Re: Our ABI diff infrastructure ignores enum SysCacheIdentifier

On Thu, Feb 12, 2026 at 11:17:37AM -0500, Tom Lane wrote:
> I discovered $SUBJECT while working on commits dbb09fd8e et al.
> The point of those commits was to back-patch addition of a syscache
> that previously existed only in v18+, so naturally I was concerned
> about not breaking ABI by changing any existing syscache's ID.
> I tested whether abidiff would bleat about it if I intentionally
> changed an ID, and found that *it didn't*.

Well. That's annoying. This one is important to be careful about.

> As for SysCacheIdentifier, the root of the problem is that
> SearchSysCache and friends are declared to take "int cacheId"
> not "enum SysCacheIdentifier cacheId". Likely we should change
> that in HEAD, but that'd be an actual not theoretical ABI break
> (the enum's not necessarily int-width).

Enforcing a type for this sounds like a good idea in the long term on
HEAD, yes, even putting the ABI argument aside for a moment. I'd
suggest the addition of an extra typedef in the code generated for
syscache_ids.h to save from a couple of enum markers in these
declarations, once refactored to use the new enum type.

> In the back branches
> I'm thinking about adding a dummy function just for this purpose,
> more or less as in the under-commented patch attached.
>
> Thoughts?

Adding a comment explaining why this function needs to exist would be
good.
--
Michael