Re: add validations for required callbacks during pgstat_register_kind()

On Thu, Jul 02, 2026 at 04:48:25PM -0500, Sami Imseih wrote:
> Also, there is an assert for snapshot_cb before it's called in
> pgstat_build_snapshot_fixed(), but we don't have the same assert for
> init_shmem_cb and reset_all_cb. The attached also adds these asserts.
> These asserts are needed for built-in stats that don't go through
> pgstat_register_kind()

Feel free to add me in CC for such changes; this would drop in my
inbox.

Hmm, why not. It does not make sense to me to define a custom stats
kinds without all three for fixed-sized kinds, as they enforce
in-memory properties (FWIW, I've wanted to add a few more safeguards,
but never got down to define a good line of enforcement). The
pending_size change with flush_pending_cb makes sense as well for
variable stats. Perhaps not something to adjust in v18, even if I
doubt that somebody relies on this behavior, it would be surprising to
get a failure on server restart.

+ ereport(ERROR,
+ (errmsg("custom cumulative statistics property is invalid"),
+ errhint("Custom cumulative statistics require init_shmem_cb, reset_all_cb, and snapshot_cb callbacks for fixed-numbered objects.")));

Function names in error messages ought to be replaced with %s
placeholders, perhaps? The two messages may be better if matching the
surroundings, with "failed to register custom .. with ID" being the
primary message, and the hint telling what's missing.

if (kind_info->fixed_amount)
+ {
+ Assert(kind_info->reset_all_cb != NULL);
kind_info->reset_all_cb(ts);
+ }
[...]
+ Assert(kind_info->reset_all_cb != NULL);
kind_info->reset_all_cb(ts);
[...]
+ Assert(kind_info->init_shmem_cb != NULL);
kind_info->init_shmem_cb(ptr);

These three don't really bring extra value. If the pointer is not
set, we just crash the line after on a NULL pointer dereference,
leading to the same result at the end.
--
Michael