Re: Assertion failure in SnapBuildInitialSnapshot()

On Mon, Nov 24, 2025 at 10:48 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> On Mon, Nov 24, 2025 at 1:46 AM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Fri, Nov 21, 2025 at 9:17 AM Zhijie Hou (Fujitsu)
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> > > On Thursday, November 13, 2025 12:56 PM Zhijie Hou (Fujitsu) <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > > >
> > >
> > > I have been thinking if there a way to avoid holding ReplicationSlotControlLock
> > > exclusively in ReplicationSlotsComputeRequiredXmin() because that could cause
> > > lock contention when many slots exist and advancements occur frequently.
> > >
> > > Given that the bug arises from a race condition between slot creation and
> > > concurrent slot xmin computation, I think another way is that, we acquire the
> > > ReplicationSlotControlLock exclusively only during slot creation to do the
> > > initial update of the slot xmin. In ReplicationSlotsComputeRequiredXmin(), we
> > > still hold the ReplicationSlotControlLock in shared mode until the global slot
> > > xmin is updated in ProcArraySetReplicationSlotXmin(). This approach prevents
> > > concurrent computations and updates of new xmin horizons by other backends
> > > during the initial slot xmin update process, while it still permits concurrent
> > > calls to ReplicationSlotsComputeRequiredXmin().
> > >
> >
> > Yeah, this seems to work.
>
> +1

Given that the computation of xmin and catalog_xmin among all slots
could be executed concurrently, could the following scenario happen
where procArray->replication_slot_xmin and
procArray->replication_slot_catalog_xmin are retreat to a non-invalid
XID?

1. Suppose the initial value procArray->replication_slot_catalog_xmin is 50.
2. Process-A updates its owned slot's catalog_xmin to 100, and
computes the new catalog_xmin as 100 while holding
ReplicationSlotControlLock in a shared mode in
ReplicationSlotsComputeRequiredLSN(). But it doesn't update the
procArray's catalog_xmin value yet.
3. Process-B updates its owned slot's catalog_xmin to 150, and
computes the new catalog_xmin as 150.
4. Process-B updates the procArray->replication_slot_catalog_xmin to 150.
5. Process-A updates the procArray->repilcation_slot_catalog_xmin to
100, which was 150.

It might be worth adding an assertion to
ProcArraySetReplicationSlotXmin(), checking if the new xmin and
catalog_xmin values are either >= the current values or an
InvalidTransactionId.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com