Re: [PATCH] Refactor SLRU to always use long file names

On Wed, Sep 11, 2024 at 04:07:06PM +0300, Aleksander Alekseev wrote:
> Commit 4ed8f0913bfd introduced long SLRU file names. The proposed
> patch removes SlruCtl->long_segment_names flag and makes SLRU always
> use long file names. This simplifies both the code and the API.
> Corresponding changes to pg_upgrade are included.

That's leaner, indeed.

> One drawback I see is that technically SLRU is an exposed API and
> changing it may affect third-party code. I'm not sure if we should
> seriously worry about this. Firstly, the change is trivial and
> secondly, it's not clear whether such third-party code even exists (we
> broke this API just recently in 4ed8f0913bfd and no one complained).

Any third-party code using custom SLRUs would need to take care of
handling their upgrade path outside pg_upgrade. Not sure there are
any of them, TBH, but let's see.

> I didn't include any tests for the new pg_upgrade code. To my
> knowledge we test it manually, with buildfarm members and during
> alpha- and beta-testing periods. Please let me know if you think there
> should be a corresponding TAP test.

Removing the old API means that it is impossible to test a move from
short to long file names. That's OK by me to rely on the pg_upgrade
paths in the buildfarm code. We have a few of them.

There is one thing I am wondering, here, though, which is to think
harder about a validity check at the end of 002_pg_upgrade.pl to make
sure that all the SLRU use long file names after running the tests.
That would mean thinking about a mechanism to list all of them from a
backend, rather than hardcode a list of them. Perhaps that's not
worth it, just dropping an idea in the bucket of ideas. I would guess
in the shape of a catalog that's able to represent at SQL level all
the SLRUs that exist in a backend.
--
Michael