Re: [HACKERS] PQhost may return socket dir for network connection

At Mon, 1 May 2017 15:48:17 -0400, Robert Haas <robertmhaas@gmail.com> wrote in
<CA+TgmoZcyGsb621i35AC9TJar__F9fQbmRPHfCOnE4aNE227HA@mail.gmail.com>
> On Mon, May 1, 2017 at 1:36 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> >> On Mon, May 1, 2017 at 12:06 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> >>> Having said that, the behavior stated in $subject does sound wrong.
> >
> >> I'm not sure.  My understanding of the relationship between host and
> >> hostaddr is that hostaddr overrides our notion of where to find host,
> >> but not our notion of the host to which we're connecting.  Under that
> >> definition, the current behavior as described by Kyotaro sounds
> >> correct.
> >
> > Perhaps.  But hostaddr also forces us to believe that we're making an
> > IP connection, so it still seems pretty dubious to return a socket
> > path.  The true situation is that we're connecting to an IP host that
> > we do not know the name of.
> 
> Yes, I think that's a reasonable interpretation.
> 
> > I notice that one of the recent changes was made to avoid situations where
> > PQhost() would return NULL and thereby provoke a crash if the application
> > wasn't expecting that (which is not unreasonable of it, since the PQhost()
> > documentation mentions no such hazard).  So I would not want to see us
> > return NULL in this case.
> > And I believe we already considered and rejected the idea of having it
> > return the hostaddr string, back in some of the older discussions.
> > (We could revisit that decision, no doubt, but let's go back and see
> > what the reasoning was first.)
> >
> > But maybe returning an empty string ("") would be OK?
> 
> Yeah, that might be OK.  But I'd be inclined not to back-patch any
> behavior changes we make in this area unless it's clear that 9.6
> regressed relative to previous releases.

I personally don't have a specific wish on this since I don't
have a specific usage of PQhost. (I think that users are
reposible for the result of contradicting host and hostaddr.)

However, it might be a problem that the documentation doesn't
mention what the returned value from PQhost is.

https://www.postgresql.org/docs/9.6/static/libpq-status.html

> Returns the server host name of the connection. This can be a
> host name, an IP address, or a directory path if the connection
> is via Unix socket. (The path case can be distinguished because
> it will always be an absolute path, beginning with /.)

I don't think more strict definition is required but the above
should describe the expected usage or obvious limitation. Anyway
it is contradicting to the current behavior. It can return a
socket path for a IP connection.

regards,

-- 
Kyotaro Horiguchi
NTT Open Source Software Center