Extension Enhancement: Buffer Invalidation in pg_buffercache

I hope this email finds you well. I am excited to share that I have
extended the functionality of the `pg_buffercache` extension by
implementing buffer invalidation capability, as requested by some
PostgreSQL contributors for improved testing scenarios.

This marks my first time submitting a patch to pgsql-hackers, and I am
eager to receive your expert feedback on the changes made. Your
insights are invaluable, and any review or comments you provide will
be greatly appreciated.

The primary objective of this enhancement is to enable explicit buffer
invalidation within the `pg_buffercache` extension. By doing so, we
can simulate scenarios where buffers are invalidated and observe the
resulting behavior in PostgreSQL.

As part of this patch, a new function or mechanism has been introduced
to facilitate buffer invalidation. I would like to hear your thoughts
on whether this approach provides a good user interface for this
functionality. Additionally, I seek your evaluation of the buffer
locking protocol employed in the extension to ensure its correctness
and efficiency.

Please note that I plan to add comprehensive documentation once the
details of this enhancement are agreed upon. This documentation will
serve as a valuable resource for users and contributors alike. I
believe that your expertise will help uncover any potential issues and
opportunities for further improvement.

I have attached the patch file to this email for your convenience.
Your valuable time and consideration in reviewing this extension are
sincerely appreciated.

Thank you for your continued support and guidance. I am looking
forward to your feedback and collaboration in enhancing the PostgreSQL
ecosystem.

The working of the extension:

1. Creating the extension pg_buffercache and then call select query on
a table and note the buffer to be cleared.
pgbench=# create extension pg_buffercache;
CREATE EXTENSION
pgbench=# select count(*) from pgbench_accounts;
count
--------
100000
(1 row)

pgbench=# SELECT *
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
bufferid | relfilenode | reltablespace | reldatabase | relforknumber
| relblocknumber | isdirty | usagecount | pinning_backends
----------+-------------+---------------+-------------+---------------+----------------+---------+------------+------------------
233 | 16397 | 1663 | 16384 | 0
| 0 | f | 1 | 0
234 | 16397 | 1663 | 16384 | 0
| 1 | f | 1 | 0
235 | 16397 | 1663 | 16384 | 0
| 2 | f | 1 | 0
236 | 16397 | 1663 | 16384 | 0
| 3 | f | 1 | 0
237 | 16397 | 1663 | 16384 | 0
| 4 | f | 1 | 0

2. Clearing a single buffer by entering the bufferid.
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1660
(1 row)

pgbench=# select pg_buffercache_invalidate(233);
pg_buffercache_invalidate
---------------------------
t
(1 row)

pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)

3. Clearing the entire buffer for a relation using the function.
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)

pgbench=# select count(pg_buffercache_invalidate(bufferid)) from
pg_buffercache where relfilenode =
pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)

pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
0
(1 row)

Best regards,
Palak