Re: Clear logical slot's 'synced' flag on promotion of standby

On Tue, Sep 9, 2025 at 4:21 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> Hi,
>
> This is a spin-off thread from [1].
>
> Currently, in the slot-sync worker, we have an error scenario [2]
> where, during slot synchronization, if we detect a slot with the same
> name and its synced flag is set to false, we emit an error. The
> rationale is to avoid potentially overwriting a user-created slot.
>
> But while analyzing [1], we observed that this error can lead to
> inconsistent behavior during switchovers. On the first switchover, the
> new standby logs an error: "Exiting from slot synchronization because
> a slot with the same name already exists on the standby."   But during
> a double switchover, this error does not occur.
>
> Upon re-evaluating this, it seems more appropriate to clear the synced
> flag after promotion, as the flag does not hold any meaning on the
> primary. Doing so would ensure consistent behavior across all
> switchovers, as the same error will be raised avoiding the risk of
> overwriting user's slots.
>
> A patch can be posted soon on the same idea.

Hi Shveta,

Here’s a patch that addresses this issue. It clears any “synced” flags
on logical replication slots when a standby is promoted. I’ve also
added handling for crashes; if the server crashes before the flags are
cleared, they are reset on restart.
The restart logic was a bit tricky, since I had to rely on the
database state to decide when the reset is needed. Documentation on
these states is sparse, but from my testing I found that
DB_IN_CRASH_RECOVERY occurs when a standby crashes during promotion.
That’s the state I use to trigger the flag reset on restart.

regards,
Ajin Cherian
Fujitsu Australia