RE: Assertion failure in SnapBuildInitialSnapshot() - Mailing list pgsql-hackers

On Tuesday, November 25, 2025 3:30 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> 
> On Mon, Nov 24, 2025 at 10:48 AM Masahiko Sawada
> <sawada.mshk@gmail.com> wrote:
> >
> > On Mon, Nov 24, 2025 at 1:46 AM Amit Kapila <amit.kapila16@gmail.com>
> wrote:
> > >
> > > On Fri, Nov 21, 2025 at 9:17 AM Zhijie Hou (Fujitsu)
> > > <houzj.fnst@fujitsu.com> wrote:
> > > >
> > > > On Thursday, November 13, 2025 12:56 PM Zhijie Hou (Fujitsu)
> <houzj.fnst@fujitsu.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.

After further investigation, I think that steps 3 and 4 cannot occur because
Process-B must have already encountered the catalog_xmin maintained by
Process-A, either 50 or 100. Consequently, Process-B will refrain from updating
the catalog_xmin to a more recent value, such as 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.

I considered this scenario and identified a potential exception in the
copy_replication_slot(). This function uses a two-phase copy process, the
original restart_lsn is directly copied to the new slot during the first phase.
However, the original slot.restart_lsn might advance between phases.
Consequently, the newly created slot initially uses the outdated restart_lsn,
which could cause the procArray->replication_slot_catalog_xmin to retreat. I
think this behavior isn't harmful, as explained in the comments, because the new
restart_lsn will be updated in the created slot during the second phase.

Best Regards,
Hou zj