Mercurial: The Definitive Guide

Tips for Finding Bugs Effectively

Give Consistent Input

The hg bisect command requires that you correctly report the result of every test you perform. If you tell it that a test failed when it really succeeded, it might be able to detect the inconsistency. If it can identify an inconsistency in your reports, it will tell you that a particular changeset is both good and bad. However, it can’t do this perfectly; it’s about as likely to report the wrong changeset as the source of the bug.

Automate As Much As Possible

When I started using the hg bisect command, I tried a few times to run my tests by hand, on the command line. This is an approach that I, at least, am not suited to. After a few tries, I found that I was making enough mistakes that I was having to restart my searches several times before finally getting correct results.

My initial problems with driving the hg bisect command by hand occurred even with simple searches on small repositories; if the problem you’re looking for is more subtle, or the number of tests that hg bisect must perform increases, the likelihood of operator error ruining the search is much higher. Once I started automating my tests, I had much better results.

The key to automated testing is twofold:

• always test for the same symptom, and

• always feed consistent input to the hg bisect command.

In my tutorial example above, the grep command tests for the symptom, and the if statement takes the result of this check and ensures that we always feed the same input to ...