Re: BUG #18988: DROP SUBSCRIPTION locks not-yet-accessed database

On Mon, Aug 4, 2025 at 7:08 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Mon, Aug 4, 2025 at 6:20 PM Álvaro Herrera <alvherre@kurilemu.de> wrote:
> >
> > On 2025-Aug-04, Dilip Kumar wrote:
> >
> > > I have worked on this and produced a first version of patch, let's see
> > > what others think about this idea.  It would have been better if we
> > > could use SysCache for rechecking the subscription, but since we are
> > > not connected to the database in the launcher we can not use the
> > > SysCache, at least that's what I think.
> >
> > I think it's reasonable to recheck after locking.  There's a comment in
> > DropSubscription that says we get AEL, which is no longer true.
>
> Right, will remove that.
>
>   In
> > is_subscription_exists() you should use the index on OID instead of
> > seqscanning the catalog without a scankey;
>
> I thought since launcher is not connected to the database we will not
> be able to open the index relation.  Otherwise we may just call
> SearchSysCache1(SUBSCRIPTIONOID, ObjectIdGetDatum(subid));  Maybe this
> is not connected because it was not required so far and we can just
> connect it to template1 ?
>
>
>  also I think the name ought
> > to be "does" rather than "is".
>
> Okay
>
>   I think it's really odd that that
> > function opens and closes a transaction; sounds to me that something
> > like that really belongs in the caller (frankly the same is true with
> > the other function that your comment references).  Why isn't
> > systable_beginscan being used to scan the catalog?
>
> You mean for this function or for get_subscription_list() as well,
> yeah logically systable_beginscan() sounds better.
>
> > I think with this coding, the resource owner for this new lock is NULL.
> > Is this really a good approach?  Maybe there should be a resowner here.
>
> As you suggested we should move the transaction to the caller and
> start it before LockSharedObject() so that we will acquire the lock
> under the TopTransactionResourceOwner ?

Here is revised version based on what I proposed here

- I have removed the comment in DropSubscription where we acquire the
lock, as mentioning the ASL is not interesting anymore, instead I am
explaining in launcher why we are acquiring shared object lock.
- Connected launcher to "postgres" database so that we can do syscache lookup
- got rid of "is_subscription_exists" as we are directly validating
using syscache lookup
- Didn't do anything with existing function i.e. get_subscription_list

--
Regards,
Dilip Kumar
Google