Re: Clear logical slot's 'synced' flag on promotion of standby - Mailing list pgsql-hackers

On Fri, Sep 19, 2025 at 7:29 PM Ashutosh Sharma <ashu.coek88@gmail.com> wrote:
>
> On Fri, Sep 19, 2025 at 3:04 PM shveta malik <shveta.malik@gmail.com> wrote:
> >
> > On Thu, Sep 18, 2025 at 5:20 PM Ashutosh Sharma <ashu.coek88@gmail.com> wrote:
> > >
> > > Hi Ajin,
> > >
> > > On Thu, Sep 18, 2025 at 4:16 PM Ajin Cherian <itsajin@gmail.com> wrote:
> > > >
> > > > On Fri, Sep 12, 2025 at 1:56 PM shveta malik <shveta.malik@gmail.com> wrote:
> > > > >
> > > > > The approach seems valid and should work, but introducing a new file
> > > > > like promote.inprogress for this purpose might be excessive. We can
> > > > > first try analyzing existing information to determine whether we can
> > > > > distinguish between the two scenarios -- a primary in crash recovery
> > > > > immediately after a promotion attempt versus a regular primary. If we
> > > > > are unable to find any way, we can revisit the idea.
> > > > >
> > > >
> > > > I needed a way to reset slots not only during promotion, but also
> > > > after a crash that occurs while slots are being reset, so there would
> > > > be a fallback mechanism to clear them again on startup. As Shveta
> > > > pointed out, it wasn’t trivial to tell apart a standby restarting
> > > > after crashing during promotion from a primary restarting after a
> > > > crash. So I decided to just reset slots every time primary (or a
> > > > standby after promotion) restarts.
> > > >
> > > > Because this fallback logic will run on every primary restart, it was
> > > > important to minimize overhead added by the patch. After some
> > > > discussion, I placed the reset logic in RestoreSlotFromDisk(), which
> > > > is invoked by StartupReplicationSlots() whenever the server starts.
> > > > Because RestoreSlotFromDisk() already loops through all slots, this
> > > > adds minimum extra work; but also ensures the synced flag is cleared
> > > > when running on a primary.
> > > >
> > > > The next challenge was finding a reliable flag to distinguish
> > > > primaries from standbys, since we really don’t want to reset the flag
> > > > on a standby. I tested StandbyMode, RecoveryInProgress(), and
> > > > InRecovery. But during restarts, both RecoveryInProgress() and
> > > > InRecovery are always true on both primary and standby. In all my
> > > > testing, StandbyMode was the only variable that consistently
> > > > differentiated between the two, which is what I used.
> > > >
> > >
> > > +/*
> > > + * ResetSyncedSlots()
> > > + *
> > > + * Reset all replication slots that have synced=true to synced=false.
> > > + */
> > >
> > > I feel this is not correct, we are force resetting sync flag status
> > > for all logical slots, not just the one that is set to true.
> > >
> > > --
> > >
> > > @@ -2664,6 +2715,10 @@ RestoreSlotFromDisk(const char *name)
> > >   memcpy(&slot->data, &cp.slotdata,
> > >      sizeof(ReplicationSlotPersistentData));
> > >
> > > + /* reset synced flag if this is a primary server */
> > > + if (!StandbyMode)
> > > + slot->data.synced = false;
> > > +
> > >
> > > I think you also need to ensure that you are only doing this for the
> > > logical slots, it's currently just checking if the slot is in-use or
> > > not.
> > >
> >
> > I think a better approach would be to reset synced only if it is
> > marked as synced. Adding a LogicalSlot check wouldn't be incorrect,
> > but IMO, it may not be necessary.
> >
>
> Thinking further on this, I believe it’s fine even if the slot is
> forcefully set to false on the primary. The slot type or its sync
> status doesn’t really matter in this case.
>

I think we need to mark the slots dirty once we set its synced-flag to
false. In such a case, we should only mark those which actually needed
synced flag resetting. IMO, we need a 'synced' flag check here.

thanks
Shveta