Removing support for FE/BE protocol version 2

PostgreSQL 14, which was released in 2021, removed protocol version 2
support. The last version supporting protocol version 2 was 13, which
was EOLed in November 2025. So, existing supported PostgreSQL versions
do not accept the protocol version 2 anymore (more precisely, they
response with "unsupported protocol version" error when the protocol
is requested).

I would like to propose removing support for FE/BE protocol version 2
in upcoming v4.8. If the version 2 startup message arrives, pgpool
responds with "unsupported protocol version" error, which is the same
behavior as PostgreSQL server.

The merits of dropping v2 protocol are:

- Simplifying our code base. There are pretty large number of code
path which handles v2 protocol.

- Removing untested codes. Our regression tests do not include v2
protocol cases.

We cannot support the version 2 protocol forever. Dropping it is only
a matter of time. As I already mentioned above, all supported
PostgreSQL versions finally drop the v2 protocol, I think v4.8 is the
good timing to remove v2 protocol for us.

Thoughts?
--
Tatsuo Ishii
SRA OSS K.K.
English: http://www.sraoss.co.jp/index_en
Japanese:http://www.sraoss.co.jp