Problem:
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  Mariabackup exits during prepare phase if it encounters
MLOG_INDEX_LOAD redo log record. MLOG_INDEX_LOAD record
informs Mariabackup that the backup cannot be completed based
on the redo log scan, because some information is purposely
omitted due to bulk index creation in ALTER TABLE.

Solution:
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Detect the MLOG_INDEX_LOAD redo record during backup phase and
exit the mariabackup with the proper error message.