pgsql: pg_rewind: Check the data checksum states of source and target

pg_rewind: Check the data checksum states of source and target

pg_rewind installs the source's control file on the target, but every
block it does not copy keeps the target's content. With checksums
enabled on the target and disabled on the source, the rewound server
claims enabled checksums while the blocks copied from the source have
none, and fails checksum verification as soon as it reads them; in the
worst case every connection attempt dies on an unverifiable catalog
page.

Comparing the control files is not enough. Replay on the rewound
server resumes from the last common checkpoint and adopts the data
checksum state recorded there, so an offline disable on the source
after the divergence leaves both control files saying "off" while the
rewound server still resumes with verification enabled. Check the
state carried by the divergence checkpoint as well, which pg_rewind
already reads.

Refuse both cases, and an interrupted online transition on either
side, same as pg_checksums does. The opposite mismatch, checksums on
the source only, is allowed with a warning: recovery under the backup
label adopts the divergence state, so the rewound server keeps
checksums disabled (or converges through the replayed WAL if the
source enabled them online), and no page can fail verification.

Author: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Daniel Gustafsson <daniel(at)yesql(dot)se>
Discussion: https://postgr.es/m/anwm6UPxoVS41QA2@bdtpg
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/227f8ea19a6b321af90607fe12d58eb5ce2c2dce

Modified Files
--------------
doc/src/sgml/ref/pg_rewind.sgml | 14 ++
src/bin/pg_rewind/parsexlog.c | 4 +-
src/bin/pg_rewind/pg_rewind.c | 93 ++++++++++++-
src/bin/pg_rewind/pg_rewind.h | 1 +
src/test/modules/test_checksums/meson.build | 2 +
.../modules/test_checksums/t/022_rewind_state.pl | 145 +++++++++++++++++++
.../test_checksums/t/023_rewind_standby_target.pl | 153 +++++++++++++++++++++
7 files changed, 410 insertions(+), 2 deletions(-)