Re: Newly created replication slot may be invalidated by checkpoint

On Mon, Jan 5, 2026 at 1:31 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Tue, Dec 30, 2025 at 5:31 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Sun, Dec 14, 2025 at 8:21 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> > >
> > > On Thu, Dec 11, 2025 at 12:39 PM Zhijie Hou (Fujitsu)
> > > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > >
> > > > >
> > > > > The other idea to fix this problem is suggested by Alexander in his
> > > > > email [1] which is to introduce a new ReplicationSlotReserveWALLock
> > > > > for this purpose. I think introducing LWLock in back branches could be
> > > > > questionable. Did you evaluate the pros and cons of using that
> > > > > approach?
> > > >
> > > > I reviewed that approach, and I think the main distinction lies in whether to
> > > > use a new LWLock to serialize the process or rely on an existing lock.
> > > > Introducing a new LWLock in back branches would alter the size of
> > > > MainLWLockArray and affect NUM_INDIVIDUAL_LWLOCKS/LWTRANCHE_FIRST_USER_DEFINED.
> > > > Although this may not directly impact user applications since users typically
> > > > use standard APIs like RequestNamedLWLockTranche and LWLockNewTrancheId to add
> > > > private LWLocks, it still has a slight risk. Additionally, using an existing
> > > > lock could keep code similarity with the HEAD, which can be helpful for future
> > > > bug fixes and analysis.
> > > >
> > >
> > > Fair enough. I'll wait for Sawada-san/Vitaly to see what their opinion
> > > on this matter is.
> >
> > While it's hacky that the proposed approach takes
> > ReplicationSlotAllocationLock before
> > XLogGetReplicationSlotMinimumLSN() during checkpoint, I find that it's
> > better than introducing a new LWLock in minor versions which could
> > lead unexpected compatibility issues.
> >
> > Regarding the v10 patch, do we need to take
> > ReplicationSlotAllocationLock also at the following place?
> >
> > /*
> > * Recalculate the current minimum LSN to be used in the WAL segment
> > * cleanup. Then, we must synchronize the replication slots again in
> > * order to make this LSN safe to use.
> > */
> > slotsMinReqLSN = XLogGetReplicationSlotMinimumLSN();
> > CheckPointReplicationSlots(shutdown);
> >
>
> No, we don't need the allocation lock here because RedoRecPtr won't
> change after the previous computation so the WAL reservation during
> creation of slots won't be impacted. I mean if the slot reservation
> starts just before this computation, it should use the latest (this
> checkpoint cycle's RedoRecPtr) whereas that was not true with the case
> where the patch acquires it.

Agreed.

>
> > I think we need to add some comments regardless of taking the lwlock.
> >
>
> I have added a comment though not sure if it is required. Do you have
> something specific in mind?

I wanted to clarify when we should and should not acquire
ReplicationSlotALlocationLock in shared mode during checkpointing. The
updated comments look good to me. Thank you!

Regards,

--
Masahiko Sawada
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