Re: [Proposal] Adding callback support for custom statistics kinds

On Wed, Dec 17, 2025 at 08:03:36AM +0100, Peter Eisentraut wrote:
> So it seems to me that either the callbacks API needs some adjustments, or
> this particular implementation of the callback function is incorrect.

Hmm, you are right that this is not aligned. This can be improved
with one change for each callback:
- It is OK with from_serialized_data() to manipulate the header data,
because we want to fill a portion of the shmem data with extra data
read from disk (the module wants to add a reference to a DSA stored in
the shmem entry, read from the second file). So we should discard the
const marker from the callback definition.
- The const usage is OK for to_serialized_data(): it is better to
encourage a policy where the header data cannot be manipulated. So
the const needs to be kept in the definition, but I also think that we
should change the module implementation so as the cast to
PgStatShared_CustomVarEntry is a const.

These changes result in the attached. Sami, what do you think?
--
Michael