Re: [INTERFACES] Peculiar behaviour in libpq

On Thu, 24 Jun 1999, Tom Lane wrote:


> Tim Allen <tim@proximity.com.au> writes:
> > I've noticed some strange behaviour when using libpq with
> > PostgreSQL 6.4.2 on RedHat Linux 5.1. I'm using libpq asynchronously, with
> > a select() loop, calling PQconsumeInput etc. What is happening is that
> > occasionally, after I have received a callback via select() and processed
> > the result, I get an extra callback. These extra callbacks have no result
> > attached to them, PQisBusy() returns false, and, by the records I'm
> > keeping in my own program, there is no database operation pending (ie I
> > already have a result for every operation I've submitted).
> 
> Hmm.  Could be that the terminating ReadyForQuery message ('Z') is
> arriving in a separate packet from the last message of the query
> response proper --- but that doesn't seem real probable.  Other
> possibilities are that backend NOTIFY or NOTICE messages are arriving;
> but if you were using NOTIFY you'd know it, and NOTICEs ought to show
> up on stderr (unless you've suppressed that).


I've performed some more experiments, after instrumenting bits of libpq.
I'm still bemused...

No, it's not the 'Z' message. What appears to be happening is that a
select condition is occurring without there being any data to read from
the socket. Inside PQconsumeInput(), which is calling pqReadData(), the
call to recv() returns -1 with errno set to EAGAIN  (I gather this is
implementation dependent, and could be EWOULDBLOCK on some platforms).

This happens after _every_ result, ie the result from a query gets
returned, with the data being successfully read when select() says we can,
and then one extra select condition occurs, with no data to read.

So the mystery is, why does select() think there is something to read from
the socket when recv() thinks not?

I was starting to suspect it could be merely an oddity of Linux's socket
implementation, but discovered that the same behaviour occurs on SGI/IRIX.

This is a local (Unix domain) socket, btw, so it's not a TCP/IP issue.

> If you really care, you could instrument the low-level guts of libpq
> to see what is arriving when (the routines in fe-misc.c would be the
> place to put debug logging).
> 
> > Will it go away in 6.5? On the whole it seems reasonably benign, I just
> > worry there may be something bad going on that will bite me eventually.
> 
> Yeah, it seems worth understanding at least.  I'm pretty sure 6.5 will
> behave the same, since nothing much has changed in the BE/FE protocol.
> I'd be interested to know what you find out, if you pursue it.

It still seems benign enough, but I would also like to understand it...

>             regards, tom lane

Tim

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Tim Allen
Proximity Pty Ltd  http://www.proximity.com.au
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