Aria engine: Undo phase failed with "Got error 121 when executing undo undo_key_delete" upon startup on datadir restored from incremental backup

Note: see also MDEV-18203 – different error, otherwise the same.

2019-01-18 21:26:44 0 [Note] mysqld: Aria engine: starting recovery

recovered pages: 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% (20.8 seconds); transactions to roll back: 1

Got error 121 when executing undo undo_key_delete

2019-01-18 21:27:05 0 [ERROR] mysqld: Aria engine: Undo phase failed

tables to flush: 8 7 6 5 4 3 2 1 0

2019-01-18 21:27:05 0 [ERROR] mysqld: Aria recovery failed. Please run aria_chk -r on all Aria tables and delete all aria_log.######## files

2019-01-18 21:27:05 0 [ERROR] Plugin 'Aria' init function returned error.

2019-01-18 21:27:05 0 [ERROR] Plugin 'Aria' registration as a STORAGE ENGINE failed.

The backup dirs are here:
ftp://ftp.askmonty.org/public/mdev18310

backup_before_prepare_0 is the initial full backup
backup_before_prepare_1 is the 1st incremental backup
backup_before_prepare_2 is the 1st incremental backup
backup_0 is the prepared backup after all prepare were executed on it.

To reproduce the failure, restore backup_0 to the datadir and run the server on it. All server default options are sufficient.

If you have doubts about the correctness of prepare, you can run it on non-prepared backups.

mariabackup logs from all stages are attached with hopefully self-explanatory names (mbackup*.log).

I have no information yet whether it affects previous versions.

perl ./runall-new.pl --basedir=/home/travis/server --vardir=/home/travis/logs/vardir --duration=350 --threads=4 --seed=1547846609 --reporters=Backtrace,ErrorLog,Deadlock --skip-gendata --gendata-advanced --views --grammar=conf/mariadb/generic-dml.yy --redefine=conf/mariadb/bulk_insert.yy --mysqld=--log_output=FILE --mysqld=--max-statement-time=20 --mysqld=--lock-wait-timeout=10 --mysqld=--loose-innodb-lock-wait-timeout=5 --mysqld=--loose-debug_assert_on_not_freed_memory=0 --filter=/home/travis/mariadb-toolbox/travis/10.4-combo-filter.ff --engine=Aria --scenario=MariaBackupIncremental