break;
+			default:
 				mtr->memset(
 					block,
 					page_offset(rec_get_field_start_offs(

storage/innobase/log/log0recv.cc

@@ -2,7 +2,7 @@
 
 Copyright (c) 1997, 2017, Oracle and/or its affiliates. All Rights Reserved.
 Copyright (c) 2012, Facebook Inc.
-Copyright (c) 2013, 2020, MariaDB Corporation.
+Copyright (c) 2013, 2021, MariaDB Corporation.
 
 This program is free software; you can redistribute it and/or modify it under
 the terms of the GNU General Public License as published by the Free Software
@@ -490,7 +490,7 @@ struct log_phys_t : public log_rec_t
         llen= len;
         if ((b & 0x70) == MEMSET)
         {
-          ut_ad(rlen < llen);
+          ut_ad(rlen <= llen);
           if (UNIV_UNLIKELY(rlen != 1))
           {
             size_t s;
@@ -499,7 +499,7 @@ struct log_phys_t : public log_rec_t
             memcpy(frame + last_offset + s, l, llen - s);
           }
           else
-          memset(frame + last_offset, *l, llen);
+            memset(frame + last_offset, *l, llen);
           goto next_after_applying_write;
         }
         const size_t slen= mlog_decode_varint_length(*l);