Re: Executing pg_createsubscriber with a non-compatible control file

On Tue, Oct 07, 2025 at 11:51:45AM -0700, Masahiko Sawada wrote:
> Just to be clear, did you mean that pg_checksums and pg_rewind already
> do such checks? IIUC pg_checksums does CRC check for the control file,
> and if we execute v18-pg_checksums against v17-cluster we end up with
> a similar error but with a different log message like "pg_checksums:
> error: pg_control CRC value is incorrect". Those log messages are not
> helpful either.

For the case of pg_checksums, we don't really have a constraint based
on the major version, currently.  However, there is a PG_VERSION_NUM
hardcoded when syncing the data folder, so we are pretty much assuming
that it is the case already.

A check based on PG_VERSION has more benefits in the long-term, IMO.
When the CRC check of the control file fails, it would be tempting to
use some of the contents read from the control file but that would
also mean that we could expose some corrupted values to the user, so
that would not be useful.

How about extracting from pg_rewind what it does with the on-disk
PG_VERSION and create an API that returns a major version number  with
a data folder given in input?  We could then reuse this API for the
other tools, at least pg_createsubscriber and pg_checksums.  I am not
sure if there is a point in backpatching any of that, but it would
lead to more user-friendly errors for all of these.
--
Michael