Re: Restrict copying of invalidated replication slots

On Fri, Feb 28, 2025 at 5:10 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Thu, Feb 27, 2025 at 12:52 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Feb 27, 2025 at 10:47 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> > >
> > > On Tue, Feb 25, 2025 at 7:33 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > > >
> > > > AFAICU, InvalidateObsoleteReplicationSlots() is not serialized with
> > > > this operation. So, isn't it possible that the source slot exists at
> > > > the later position in ReplicationSlotCtl->replication_slots and the
> > > > loop traversing slots is already ahead from the point where the newly
> > > > copied slot is created?
> > >
> > > Good point. I think it's possible.
> > >
> > > > If so, the newly created slot won't be marked
> > > > as invalid whereas the source slot will be marked as invalid. I agree
> > > > that even in such a case, at a later point, the newly created slot
> > > > will also be marked as invalid.
> > >
> > > The wal_status of the newly created slot would immediately become
> > > 'lost' and the next checkpoint will invalidate it. Do we need to do
> > > something to deal with this case?
> > >
> >
> > + /* Check if source slot became invalidated during the copy operation */
> > + if (second_slot_contents.data.invalidated != RS_INVAL_NONE)
> > + ereport(ERROR,
> > + errmsg("cannot copy replication slot \"%s\"",
> > + NameStr(*src_name)),
> > + errdetail("The source replication slot was invalidated during the
> > copy operation."));
> >
> > How about adding a similar test as above for MyReplicationSlot? That
> > should suffice the need because we have already acquired the new slot
> > by this time and invalidation should signal this process before
> > marking the new slot as invalid.
>
> IIUC in the scenario you mentioned, the loop traversing slots already
> passed the position of newly created slot in
> ReplicationSlotCtl->replication_slots array, so
> MyReplicationSlot->data.invalidated is still RS_INVAL_NONE, no?
>

Right, I don't have a simpler solution for this apart from making it
somehow serialize this operation with
InvalidateObsoleteReplicationSlots(). I don't think we need to go that
far to handle the scenario being discussed. It is better to add a
comment for this race and why it won't harm.

--
With Regards,
Amit Kapila.