pgsql: pg_checksums: Refuse interrupted transitions, note change is loc

pg_checksums: Refuse interrupted transitions, note change is local

An inprogress-on or inprogress-off control file means an online
transition was cut short, running the offline tool on top of it mixes
two procedures. Refuse it in every mode, with a hint pointing at the
start-stop cycle (or, on a standby, at letting replication finish)
that resets the state. Also print that the change applies to one
data directory only, with standby-aware wording, since in a
replication setup the same change must be made on every node.

On a primary, inprogress-on cannot survive a graceful stop: the
checksums launcher resolves it from its exit cleanup, and a crashed
primary is already rejected by the existing "cluster must be shut
down" check. inprogress-off can, however, it is set by the backend
running pg_disable_data_checksums(), and a fast shutdown arriving
between its two barriers leaves it behind in a cleanly shut down
control file, where the next start-stop cycle resolves it at end of
recovery. A standby can be stopped with either state, since it has
no launcher and only carries forward whatever state the last replayed
WAL record left it in, with a restartpoint persisting that as-is.
The standby is also the deterministic way to reach the new guard,
which is why the test coverage uses one; the primary window would
need an injection point between the two barriers.

The pg_checksums page said the tool still processes all relation files
regardless of an interrupted online transition; describe the refusal
instead.

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/3e70c527cc399e3d7f569f6d01b6601f20b82255

Modified Files
--------------
doc/src/sgml/ref/pg_checksums.sgml | 11 ++++----
src/bin/pg_checksums/pg_checksums.c | 21 ++++++++++++++++
.../test_checksums/t/012_offline_standby.pl | 29 +++++++++++++++++-----
3 files changed, 50 insertions(+), 11 deletions(-)