Subject: pg_waldump vs. all-zeros WAL files; server creation of such files

From: Noah Misch <[email-masked]>
Date: Sat, 12 Aug 2023 20:15:31 -0700

The attached 010_zero.pl, when run as part of the pg_waldump test suite, fails
at today's master (c36b636) and v15 (1bc19df).  It passes at v14 (5a32af3).
Command "pg_waldump --start 0/01000000 --end 0/01000100" fails as follows:

  pg_waldump: error: WAL segment size must be a power of two between 1 MB and 1 GB, but the WAL file "000000010000000000000002" header specifies 0 bytes

Where it fails, the server has created an all-zeros WAL file under that name.
Where it succeeds, that file doesn't exist at all.  Two decisions to make:

- Should a clean server shutdown ever leave an all-zeros WAL file?  I think
  yes, it's okay to let that happen.
- Should "pg_waldump --start $X --end $Y" open files not needed for the
  requested range?  I think no.

Bisect of master got:
30a53b7 Wed Mar 8 16:56:37 2023 +0100 Allow tailoring of ICU locales with custom rules
Doesn't fail at $(git merge-base REL_15_STABLE master).  Bisect of v15 got:
811203d Sat Aug 6 11:50:23 2022 -0400 Fix data-corruption hazard in WAL-logged CREATE DATABASE.

I suspect those are innocent.  They changed the exact WAL content, which I
expect somehow caused creation of segment 2.

Oddly, I find only one other report of this:
https://www.postgresql.org/message-id/CAJ6DU3HiJ5FHbqPua19jAD%3DwLgiXBTjuHfbmv1jCOaNOpB3cCQ%40mail.gmail.com

Thanks,
nm

---

From: Michael Paquier <[email-masked]>
Date: Mon, 14 Aug 2023 10:37:37 +0900

On Sat, Aug 12, 2023 at 08:15:31PM -0700, Noah Misch wrote:
>   pg_waldump: error: WAL segment size must be a power of two between
>   1 MB and 1 GB, but the WAL file "000000010000000000000002" header
>   specifies 0 bytes

So this depends on the ordering of the entries retrieved by readdir()
as much as the segments produced by the backend.

>   yes, it's okay to let that happen.

It doesn't hurt to leave that around.  On the contrary, it makes any
follow-up startup cheaper the bigger the segment size.

>   requested range?  I think no.

So this is a case where identify_target_directory() is called with a
fname of NULL.  Agreed that search_directory could be smarter with the
files it should scan, especially if we have start and/or end LSNs at
hand to filter out what we'd like to be in the data folder.
--
Michael