Before MDEV-12113 (MariaDB Server 10.1.25), on shutdown InnoDB would write
the current LSN to the first page of each file of the system tablespace.
This is incompatible with MariaDB's InnoDB table encryption, because
encryption repurposed the field for an encryption key ID and checksum.
buf_page_is_corrupted(): For the InnoDB system tablespace, skip
FIL_PAGE_FILE_FLUSH_LSN when checking if a page is all zero,
because the first page of each file in the system tablespace can
contain nonzero bytes in the field.
Simplify, and make it work with system tablespace outside of
innodb data home.
Also, do not reread TRX_SYS page in endless loop,
if it appears to be corrupted.
Use finite number of attempts.
System tablespace can be specified with absolute path, if innodb_data_home_dir
is empty.
This was not handled well by mariabackup
1. In backup phase, empty innodb_data_home_dir variable from server was
not recognized. full paths were stored in backup-my.ini, even if
it stored all files locally. thus prepare phase would not find the system
tablespace files.
2. In copy-back phase, copy would not be done to the absolute destination
path, as path would be stripped with trim_dotslash
This patch fixes the above defects.
