INSERT into a secondary index with zero-data-length key is not crash-safe

Problem found during RQG testing invoking the RQG reporter "Mariabackup".

Scenario:

1. More or less heavy DDL/DML load

     During that some call   mariabackup ... --backup ... is made.

2. mariabackup ... --prepare  ...  is called first time.

2020-03-06 19:20:38 0 [Note] InnoDB: Starting final batch to recover 491 pages from redo log.

2020-03-06 19:20:39 0 [ERROR] InnoDB: Not applying DELETE_ROW_FORMAT_DYNAMIC due to corruption on [page id: space=167, page number=36]

2020-03-06 19:20:39 0 [ERROR] InnoDB: Set innodb_force_recovery=1 to ignore corruption.

2020-03-06 19:20:39 0 [ERROR] InnoDB: Plugin initialization aborted at srv0start.cc[1580] with error Data structure corruption

[00] FATAL ERROR: 2020-03-06 19:20:40 mariabackup: innodb_init() returned 39 (Data structure corruption).

origin/HEAD, origin/10.5 23685378bafa38a74957e8f07e2d56b57e90fa53 2020-03-06T11:06:59+02:00

Some second variant for the same failure is

1. Only on session and that runs

    REPLACE INTO <random table which was created earlier> ( _field_no_pk ) VALUES ( <random _mediumint  value) ;

2. At some point of time the server gets killed (SIGKILL)

3. Attempt to restart the server

2020-03-23 14:12:49 0 [Note] InnoDB: Completed initialization of buffer pool

2020-03-23 14:12:49 0 [Note] InnoDB: If the mysqld execution user is authorized, page cleaner thread priority can be changed. See the man page of setpriority().

2020-03-23 14:12:49 0 [Note] InnoDB: Starting crash recovery from checkpoint LSN=335923252

2020-03-23 14:12:49 0 [Note] InnoDB: 1 transaction(s) which must be rolled back or cleaned up in total 2 row operations to undo

2020-03-23 14:12:49 0 [Note] InnoDB: Trx id counter is 21150

2020-03-23 14:12:49 0 [Note] InnoDB: Starting final batch to recover 272 pages from redo log.

2020-03-23 14:12:49 0 [ERROR] InnoDB: Not applying DELETE_ROW_FORMAT_DYNAMIC due to corruption on [page id: space=5, page number=22]

2020-03-23 14:12:49 0 [ERROR] InnoDB: Set innodb_force_recovery=1 to ignore corruption.

2020-03-23 14:12:49 0 [ERROR] InnoDB: Plugin initialization aborted at srv0start.cc[1574] with error Data structure corruption

origin/10.5 02fe99750536aad7c9e75513be47c91c8c568419 2020-03-23T10:45:08+01:00

The bug affects builds with and without debug.