[Mapserver-users] mod_perl segfaults

[Eric Bridger]

On Tue, 2004-06-15 at 10:44, Lowell Filak wrote:
> The following message was sent by Robert Marcus Evans
> <rob at ert.dyndns.org> on Tue, 15 Jun 2004 10:24:56 -0400.
> 
> > Actually, we're using mod_perl/HTML::Mason in one place, and straight
> cgi's
> > in another. The CGI's don't ever seem to crash, whereas the mason is
> > unstable. I don't think it would be a threading issue, as I create a
> new map
> > object for each request. However, it would be possible for multiple
> > simultaneous map objects to exist for the same base file. Furthermore,
> I use
> > the static .map file as a base, and draw different dynamic objects on top,
> > depending on the request. I don't explicitly kill the map objects (is
> there
> > some kind of locking mechanism?), but then they're in a lexical scope and
> > should be garbage collected. You guys seem to be suggesting that if I
> used a
> > static map object for each apache process, the problem would still exist.
> > Anyone have any other ideas?
> > 
> > -rob
> > 
> > Lowell Filak wrote this on Tue, Jun 15, 2004 at 09:47:38AM -0400:
> > > The following message was sent by covad at ert.dyndns.org on Mon, 14 Jun
> > > 2004 15:30:43 -0400.
> > > 
> > > > I'm getting errors like the following:
> > > > 
> > > > [Mon Jun 14 14:22:49 2004] [notice] child pid 19412 exit signal
> > > Segmentation fault (11)
> > > > 
> > > > in my apache error log, but only intermittently. The site is
> usable, but
> > > > craps out ~25% of the time.
> > > > 
> > > > I'm running perl mapscript 4.2.0, apache 1.3.31, and mod_perl 1.29.
> > > > 
> > > > Has anyone else seen behavior like this?
> > > > 
> > > > Would rolling back to mapscript 4.0 help?
> > > > 
> > > > Thanks,
> > > > 
> > > > -rob
> > > > 
> > > 
> > > Rob
> > > 
> > > As Sean mentioned before - it is probably due to multiple requests into
> > > the same map/mapObj.
> > > Starting in 4.2 it should be safe to clone the map to avoid this but it
> > > does require more testing then what has been done.
> > > If that doesn't work I know that GOMOOS (currently very busy so I'm not
> > > sure they will reply) currently uses mod_perl but opens a new map for
> > > each request to ensure that two requests do not conflict.
> > > 
> > > Lowell
> > 
> 
> Rob
> 
> Until I have a chance to switch our server to mod_perl I can't reproduce
> the issue but I meant to suggest that you Do use a static map object for
> each request to try and eliminate the problem.
> Dynamic objects being drawn via MapScript only change the map object and
> not the map file so it sounds like your expecting the map object to not
> be shared between requests but in fact it is.?
> You do not need to explicitly kill map objects and as long as seperate
> requests are not sharing the same map object the process of 'reading'
> the same static map file is read-only so no locking should be
> required... unless your doing a saveMapContext.?

I do see this type of error frequently in our development server Apache
logs:
> [Mon Jun 14 14:22:49 2004] [notice] child pid 19412 exit signal
> > > Segmentation fault (11)

in fact as recently as last Monday. Unfortunately I cannot remember what
the circumstances were and I have not been able to reproduce it. When it
happens again (it will) and my memory gets jogged I'll report the
circumstances.

We never see this in our production server or even in out development
staging server.

My guess is that it has nothing to do with mod_perl or thread safety
issues since you load your map file for each request (as we do). There
is no issue in multiple processes reading the map file simultaneously.

Eric