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

From: eem21(at)cam(dot)ac(dot)uk
To: scrappy(at)hub(dot)org
Cc: pgsql-interfaces(at)postgresql(dot)org, tgl(at)sss(dot)pgh(dot)pa(dot)us
Subject: Re: [INTERFACES] libpq + multiple connections ...
Date: 1999-11-29 21:18:58
Message-ID: E11sYAg-0002xd-00@orange.csi.cam.ac.uk (view raw or flat)
Thread:
Lists: pgsql-interfaces
On 29 Nov, The Hermit Hacker wrote:
> 
> Is this something that could *safely* be back-patched to a v6.5.4 release?

Well, it's not _supposed_ to have changed the behaviour for existing
functions.  However, there's been quite a lot of chopping and changing,
and I might have lost something somewhere.  That said, it would be of
great help to me if it made it into a 6.5.x release, and I would thus
be willing to test it.  I can only test ix86/Linux, unfortunately. 
When would it be released, if that were the case?

BTW, how do I find out about the SGML tags we are using (I've never
written any SGML before, only HTML).

Ewan.

> 
> 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 @ hub.org 
> primary: scrappy(at)hub(dot)org           secondary: scrappy(at){freebsd|postgresql}.org 
> 



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