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

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.

thanks
Shveta