libpq host/hostaddr/conninfo inconsistencies

Hello devs,

While reviewing various patches by Tom which are focussing on libpq
multi-host behavior,

https://commitfest.postgresql.org/19/1749/
https://commitfest.postgresql.org/19/1752/

it occured to me that there are a few more problems with the
documentation, the host/hostaddr feature, and the consistency of both.
Namely:

* According to the documentation, either "host" or "hostaddr" can be
specified. The former for names and socket directories, the later for ip
addresses. If both are specified, "hostaddr" supersedes "host", and it may
be used for various authentication purposes.

However, the actual capability is slightly different: specifying an ip
address to "host" does work, without ensuing any name or reverse name
look-ups, even if this is undocumented. This means that the host/hostaddr
dichotomy is somehow moot as host can already be used for the same
purpose.

* \conninfo does not follow the implemented logic, and, as there is no
sanity check performed on the specification, it can display wrong
informations, which are not going to be helpful to anyone with a problem
to solve and trying to figure out the current state:

sh> psql "host=/tmp hostaddr=127.0.0.1"
psql> \conninfo
You are connected to database "fabien" as user "fabien" via socket in "/tmp" at port "5432"
# wrong, it is really connected to 127.0.0.1 by TCP/IP

sh> psql "host=127.0.0.2 hostaddr=127.0.0.1"
psql> \conninfo
You are connected to database "fabien" as user "fabien" on host "127.0.0.2" at port "5432".
# wrong again, it is really connected to 127.0.0.1

sh> psql "hostaddr=127.0.0.1"
psql> \conninfo
You are connected to database "fabien" as user "fabien" via socket in "/var/run/postgresql" at port "5432".
# wrong again

* Another issue with \conninfo is that if a host resolves to multiple ips,
there is no way to know which was chosen and/or worked, although on errors
some messages show the failing ip.

* The host/hostaddr dichotomy worsens when several targets are specified,
because according to the documentation you should specify either names &
dirs as host and ips as hostaddr, which leads to pretty strange spec each
being a possible source of confusion and unhelpful messages as described
above:

sh> psql "host=localhost,127.0.0.2,, hostaddr=127.0.0.1,,127.0.0.3,"
# attempt 1 is 127.0.0.1 identified as localhost
# attempt 2 is 127.0.0.2
# attempt 3 is 127.0.0.3 identified as the default, whatever it is
# attempt 4 is really the default

* The documentation about host/hostaddr/port accepting lists is really
added as an afterthought: the features are presented for one, and then the
list is mentionned. Moreover there are quite a few repeats between the
paragraph about defaults and so.

Given this state of affair ISTM that the situation would be clarified by:

(1) describing "host" full capability to accept names, ips and dirs.

(2) describing "hostaddr" as a look-up shortcut. Maybe the "hostaddr"
could be renamed in passing, eg "resolve" to outline that it is just a
lookup shortcut, and not a partial alternative to "host".

(3) checking that hostaddr non empty addresses are only accepted if the
corresponding host is a name. The user must use the "host=ip" syntax
to connect to an ip.

(4) teaching \conninfo to show the real connection, which probably require
extending libpq to access the underlying ip, eg PQaddr or PQhostaddr or
whatever.

The attached patch does 1-3 (2 without renaming, though).

Thoughts?

--
Fabien.