Re: Offline data checksum changes can cause incorrect checksum state on standbys

Hi Raja,

Thanks for the patch. I tried v1 (cfbot branch cf/7587, commit
8dda72e) on Ubuntu 26.04 (WSL2), applied on master c1694b7. It applied
cleanly with git apply and pg_combinebackup rebuilt fine. The build
was --without-icu and without TAP tests.

I used a small script (attached as repro.sh) with initdb
--no-data-checksums, summarize_wal=on, a full backup, pg_checksums
--enable on the stopped cluster, then an incremental backup and
pg_combinebackup.

Before (master):
pg_combinebackup: error: only some backups have checksums enabled
pg_combinebackup: hint: Take a new full backup after changing the
data checksum state with pg_checksums.
(exit=1)

After (v1):
pg_combinebackup: error: only some backups have checksums enabled
pg_combinebackup: hint: Take a new full backup after changing the
data checksum state.
(exit=1)

I got the same before/after result for the online case. There I took a
full backup with checksums off, ran pg_enable_data_checksums(), waited
for data_checksums=on, took an incremental backup, and ran
pg_combinebackup. The full backup's control file shows
data_checksum_version 0 and the incremental shows 1.

Observations:
- The new hint matches the behaviour, since the check also triggers
after online enabling and the old hint pointed only at pg_checksums.
- The hint is now less actionable. A reader gets the "what" (take a
new full backup) but not how the state changed. That seems fine to me,
but you could keep it explicit, for example "after enabling or
disabling data checksums". I did not test a backup chain across a
disable.
- The doc xref checksums-online-enable-disable exists in wal.sgml. I
could not build the docs here (xmllint is missing), so the SGML change
is only read, not rendered.
- The new doc text says the chain is refused for backups taken "while
they were still being enabled". I did not test that intermediate
state, so I cannot confirm that sentence from a run.
- I did not run the pg_combinebackup TAP tests (IPC::Run not
available), and a grep for "data checksum state" in
src/bin/pg_combinebackup/t finds no test for this hint, so a small
test for the error/hint might be worth adding.
- src/bin/pg_combinebackup/po exists, so the translations may need an
update; I did not check the .po files.

Thanks again,
João Rito