Re: wal_consistency_checking reports an inconsistency on master branch

On 13/04/18 13:08, Michael Paquier wrote:
> On Fri, Apr 13, 2018 at 02:15:35PM +0530, amul sul wrote:
>> I have looked into this and found that the issue is in heap_xlog_delete -- we
>> have missed to set the correct offset number from the target_tid when
>> XLH_DELETE_IS_PARTITION_MOVE flag is set.
>
> Oh, this looks good to me. So when a row was moved across partitions
> this could have caused incorrect tuple references on a standby, which
> could have caused corruptions.

Hmm. So, the problem was that HeapTupleHeaderSetMovedPartitions() only
sets the block number to InvalidBlockNumber, and leaves the offset
number unchanged. WAL replay didn't preserve the offset number, so the
master and the standby had a different offset number in the ctid.

Why does HeapTupleHeaderSetMovedPartitions() leave the offset number
unchanged? The old offset number is meaningless without the block
number. Also, bits and magic values in the tuple header are scarce.
We're squandering a whole range of values in the ctid, everything with
ip_blkid==InvalidBlockNumber, to mean "moved to different partition",
when a single value would suffice.

Let's tighten that up. In the attached (untested) patch, I changed the
macros so that "moved to different partition" is indicated by the magic
TID (InvalidBlockNumber, 0xfffd). Offset number 0xfffe was already used
for speculative insertion tokens, so this follows that precedent.

I kept using InvalidBlockNumber there, so ItemPointerIsValid() still
considers those item pointers as invalid. But my gut feeling is actually
that it would be better to use e.g. 0 as the block number, so that these
item pointers would appear valid. Again, to follow the precedent of
speculative insertion tokens. But I'm not sure if there was some
well-thought-out reason to make them appear invalid. A comment on that
would be nice, at least.

(Amit hinted at this in
https://www.postgresql.org/message-id/CAA4eK1KtsTqsGDggDCrz2O9Jgo7ma-Co-B8%2Bv3L2zWMA2NHm6A%40mail.gmail.com.
He was OK with the current approach, but I feel pretty strongly that we
should also set the offset number.)

- Heikki