Fri, 25 Jun 2010

Professors' Open Source Summer Experience @ RIT

Last week Red Hat put on a Professors' Open Source Summer Experience (POSSE) at RIT. Being an alumni, I was excited by the opportunity to be able to go back up to The ROC and teach some of the people that taught me. Going into it, I really had no idea what to expect. All I knew is that I was going to help lead the 'deep dive' section of the course, where I would teach professors how to dive in head first and get productively lost in a strange codebase. This is not something that can be accomplished with a set of powerpoint slides. Teaching how to hack on open source requires that you emerse yourself into a codebase, and bring your students with you.

The previous POSSE at Worcester State dove into the Sugar Measure Activity, and we were going to do the same.

Measure is an activity that turns the computer into an oscilloscope. Signals from the microphone (and sensors) can be plotted in time and frequency domains.

I had never used this activity, let alone hacked on it before. I've also never done any sugar activity development, aside from some tweaking of the OpenVideoChat, so I really had no idea what I was getting myself into. The obvious first step was to get it running. All of us were able to start the activity in virtual machines or emulators, except for one install which hit some odd errors upon startup. We were able to quickly track the bug down to a stray return statement in __init__ before some critical initialization code. Right after we fixed the problem we noticed that Walter Bender had already fixed this issue a few hours earlier. After a git pull, we were up and running.

Once we all got the activity running, we took a look at the bug list to see if there was any low-hanging fruit for us to tackle. Since the previous POSSE had already done some work on this activity the week before, there were not any trivial tickets left in the queue. So, in that case, we dove head first into the hardest one, "Measure activity gets stuck after recording". This ticket had very little information, and no log output, so we were on our own to try and track it down. We were able to reliably reproduce the issue on the XO-1.5, but not on the 1.0. In our virtual machines we hit it sporadically. We all agreed that it felt like a race-condition, most likely due to threading. So we started instrumenting the code and adding some debugging statements to try and figure out which line of code was the culprit.

In our efforts to scatter print statements all over the place to try and determine the code path, we noticed that none of our output was hitting the logs. When you have no idea if your code is even being run or not, don't be ashamed to throw in a raise Exception("WTF?!"). We finally realized that since the activity would freeze, it was never able to flush stderr/stdout to the log. A quick find/replace regex later, and we were using the proper logging module and seeing our debugging output hit the logs.

We were then able to track the bug down to a line of code that uses GTK to try and get the coordinates of the parent window. At this point, since none of us were GTK experts, we had to go upstream. So, I dropped into #fedora-devel and asked. Within 10 minutes I had responses from 3 different GTK hackers. One was a typical RTFM response, which humored the professors (who are very used to hearing/saying this), but the others pointed us in the right direction. One mentioned that gtk.gdk calls probably should not be done in a seperate thread. So, a suggested workaround was to add gtk.threads_enter()/gtk.threads_leave() calls before running any gtk.gdk code in the thread. A 2-line patch later, and we had squashed the bug.

We eventually bumped into the inevitable typo in some comments. So, we made the fix locally, and committed it to our git repo. A few minutes later and we found another one. I saw this as a great opportunity to show off some of my git-fu. Instead of sending two "Fix typo" patches upstream, I showed the professors how to use git interactive rebasing to squash multiple commits into a single one. They all followed along closely, and the workflow made sense to them.

While we were looking through the ticket queue, we saw an issue where Measure would apparently leak memory and crash when running it for a long period of time. While keeping this in mind, we kept our eyes peeled while wandering around the codebase to see if we could track the issue down. When looking at the code that takes screenshots of the waveform, I noticed that it created a temporary file with tempfile.mkstemp, saved the pixmap to it, injected it into the Journal, and then deleted the directory. This looked fine at a first glance, until I realized that it never closed the temporary file descriptor. Another 2-line patch later, and this issue was solved.

The next day both of the patches that we sent upstream were applied by Walter Bender.

Overall, POSSE definitely exceeded my expectations, and I'm extremely satisfied with how the 'deep dive' section went. I went into it feeling completely unprepared to teach, but by trusting my "hacker intuition", I feel that it turned out to be a fantastic learning experience for all of us.

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