[Mesa-dev] Truncated extensions string

[Kenneth Graunke]

On Friday, March 11, 2011 10:46:31 AM José Fonseca wrote:
> On Fri, 2011-03-11 at 09:04 -0800, Eric Anholt wrote:
> > On Fri, 11 Mar 2011 10:33:13 +0000, José Fonseca <jfonseca at vmware.com> 
wrote:
> > > The problem from
> > > 
> > > http://www.mail-archive.com/mesa3d-dev@lists.sourceforge.net/msg12493.h
> > > tml
> > > 
> > > is back, and now a bit worse -- it causes Quake3 arena demo to crash
> > > (at least the windows version). The full version works fine. I'm not
> > > sure what other applications are hit by this. See the above thread for
> > > more background.
> > > 
> > > 
> > > There are two major approaches:
> > > 
> > > 1) sort extensions chronologically instead of alphabetically. See
> > > attached patch for that
> > > 
> > >   - for those who prefer to see extensions sorted alphabetically in
> > > 
> > > glxinfo, we could modify glxinfo to sort then before displaying
> > > 
> > > 2) detect broken applications (i.e., by process name), and only sort
> > > extensions strings chronologically then
> > > 
> > > Personally I think that varying behavior based on process name is a
> > > ugly and brittle hack, so I'd prefer 1), but I just want to put this
> > > on my back above all, so whatever works is also fine by me.
> > 
> > If this is just a hack for one broken application, and we think that
> > building in a workaround for this particular broken application is
> > important (I don't), I still prefer an obvious hack for that broken
> > application like feeding it a tiny extension string that it cares about,
> > instead of reordering the extension list.
> 
> There are many versions of Quake3 out there, some fixed, others not, and
> others enhanced. This means a tiny string would prevent any Quake3
> application from finding newer extensions. So I think that if we go for
> the application name detection then we should present the whole
> extension string sorted chronologically, instead of giving a tiny
> string.
> 
> Jose

I agree with José - it's not one broken application, it's a number of old, 
sometimes closed-source games that we can't change.

I'm not sure how changing the sorting solves the problem, anyway - the amount 
of data returned would still overflow the buffer, possibly wreaking havoc.  I'd 
rather avoid that.

Ian and I talked about this a year ago, and the solution I believe we came up 
with was to use a driconf option or environment variable:

If MESA_MAX_EXTENSION_YEAR=2006, then glGetString would only return extensions 
created in 2006 or earlier.  The rationale is that if a game came out in 2006, 
it won't know about any extensions from 2007 anyway, so advertising them is 
useless.  The fixed-size buffer is also almost certainly large enough to handle 
this cut-down list of extensions.

This should be trivial to do now that you already have the years for each 
extension...just store them in the table, rather than in comments, and check 
before listing an extension.

A driconf option is nice because it allows this to be overridden in .drirc on 
a per-app basis, rather than having to set an environment variable.  It might 
be a bit more work though.

--Kenneth