SEQUENCE values (duplicated) in some corner cases when crash happens

Hello PG Hackers

Hope you are well and safe!

I'm opening this thread to clarify something that I can observe: duplicated
values for sequences.

[My understanding is that duplication is not something we desire. In fact
it does NOT happen in the majority of cases, for example, when you
immediately insert the value and commit it. But it can eventually happen in
some specific scenarios...describe below]

The duplication can be observed when you only makes use of "nextval" (which
calls sequence.c / nextval_internal function) without - inserting it for
any reason - and the database crashes.
This is reproducible using the steps described on this link:

https://gist.github.com/vinnix/2fe148e3c42e11269bac5fcc5c78a8d1

There are two variants where this is reproducible (steps on the link above):

- Autocommit + Suspending I/O (simulating a storage issue)
- Explicitly opening transaction + Not Suspending I/O

** To simulate the "crash" I'm running: `killall -9 postgres`.

I've being debugging sequence.c code, I can see we only want to flush the
WAL segments once every 32 requests (per connection) - this is defined by
SEQ_LOG_VALS. Since, these values are "persisted" in the WAL file
containing the "advanced"/future values; not the current value being
retrieved by nextval_internal().

What I'm trying to understand now if this is a "bug" or a "new feature"...

Kind regards,
Vini