RE: Slow catchup of 2PC (twophase) transactions on replica in LR

Dear Amit,

Sorry, I forgot to say one content.

> > But that is not a good reason for this operation to stop workers
> > first. Instead, we should prohibit this operation if any worker is
> > present. The reason is that there is always a chance that if any
> > worker is alive, it can prepare a new transaction after we have
> > checked for the presence of any prepared transactions.
> 
> I used the function because it internally waits until all workers are exited.
> But OK, I modified like you suggested (logicalrep_workers_find() is used).

Based on the reason, after the above modification, test codes prior to v14
sometimes failed because backend could execute ALTER SUBSCRIPTION ... SET (two_phase).
So I added lines in test codes to poll until workers are exited, e.g.,

```
+# Alter subscription two_phase to false
+$node_subscriber->safe_psql('postgres',
+    "ALTER SUBSCRIPTION tap_sub_copy DISABLE;");
+$node_subscriber->poll_query_until('postgres',
+    "SELECT count(*) = 0 FROM pg_stat_activity WHERE backend_type = 'logical replication worker'"
+);
+$node_subscriber->safe_psql(
+       'postgres', "
+    ALTER SUBSCRIPTION tap_sub_copy SET (two_phase = false);
+    ALTER SUBSCRIPTION tap_sub_copy ENABLE;");
```

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/