Re: promotion related handling in pg_sync_replication_slots()

On Fri, Apr 19, 2024 at 10:53 AM Bertrand Drouvot
<bertranddrouvot(dot)pg(at)gmail(dot)com> wrote:
>
> Hi,
>
> On Thu, Apr 18, 2024 at 05:36:05PM +0530, shveta malik wrote:
> > Please find v8 attached. Changes are:
>
> Thanks!
>
> A few comments:

Thanks for reviewing.

> 1 ===
>
> @@ -1440,7 +1461,7 @@ ReplSlotSyncWorkerMain(char *startup_data, size_t startup_data_len)
> * slotsync_worker_onexit() but that will need the connection to be made
> * global and we want to avoid introducing global for this purpose.
> */
> - before_shmem_exit(slotsync_failure_callback, PointerGetDatum(wrconn));
> + before_shmem_exit(slotsync_worker_disconnect, PointerGetDatum(wrconn));
>
> The comment above this change still states "Register the failure callback once
> we have the connection", I think it has to be reworded a bit now that v8 is
> making use of slotsync_worker_disconnect().
>
> 2 ===
>
> + * Register slotsync_worker_onexit() before we register
> + * ReplicationSlotShmemExit() in BaseInit(), to ensure that during exit of
> + * slot sync worker, ReplicationSlotShmemExit() is called first, followed
> + * by slotsync_worker_onexit(). Startup process during promotion waits for
>
> Worth to mention in shmem_exit() (where it "while (--before_shmem_exit_index >= 0)"
> or before the shmem_exit() definition) that ReplSlotSyncWorkerMain() relies on
> this LIFO behavior? (not sure if there is other "strong" LIFO requirement in
> other part of the code).

I see other modules as well relying on LIFO behavior.
Please see applyparallelworker.c where
'before_shmem_exit(pa_shutdown)' is needed to be done after
'before_shmem_exit(logicalrep_worker_onexit)' (commit id 3d144c6).
Also in postinit.c, I see such comments atop
'before_shmem_exit(ShutdownPostgres, 0)'.
I feel we can skip adding this specific comment about
ReplSlotSyncWorkerMain() in ipc.c, as none of the other modules has
also not added any. I will address the rest of your comments in the
next version.

> 3 ===
>
> + * Startup process during promotion waits for slot sync to finish and it
> + * does that by checking the 'syncing' flag.
>
> worth to mention ShutDownSlotSync()?
>
> 4 ===
>
> I did a few tests manually (launching ShutDownSlotSync() through gdb / with and
> without sync worker and with / without pg_sync_replication_slots() running
> concurrently) and it looks like it works as designed.

Thanks for testing it.

> Having said that, the logic that is in place to take care of the corner cases
> described up-thread seems reasonable to me.

thanks
Shveta