Re: Wrong usage of RelationNeedsWAL

On Thu, Jan 21, 2021 at 12:28:44AM +0900, Kyotaro Horiguchi wrote:
> At Wed, 20 Jan 2021 17:34:44 +0900 (JST), Kyotaro Horiguchi <horikyota.ntt@gmail.com> wrote in 
> > Anyway, it seems actually dangerous that cause pruning on wal-skipped
> > relation.
> > 
> > > with your patch versions.  Could you try implementing both test procedures in
> > > src/test/modules/snapshot_too_old?  There's no need to make the test use
> > > wal_level=minimal explicitly; it's sufficient to catch these bugs when
> > > wal_level=minimal is in the TEMP_CONFIG file.
> > 
> > In the attached, TestForOldSnapshot() considers XLogIsNeeded(),
> > instead of moving the condition on LSN to TestForOldSnapshot_impl for
> > performance.
> > 
> > I'll add the test part in the next version.

That test helped me.  I now see "there's not a single tuple removed due to
old_snapshot_threshold in src/test/modules/snapshot_too_old"[1], which limits
our ability to test using this infrastructure.

> However, with the previous patch, two existing tests sto_using_cursor
> and sto_using_select behaves differently from the master.  That change
> is coming from the omission of actual LSN check in TestForOldSnapshot
> while wal_level=minimal. So we have no choice other than actually
> updating page LSN.
> 
> In the scenario under discussion there are two places we need to do
> that. one is heap_page_prune, and the other is RelationCopyStorge. As
> a PoC, I used gistXLogAssignLSN() as is for thie purpose. See the
> attached third file.

Fake LSNs make the system harder to understand, so I prefer not to spread fake
LSNs to more access methods.  What I had in mind is to simply suppress early
pruning when the relation is skipping WAL.  Attached.  Is this reasonable?  It
passes the older tests.  While it changes the sto_wal_optimized.spec output, I
think it preserves the old_snapshot_threshold behavior contract.

[1] https://www.postgresql.org/message-id/20200403001235.e6jfdll3gh2ygbuc%40alap3.anarazel.de