During the execution of the "delete" command, the oldest start LSN is saved
and used to determine what is the oldest WAL segment that needs to be kept
for the existing backups. Like pg_archivecleanup, the implemented logic
ignores the timeline part of the WAL segment name, ensuring that a segment
from a parent timeline will not be removed prematurely. This logic could
be made more complicated regarding this matter, but in order to take backups
from a node on a given timeline this is far enough.
Performance of compression is quite questionable on many objects like
that and makes the routines aimed at managing file copy, backup and restore
more complicated than they should be.
This commit results in a largely simplified code in data.c, which will
be helpful when integrating differential backup using WAL file lookup.
This new facility has the advantage on not relying on static data when
generating the tests making the whole facility more robust. This is
basically taken from the upstream project pg_rman and adapted for the
sake of this pet project, so most of the credit go to Kyotaro Horiguchi
and Amit Langote regarding this facility. However I have adapted a bunch
of things and fixed a lot of redundancy and code duplication.
This mode is not actually necessary if we consider that the core of
pg_rman is the obtention of differential and full backups, the server
being afterwards in charge to recover necessary WAL segments from the
archive.
Regression tests and documentation are updated in accordance to the
changes.
In order to keep only the core of pg_rman for incremental/differential
backup, this looks necessary and makes the code more simple. Including
server log files in a backup could be subject to discussion as well,
as for example a Postgres base backup does not include them, just
because in this case server instance is not aware of the log files.
Diffs were generated because of wc that puts some spaces before the
output which is in this case a number of lines. This does not impact
regression tests on Linux/Unix.
