Re: Improve pg_sync_replication_slots() to wait for primary to advance

On Wed, Aug 20, 2025 at 10:53 AM Ajin Cherian <itsajin@gmail.com> wrote:
>
>
> I've removed them.
> Attaching patch v8 addressing the above comments.
>

Thanks for the patch. Please find a few comments:

1)
When the API is in progress, and meanwhile in another session we turn
off hot_standby_feedback, the API session terminates abnormally.

postgres=# SELECT pg_sync_replication_slots();
server closed the connection unexpectedly

It seems slotsync_reread_config() is not adjusted for API. It does
proc_exit assuming it is a worker process.

2)
slotsync_worker_onexit():

        SlotSyncCtx->slot_persistence_pending = false;

        /*
         * If syncing_slots is true, it indicates that the process errored out
         * without resetting the flag. So, we need to clean up shared memory and
         * reset the flag here.
         */
        if (syncing_slots)
        {
                SlotSyncCtx->syncing = false;
                syncing_slots = false;
        }

Shall we reset slot_persistence_pending inside 'if (syncing_slots)'?
slot_persistence_pending can not be true without syncing_slots being
true.

3)
reset_syncing_flag():

  SpinLockAcquire(&SlotSyncCtx->mutex);
  SlotSyncCtx->syncing = false;
+ SlotSyncCtx->slot_persistence_pending = false;
  SpinLockRelease(&SlotSyncCtx->mutex);

Here we are changing slot_persistence_pending under mutex, while at
other places, it is not protected by mutex. Is it intentional here?

4)
On rethinking, we maintain anything in shared memory if it has to be
shared between a few processes. 'slot_persistence_pending' OTOH is
required to be set and accessed by only one process at a time. Shall
we move it out of SlotSyncCtxStruct and keep it static similar to
'syncing_slots'? Rest of the setting, resetting flow remains the same.
What do you think?


5)
/* Build the query based on whether we're fetching all or refreshing
specific slots */

Perhaps we can shift this comment to where we actually append
target_slot_list. Better to have it something like:
'If target_slot_list is provided, construct the query only to fetch given slots'


thanks
Shveta