[libxml-devel] Crash in ruby_xml_xpath_object_mark

[Luc Heinrich]

Greetings,

I have recently switched from REXML to libxml-ruby (0.5.2) because I  
really needed a major speed boost for a web application and libxml- 
ruby really delivered. Some parts of the code which were taking more  
than 30 seconds are now running under 0.2 seconds, I'll take a 150x  
speed boost any day ;)

HOWEVER, I also discovered that this comes at a price: libxml-ruby is  
*VERY* unstable and crashes very frequently under high(-ish) memory  
pressures. I have seen mentions in past messages about a new memory  
model being used, but I still get very frequent crashes.

For example, I was able to track one down in  
'ruby_xml_xpath_object_mark' (that's in ext/xml/ 
ruby_xml_xpath_object.c) where the use of 'xpop->nodesetval->nodeNr'  
causes the crash because 'nodesetval' is NULL. The top of the stack  
always looks like this:

0   libxml_so.bundle  	0x002f2484 ruby_xml_xpath_object_mark + 92
1   libruby.dylib     	0x00238dd8 gc_mark_children + 1628
2   libruby.dylib     	0x00238744 gc_mark + 196
3   libruby.dylib     	0x002383e8 mark_locations_array + 148
4   libruby.dylib     	0x00239c24 garbage_collect + 340
5   libruby.dylib     	0x00237f80 rb_newobj + 44
...

I don't know the libxml-ruby code well enough yet to propose a clean  
fix (I'm not sure that simply testing for NULL on nodesetval would be  
enough or appropriate), so if anyone could chime in... :)

Thanks.

-- 
Luc Heinrich