Re: Issue with logical replication slot during switchover

I propose to modify 

get_local_synced_slots() => to get the local logical slots (the name of the function must be renamed)
and 

local_sync_slot_required() => to get also the slots that have the flag synced = false

then in both our use cases the local slot will be dropped and recreated if it must be resynchronized.
Changes in synchronize_one_slot()  function are not necessary anymore.

static List *

get_local_synced_slots(void)

{

List    *local_slots = NIL;

LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);

for (int i = 0; i < max_replication_slots; i++)

{

ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];

/* Check if it is a synchronized slot */

if (s->in_use && (s->data.synced || s->data.failover))

{

Assert(SlotIsLogical(s));

local_slots = lappend(local_slots, s);

}

}

LWLockRelease(ReplicationSlotControlLock);

return local_slots;

}

static bool

local_sync_slot_required(ReplicationSlot *local_slot, List *remote_slots)

{

bool remote_exists = false;

bool locally_invalidated = false;

bool synced_slot = false;

foreach_ptr(RemoteSlot, remote_slot, remote_slots)

{

if (strcmp(remote_slot->name, NameStr(local_slot->data.name)) == 0)

{

remote_exists = true;

/*

* If remote slot is not invalidated but local slot is marked as

* invalidated, then set locally_invalidated flag.

*/

SpinLockAcquire(&local_slot->mutex);

locally_invalidated =

(remote_slot->invalidated == RS_INVAL_NONE) &&

(local_slot->data.invalidated != RS_INVAL_NONE);

synced_slot = local_slot->data.synced;

SpinLockRelease(&local_slot->mutex);

break;

}

}

return (remote_exists && !locally_invalidated && synced_slot);

}

I could propose a new patch in that sense.

Regards,

Fabrice

On Tue, Nov 4, 2025 at 2:01 PM Alexander Kukushkin <cyberdemn@gmail.com> wrote:

On Tue, 4 Nov 2025 at 12:15, Fabrice Chapuis <fabrice636861@gmail.com> wrote:

Hi Alexander,

I understand your use case but I think it won't work like this because of how is implemented get_local_synced_slots()

this function will return slots which synced flag to true only. 

Yeah. Sorry, my bad, you are absolutely right about get_local_synced_slots() function.

I didn't validate this part of the patch, because first I wanted to get feedback from hackers.

However, it looks like the people who build it never run Postgres in production and therefore don't understand the problem and the pain.

I will probably just add yet another workaround to Patroni and start dropping logical replication slots on standby with failover=true and synced=false.

--

Regards,

--

Alexander Kukushkin