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Re: [INTERFACES] libpq + multiple connections ...

From: The Hermit Hacker <scrappy(at)hub(dot)org>
To: "E(dot)E(dot) Mellor" <eem21(at)cam(dot)ac(dot)uk>
Cc: pgsql-interfaces(at)postgreSQL(dot)org, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: [INTERFACES] libpq + multiple connections ...
Date: 1999-11-29 21:05:55
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-interfaces
Is this something that could *safely* be back-patched to a v6.5.4 release?

On Mon, 29 Nov 1999, E.E. Mellor wrote:

> --On 29 November 1999, 02:45 -0500 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > The Hermit Hacker <scrappy(at)hub(dot)org> forwards:
> >> Hmm, sorta, I'm a bit troubled, I was trying to add an async connection
> >> function to libpq and I stumbled across some problems.
> > 
> > Someone else was already working on that --- check the archives from
> > a few months back.
> 'twas me.  In fact, it's been ready for ages, but I've had so much on that
> I've not had time to write the docs.  I'll do them today for you.
> >> It seems that libpq makes use of some static variables, meaning i'm not
> >> sure if it's safe to use libpq for multiple database connections.
> >> What i'm refering to is:
> >> postgresql-6.5.3/src/interfaces/libpq/fe-connect.c
> >> line 79 has a structure that seems to be shared amongst the entire
> >> library, am I likely to stumble upon more stuff that makes it somewhat
> >> dangerous to have more than one active database connection in my program?
> > 
> > The PQconnectdb() function uses that static array, meaning that you can't
> > safely run two PQconnectdb() calls in parallel.  But you can open two
> > connections in sequence and then use them in parallel; and you can do
> > the opens in parallel if you use one of the older, less-friendly
> > connection-opening calls.  There aren't any other non-constant statics
> > in libpq AFAIR.
> This is correct.  Someone else was looking at thread-safety across the
> whole of libpq, but as far as I'm aware, it's not _guaranteed_ to be
> thread-safe anywhere.  My changes allow one to use libpq in a single
> thread, but with multiple connections and without blocking the thread at
> all.
> > The static array sucks, I agree, but I don't see any way to get rid of
> > it without changing libpq's API for PQconnectdb() and PQconndefaults().
> > Do we want to consider doing that (and breaking some apps) for 7.0?
> I think it should be possible to arrange it so that these functions still
> exist (but are deprecated and marked as non-thread-safe).  I'll leave that
> for others to decide though.
> Ewan Mellor.

Marc G. Fournier                   ICQ#7615664               IRC Nick: Scrappy
Systems Administrator @ 
primary: scrappy(at)hub(dot)org           secondary: scrappy(at){freebsd|postgresql}.org 

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