Re: [Proposal] Fully WAL logged CREATE DATABASE - No Checkpoints - Mailing list pgsql-hackers

On Mon, Mar 21, 2022 at 11:53 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Mon, Mar 21, 2022 at 11:21 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > I tried to debug the case but I realized that somehow
> > CHECK_FOR_INTERRUPTS() is not calling the
> > AcceptInvalidationMessages() and I could not find the same while
> > looking into the code as well.   While debugging I noticed that
> > AcceptInvalidationMessages() is called multiple times but that is only
> > through LockRelationId() but while locking the relation we had already
> > closed the previous smgr because at a time we keep only one smgr open.
> > And that's the reason it is not hitting the issue which we think it
> > could. Is there any condition under which it will call
> > AcceptInvalidationMessages() through CHECK_FOR_INTERRUPTS() ? because
> > I could not see while debugging as well as in code.
>
> Yeah, I think the reason you can't find it is that it's not there. I
> was confused in what I wrote earlier. I think we only process sinval
> catchups when we're idle, not at every CHECK_FOR_INTERRUPTS(). And I
> think the reason for that is precisely that it would be hard to write
> correct code otherwise, since invalidations might then get processed
> in a lot more places. So ... I guess all we really need to do here is
> avoid assuming that the results of smgropen() are valid across any
> code that might acquire relation locks. Which I think the code already
> does.
>
> But on a related note, why doesn't CreateDatabaseUsingWalLog() acquire
> locks on both the source and destination relations? It looks like
> you're only taking locks for the source template database ... but I
> thought the intention here was to make sure that we didn't pull pages
> into shared_buffers without holding a lock on the relation and/or the
> database? I suppose the point is that while the template database
> might be concurrently dropped, nobody can be doing anything
> concurrently to the target database because nobody knows that it
> exists yet. Still, I think that this would be the only case where we
> let pages into shared_buffers without a relation or database lock,
> though maybe I'm confused about this point, too. If not, perhaps we
> should consider locking the target database OID and each relation OID
> as we are copying it?
>
> I guess I'm imagining that there might be more code pathways in the
> future that want to ensure that there are no remaining buffers for
> some particular database or relation OID. It seems natural to want to
> be able to take some lock that prevents buffers from being added, and
> then go and get rid of all the ones that are there already. But I
> admit I can't quite think of a concrete case where we'd want to do
> something like this where the patch as coded would be a problem. I'm
> just thinking perhaps taking locks is fairly harmless and might avoid
> some hypothetical problem later.
>
> Thoughts?

I think this make sense.  I haven't changed the original patch as you
told you were improving on some comments, so in order to avoid
conflict I have created this add on patch.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com