Re: Proposal: manipulating pg_control file from Perl

Aleksander Alekseev <aleksander(at)timescale(dot)com> writes:

> Hi Dagfinn,
>
>> Notwitstanding Tom's objections, these are not reasons for not being
>> able to manipulate these values from Perl. The `i` and `I` formats for
>> pack/unpack (see https://perldoc.perl.org/functions/pack) use what the C
>> compiler calls `int`, in terms of both endianness and size.
>>
>> > For named reasons, manipulating pg_upgrade from Perl is impractical,
>> > considering the number of environments we support.
>> >
>> > I see two possible solutions:
>> >
>> > 1) Provide a tool written in C that allows changing pg_control, e.g.
>> > `pg_writecontoldata` or maybe a flat like `pg_controldata -w`. The
>> > tool can be executed from Perl, so it shouldn't know about
>> > sizeof(int), alignment and endiness.
>>
>> 1.5) Use Perl's pack/unpack functions, which are explicitly desgined for
>> exactly this use case.
>
> Thanks for your reply.
>
> It is my understanding that Perl is not extremely aware of alignment.
> For instance if I want to modify the checksum of the file and the
> offset of the checksum is let's say 200 bytes on one platform, 204 on
> another and 208 on a third, pack/unpack will not help me.
>
> Or did I miss something?

You have to specify alignment manually using the `x![$type]` notation.
I actually wrote up a pack template for the entire control file
(assuming all types have alignof==sizeof), and it seems to work. Run
the attached with a Postgres 17 data directory as the arguemnt, and it
should show all the fields.

- ilmari