Re: promotion related handling in pg_sync_replication_slots()

On Fri, Apr 19, 2024 at 11:37 AM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> 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.

Please find v9 with the above comments addressed.

thanks
Shveta