Re: pg_logical_slot_get_changes waits continously for a partial WAL record spanning across 2 pages

On Wed, Jun 25, 2025 at 10:19:55PM +0530, vignesh C wrote:
> Currently, the logic attempts to read the complete WAL record based on
> the size obtained before the crash—even though only a partial record
> was written. It then checks the page header to determine whether the
> XLP_FIRST_IS_OVERWRITE_CONTRECORD flag is set only  after reading the
> complete WAL record at XLogDecodeNextRecord function, but since that
> much WAL data was not available in the system we never get a chance to
> check the header after this.. To address this issue, a more robust
> approach would be to first read the page header, check if the
> XLP_FIRST_IS_OVERWRITE_CONTRECORD flag is set, and only then proceed
> to read the WAL record size if the record is not marked as a partial
> overwrite. This would prevent the system from waiting for WAL data
> that will never arrive. Attached partial_wal_record_fix.patch patch
> for this.

So you are suggesting the addition of an extra ReadPageInternal() that
forces a read of only the read, perform the checks on the header, then
read the rest.  After reading SizeOfXLogShortPHD worth of data,
shouldn't the checks on xlp_rem_len be done a bit earlier than what
you are proposing in this patch?

> I don't have a consistent test to reproduce this issue, currently this
> issue can be reproduced by running the 046_checkpoint_logical_slot
> test about 50 times. This test 046_checkpoint_logical_slot was
> reverted recently after it caused a few buildfarm failures discussed
> at [2]. The same test is also attached as
> reverted_test_046_checkpoint_logical_slot.patch.

Seeing the noise that this originally created in the CFbot and the
buildfarm, even if the issue would be only triggered after a timeout,
I'd vote for relying on this test being good enough for the purpose of
this race condition.  Another reliable approach would be to make the
code wait before reading the record in the internal loop of
ReadPageInternal() with an injection point when we know that we have a
contrecord, but I'm not really excited about this prospect in
xlogreader.c which can be also used in the frontend.
--
Michael