Re: libpq host/hostaddr/conninfo inconsistencies

From: Arthur Zakirov <a(dot)zakirov(at)postgrespro(dot)ru>
To: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
Cc: PostgreSQL Developers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: libpq host/hostaddr/conninfo inconsistencies
Date: 2018-09-29 21:56:38
Message-ID: 20180929215637.GA22226@artur-book.localdomain
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On Fri, Aug 24, 2018 at 11:22:47AM +0200, Fabien COELHO wrote:
> Attached is a rebase after 5ca00774.

I looked a little bit the patch. And I have a few notes.

> 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.

Agree it may have more details within the documentation.

> sh> psql "host=/tmp hostaddr="

Yeah this example shows that user may be confused by output of
\conninfo. I think it is psql issue and libpq issue. psql in
exec_command_conninfo() rely only on the PQhost() result. Can we add a
function PQhostType() to solve this issue?

> sh> psql "host= hostaddr="

I'm not sure that is is the issue. User defined the host name and psql
show it.

> sh> psql "hostaddr="

I cannot reproduce it. It gives me the message:

You are connected to database "artur" as user "artur" on host "" at port "5432".

I think it is because of the environment (I didn't define PGHOST
variable, for example). If so, depending on PGHOST variable value
("/tmp" or "") it is related with first or second issue.

> * 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.

Can you explain it please? You can use PQhost() to know choosed host.

> * 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.

I cannot agree with you. When I've learned libpq before I found
host/hostaddr rules description useful. And I disagree that it is good
to remove it (as the patch does).
Of course it is only my point of view and others may have another opinion.

> (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.

Patch gives me an error if I specified only hostaddr:

psql -d "hostaddr="
psql: host "/tmp" cannot have an hostaddr ""

It is wrong, because I didn't specified host=/tmp.

Arthur Zakirov
Postgres Professional:
Russian Postgres Company

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