That was exactly the right neighborhood, thanks for the quick pointer.
The mechanism, as far as I can tell (I'm new to this code, so corrections welcome):
The autovacuum launcher scans pg_database in get_database_list() with a
catalog scan (rel_read_only = false). On a full, prunable page,
heap_page_prune_opt() calls visibilitymap_pin(), which extends the VM fork
if it doesn't exist. The launcher isn't allowed to do IOOP_EXTEND, so
pgstat_count_io_op() trips the assertion at pgstat_io.c:74 and it aborts.
Two commits combine to cause it: 4f7ecca84dd added the unconditional
(extending) visibilitymap_pin() in the on-access prune path, and 378a216187a
made INSERT set pd_prune_xid, so on-access pruning now fires on insert-mostly
catalogs like pg_database. That's also why it was hard to reduce: any
regular backend or autovacuum worker that scans pg_database recreates the
fork harmlessly (they may extend), so the launcher only crashes in the brief
window before that happens. I can now reproduce it deterministically; happy
to share the script.
Patch attached (one file). Only read-only scans actually set the VM, and
those run only in regular backends (which may extend), so it extends the
fork only on read-only scans and, for other scans, pins an existing VM page
without extending (via visibilitymap_get_status()). Corruption detection is
unaffected; we just leave fork creation to the next VACUUM. Passes
check/installcheck, isolation, and contrib/pg_visibility.
This is just my own attempt at a fix and I'm not sure it's correct, so
please don't hesitate to point out anything I've got wrong.
Thanks,
Ewan
On Sun, May 31, 2026 at 12:36:45PM +0800, Ewan Young wrote:
> I haven't been able to boil this down to a clean standalone repro yet -- it
> seems to need the launcher to hit get_database_list() at the moment a
> pg_database page is prunable and the VM fork still has to grow -- but the
> path looks pretty clear from the stack.
My first suspicion would be something in the area of b46e1e54d078 and
some of the VM improvements. Melanie?
--
Michael
