Re: Add LSN along with offset to error messages reported for WAL file read/write/validate header failures

On Mon, Dec 5, 2022 at 6:34 AM Michael Paquier <michael@paquier.xyz> wrote:
>
> Regarding pg_walfile_offset_lsn(), I am not sure that this is the best
> move we can do as it is possible to compile a LSN from 0/0 with just a
> segment number, say:
> select '0/0'::pg_lsn + :segno * setting::int + :offset
>   from pg_settings where name = 'wal_segment_size';

Nice.

> +   resultTupleDesc = CreateTemplateTupleDesc(2);
> +   TupleDescInitEntry(resultTupleDesc, (AttrNumber) 1, "lsn",
> +                      PG_LSNOID, -1, 0);
> +   TupleDescInitEntry(resultTupleDesc, (AttrNumber) 2, "timeline_id",
> +                      INT4OID, -1, 0);
> Let's use get_call_result_type() to get the TupleDesc and to avoid a
> duplication between pg_proc.dat and this code.
>
> Hence I would tend to let XLogFromFileName do the job, while having a
> SQL function that is just a thin wrapper around it that returns the
> segment TLI and its number, leaving the offset out of the equation as
> well as this new XLogIdFromFileName().

So, a SQL function pg_dissect_walfile_name (or some other better name)
given a WAL file name returns the tli and seg number. Then the
pg_walfile_offset_lsn can just be a SQL-defined function (in
system_functions.sql) using this new function and pg_settings. If this
understanding is correct, it looks good to me at this point.

That said, let's also hear from others.

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com