When I last tested my application on Mono, I discovered a very annoying problem: each time I tried to type something in a markdown document, a new nautilus file browser window would be launched! I had absolutely no idea why this would happen or where to start debugging. To make matters worse, I hadn't tested on Mono for about 40 revisions so there were a lot of possible changes that might have introduced the problem.
I have the source code history in a mercurial repository, so I decided to give the "bisect" feature a try. I started by marking the latest revision as "known bad":
hg bisect --bad
I also remembered making some mono-specific bug fixes, and the problem didn't exist at that point. So I grepped the output of hg log for commit messages containing the word "mono" and marked the revision as good:
hg bisect --good 627d6
At this point the bisect command has a revision range to work with. It will automatically update your working copy to a revision right in the middle of that range. You just have to rebuild your application, verify whether the problem is still there, and mark the revision as good or bad. (It is also possible to do this test automatically with a script.) The bisect command will then automatically cut the revision range to investigate in half again, and select yet another revision in the middle, etcetera.
Sure enough, after about 5 or 6 tests I got this message:
The first bad revision is:
date: Sun Nov 07 23:31:32 2010 +0100
summary: Links are now handled by opening .md or by external application.
Aha! Looks like I added some code to handle link clicks by passing the URL to the OS (to open it with a suitable external application). And obviously this code is now being triggered erroneously on mono whenever the document is updated. Thank you bisect!