A bug in recovery prefetcher, triggering asserts / FATAL: checkpoint request failed

Hi,

While hacking on an improvement for the recovery prefetcher, I ran into
a pre-existing bug present since PG 15. The brief explanation is that
XLogPrefetcherBeginRead resets a bunch of stuff, but not the pointer
prefetcher->record. In some circumstances (full I/O queue, ...) that
happens to confuse XLogPrefetcherReadRecord, triggering failures:

- assert failures on development builds

Assert(lrq_completed(prefetcher->streaming_read) == 0);

- a bunch of weird/uncommon errors on regular builds

LOG: request to flush past end of generated WAL; request 0/030BBD90,
current position 0/030B9D48
ERROR: xlog flush request 0/030BBD90 is not satisfied --- flushed
only to 0/030B9D48
FATAL: checkpoint request failed

AFAICS it can happen only with reduced maintenance_io_concurrency, but
I'm not sure about that. Maybe there's a way to generate WAL records
with enough block images.

The attached patch fixes this by simply resetting the record to NULL in
XLogPrefetcherBeginRead. It also introduces a new TAP test triggering
this, using a SP-GiST index built etc.

I'm not sure if we want to add the TAP test, I'm attaching it more as a
way to reproduce the issue. Without the fix in XLogPrefetcherBeginRead,
it crashes with errors similar to the attached logs.

regards

--
Tomas Vondra