Re: promotion related handling in pg_sync_replication_slots()

On Fri, Apr 12, 2024 at 5:25 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> Please find v3 addressing race-condition and one other comment.
>
> Up-thread it was suggested that,  probably, checking
> SlotSyncCtx->syncing should be sufficient in ShutDownSlotSync(). On
> re-thinking, it might not be. Slot sync worker sets and resets
> 'syncing' with each sync-cycle, and thus we need to rely on worker's
> pid in ShutDownSlotSync(), as there could be a window where promotion
> is triggered and 'syncing' is not set for worker, while the worker is
> still running. This implementation of setting and resetting syncing
> with each sync-cycle looks better as compared to setting syncing
> during the entire life-cycle of the worker. So, I did not change it.
>

To retain this we need to have different handling for 'syncing' for
workers and function which seems like more maintenance burden than the
value it provides. Moreover, in SyncReplicationSlots(), we are calling
a function after acquiring spinlock which is not our usual coding
practice.

One minor comment:
 * All the fields except 'syncing' are used only by slotsync worker.
 * 'syncing' is used both by worker and SQL function pg_sync_replication_slots.
 */
typedef struct SlotSyncCtxStruct
{
pid_t pid;
bool stopSignaled;
bool syncing;
time_t last_start_time;
slock_t mutex;
} SlotSyncCtxStruct;

I feel the above comment is no longer valid after this patch. We can
probably remove this altogether.

--
With Regards,
Amit Kapila.