An interface to use memory-mapped I/O on the InnoDB redo log that
is stored in persistent memory was introduced
in commit 685d958e38 (MDEV-14425).
log_t::attach(): In mariadb-backup --backup, never attempt to
use memory-mapped I/O for reading the log file of the server.
xtrabackup_copy_logfile(): Assert !log_sys.is_pmem() and remove
the code to deal with a memory-mapped log.
This fixes a race condition scenario of the following type:
1. Backup parsed a mini-transaction from the memory-mapped buffer.
This took some time.
2. Meanwhile, the server might have overwritten this portion
of the circular log_sys.buf.
3. Backup copied the data to the output file while or after
the server had overwritten this portion of the file.
4. Backup failed to notice that a log overrun occurred.
The symptom of this was that a mariadb-backup --prepare of the
log failed. In the analyzed case, the error message was:
[ERROR] InnoDB: Missing FILE_CHECKPOINT(...)
This will also make it possible to run mariadb-backup --backup
under "rr replay".
155019b96b