Re: persist logical slots to disk during shutdown checkpoint

On Wed, Sep 6, 2023 at 9:47 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Sep 5, 2023 at 6:00 PM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:

>
> Right, it can change and in that case, the check related to
> confirm_flush LSN will fail during the upgrade. However, the point is
> that if we don't take spinlock, we need to properly write comments on
> why unlike in other places it is safe here to check these values
> without spinlock.

I agree with that, but now also it is not true that we are alway
reading this under the spin lock for example[1][2], we can see we are
reading this without spin lock.
[1]
StartLogicalReplication
{
/*
* Report the location after which we'll send out further commits as the
* current sentPtr.
*/
sentPtr = MyReplicationSlot->data.confirmed_flush;
}
[2]
LogicalIncreaseRestartDecodingForSlot
{
/* candidates are already valid with the current flush position, apply */
if (updated_lsn)
LogicalConfirmReceivedLocation(slot->data.confirmed_flush);
}

We can do that but I feel we have to be careful for
> all future usages of these variables, so, having spinlock makes them
> follow the normal coding pattern which I feel makes it more robust.
> Yes, marking dirty via common function also has merits but personally,
> I find it better to follow the normal coding practice of checking the
> required fields under spinlock. The other possibility is to first
> check if we need to mark the slot dirty under spinlock, then release
> the spinlock, and then call the common MarkDirty function, but again
> that will be under the assumption that these flags won't change.

Thats true, but we are already making the assumption because now also
we are taking the spinlock and taking a decision of marking the slot
dirty. And after that we are releasing the spin lock and if we do not
have guarantee that it can not concurrently change the many things can
go wrong no?

Anyway said that, I do not have any strong objection against what we
are doing now. There were discussion around making the code so that
it can use common function and I was suggesting how it could be
achieved but I am not against the current way either.

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