RE: Newly created replication slot may be invalidated by checkpoint

Dear Vignesh,

> Here is a version which includes back branch version patches with
> pgindent changes. I have verified the following scenario in PG17,
> PG16, PG15 and PG14 branches and works as expected with the patches:

FYI, I also confirmed that the invalidation has not happened anymore with given
workload, for PG14+. Reserving a WAL record is protected by AllocationLock thus
concurrent read by checkpointer cannot happen.

```
+       if (XLogGetLastRemovedSegno() >= segno)
+               elog(ERROR, "WAL required by replication slot %s has been removed concurrently",
+                        NameStr(slot->data.name));
```

Slightly I feel that we may be able to add a HINT to make sure user can retry
creating the slot.
But making them similar with HEAD is more important the code change.

Best regards,
Hayato Kuroda
FUJITSU LIMITED