Bisect

Have you ever been in a situation in which something has “gone wrong” (intentionally vague) between two git commits, say c1 and c2, and you’re trying to figure out which commit caused the issue? In other words, your code works fine at c1, but not at c2. Thus, a commit in the range (c1, c2] resulted in your code being in a “bad” (for some definition of “bad”) state.

One approach is to look at all the commits between (c1, c2] and see if any commit stands out as something that might have caused the issue. But there are times when looking at the changes is not enough, or it’s not clear why any of the changes would have broken anything, and you need to do some other work (run integration tests, run performance suites, run UI tests, etc.) in order to pinpoint the breaking commit.

“Why, this seems like a perfect opportunity to use binary search to figure out which commit caused a problem! All I need to do is a binary search in the range (c1, c2]. For a particular commit in this range (starting in the middle) I simply need to git checkout the code at that point, do whatever work I need to (explained above), and then make a decision on whether I need to search in the ‘upper half’ or ‘lower half'”

Enter git bisect. It allows you to focus on what went wrong, without having to manage the git + binary search state. In our scenario we’d simply mark c1 as a good commit, and c2 as a bad one, and then let git bisect work its magic in enabling us to discover what went wrong between (c1, c2].

I love git.

One thought on “Bisect

  1. ktal90 says:

    Sounds daunting because it’s outside of a typical git workflow, but you make this sound really straightforward. I’ll have to give it a shot when I’m in such a situation.

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