Redland Bug Squish

Chris: 2, Redland: 0

For quite a while, I’ve been having some problems with installing the latest version of the Redland RDF Application Framework. I’m sure that these were problems of my own: I know that my system is a little goofy, in part because I’ve got a combination of ebuild-built and non-ebuild versions of things. So, for a while, I’ve been unable to compile the lateset Redland, because whenever I’d try and compile, the system-wide redland-config would get in the way, and then Redland would try to link against the already installed libraries, etc. etc.

However, yesterday I started having issues with julie crashing, returning a glibc exception. I wrote a small test script, and found that the issue was occuring any time I was using two “AND” clauses in an RDQL query. I recompiled glibc with debug info this morning, and was amazed to find that my backtraces suddenly became much more informative: Instead of gdb returning ?? in ?? in my backtrace, it actually gave me function names and line numbers. I didn’t think that just recompiling glibc would have helped so much.

After I was able to do this, I sent mail to the Redland-dev list: Here’s my problem, here’s my backtrace, where do I go from here? That Email got several responses back and forth from Dave Beckett (maintainer of Redland) and Morten Fredrickson (maintainer of the MySQL storage backend). I ended up stopping all services which used Redland, uninstalling redland completely, and making the latest release successfully. In combination with discussion on IRC, I moved onto testing with valgrind, an open-source memory debugger for x86-GNU/Linux and ppc-GNU/Linux. Sending the data dump from that helped Dave to find the problem, which was related to the way that the mysql backend frees results.

One patch later, the problem was cleared up. There’s still the question of why it broke, which seems to be related to some broken statements in my database. However, it’s good to know that I helped to find a bug in Redland, rather than just finding something broken locally.

In the end, I did learn a few things:

1. Having glibc debug symbols on is a Good Thing.
2. If you can’t install the latest version, try uninstalling the previous version first.
3. valgrind is an app which may help you locate the source of bad free() calls
4. glibc has some extra sanity check memory detection: you can adjust its behavior via MALLOC_CHECK_. Levels of sanity are 0->3, with 0 being “display error message and kill application” and 3 being “do nothing”. (I found this via fedora core release notes.)

I also was able to compile new Redland bindings: this means that I fixed my Perl Query problem (which has been fixed for a while, but I couldn’t install new bindings), as well as adding Sparql support to my local Redland tools. Now, I just need to write something that uses it.

So, in the end, I helped squish a bug, and upgraded all my Redland related stuff on my system. That’s what I call a good day.

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