libpq: decouple the .pgpass lookup port from the connection port

Hello hackers,

I would like to float an idea before writing a patch, to find out whether
it is wanted and to get the design right.

Problem
-------

libpq looks up a password in .pgpass using the connection's host and port
as part of the key (host:port:database:user:password). When a client
connects through an SSH tunnel, or through a connection pooler that
listens on a different local port, the port that libpq actually connects
to is not the port of the real server. As a result, the .pgpass lookup is
done against the local/tunnel port and fails to match entries written for
the real server port.

Concretely, suppose the real server is db.example.com:5432 and a user
opens an SSH tunnel so that 127.0.0.1:54321 forwards to it. The natural
.pgpass entry is:

    db.example.com:5432:appdb:alice:secret

The client then connects with host=db.example.com (kept for .pgpass and
TLS), hostaddr=127.0.0.1 and port=54321 (the tunnel). libpq looks up

    db.example.com:54321:appdb:alice

which does not match the 5432 entry, so no password is found and the user
is prompted (or the connection fails under -w).

The host side of this exact problem was already solved
-------------------------------------------------------

libpq already decouples the *host* used for the .pgpass lookup from the
real network endpoint: hostaddr gives the address actually connected to,
while host remains the logical name used for the .pgpass lookup and for
TLS verification. This is the pwhost logic in fe-connect.c, which goes
back to the 2018 thread "Bizarre behavior in libpq's searching of
~/.pgpass":

https://www.postgresql.org/message-id/30805.1532749137%40sss.pgh.pa.us

The port has no equivalent. passwordFromFile() is called with
conn->connhost[i].port, i.e. the real connection port, with no way to say
"connect to this port, but look up .pgpass under that port". The host has
host/hostaddr; the port only has port. This proposal is to close that
asymmetry.

Why the port wildcard is not enough
-----------------------------------

One can write the entry with a wildcard port:

    db.example.com:*:appdb:alice:secret

and it does match the tunnel. But the wildcard over-matches: a single
local forwarding port (say 54321, or even a fixed local port reused for
several tunnels at different times) ends up matching every server reached
through that port, so the same password line can be applied to different
servers. That is precisely the kind of "password sent to the wrong
server" situation the 2018 host fix was trying to avoid. The wildcard
trades safety for convenience; it is not a substitute for matching the
real server port.

Proposal
--------

Add a libpq connection parameter that specifies the port to be used for
the .pgpass lookup, independently of the port libpq connects to. The
connection still uses port (and hostaddr); only the password-file lookup
key uses the new value. When the new parameter is not set, behavior is
unchanged: the lookup uses port exactly as today.

I do not have a strong opinion on the name and would rather not bikeshed
it before the idea itself is judged. Candidates that came to mind:

    - pgpassport / passfileport (it only affects the password file)
    - portaddr (mirrors hostaddr: "port stays logical, portaddr is the
      real endpoint"), though that would invert today's meaning of port,
      which is probably too invasive

A dedicated parameter that affects only the .pgpass lookup (the first
option) seems the least surprising and the smallest change. It is also
easy to reason about for security: it is an explicit, opt-in assertion by
the user, exactly like hostaddr/host.

This is not hypothetical. I ran into it myself while adding SSH tunnel
support to pgcli (a widely used Postgres CLI): with the tunnel active, an
explicit-port .pgpass entry never matches, because the lookup happens
against the random local forwarding port. The user is prompted for a
password even though the matching entry is right there, and only a
wildcard port papers over it. Other tools hit the same wall:

    - pgcli: SSH tunnel rewrites the port before the .pgpass lookup
https://github.com/dbcli/pgcli/pull/1546
    - DBeaver: .pgpass looked up by 127.0.0.1 through an SSH tunnel
https://github.com/dbeaver/dbeaver/issues/16499
    - pgAdmin 4: control the SSH tunnel local port for .pgpass matching
https://github.com/pgadmin-org/pgadmin4/issues/6903

Questions for the list
----------------------

  1. Is decoupling the .pgpass-lookup port from the connection port
     something libpq wants, given that host/hostaddr already does the
     equivalent for the host?
  2. Is a dedicated lookup-only parameter the right shape, or would you
     prefer a different model?
  3. Naming preferences?

If there is interest, I am happy to write the patch (code, docs and
tests).

Thanks for reading,
Diego