Re: Single transaction in the tablesync worker?

On Sun, Jan 31, 2021 at 12:19 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> I have made the below changes in the patch. Let me know what you think
> about these?
> 1. It was a bit difficult to understand the code in DropSubscription
> so I have rearranged the code to match the way we are doing in HEAD
> where we drop the slots at the end after finishing all the other
> cleanup.

There was a reason why the v22 logic was different from HEAD.

The broken connection leaves dangling slots which is unavoidable. But,
whereas the user knows the name of the Subscription slot (they named
it), there is no easy way for them to know the names of the remaining
tablesync slots unless we log them.

That is why the v22 code was written to process the tablesync slots
even for wrconn == NULL so their name could be logged:
elog(WARNING, "no connection; cannot drop tablesync slot \"%s\".",
syncslotname);

The v23 patch removed this dangling slot name information, so it makes
it difficult for the user to know what tablesync slots to cleanup.

> 2. In AlterSubscription_refresh(), we can't allow workers to be
> stopped at commit time as we have already dropped the slots because
> the worker can access the dropped slot. We need to stop the workers
> before dropping slots. This makes all the code related to
> logicalrep_worker_stop_at_commit redundant.

OK.

> 3. In AlterSubscription_refresh(), we need to acquire the lock on
> pg_subscription_rel only when we try to remove any subscription rel.

+ if (!sub_rel_locked)
+ {
+ rel = table_open(SubscriptionRelRelationId, AccessExclusiveLock);
+ sub_rel_locked = true;
+ }

OK. But the sub_rel_locked bool is not really necessary. Why not just
check for rel == NULL? e.g.
if (!rel)
    rel = table_open(SubscriptionRelRelationId, AccessExclusiveLock);

> 4. Added/Changed quite a few comments.
>

@@ -1042,6 +1115,31 @@ DropSubscription(DropSubscriptionStmt *stmt,
bool isTopLevel)
  }
  list_free(subworkers);

+ /*
+ * Tablesync resource cleanup (slots and origins).

The comment is misleading; this code is only dropping origins.

----
Kind Regards,
Peter Smith.
Fujitsu Australia