Discovering Problematic Commits With Git Bisect

Ryan James Spencer

June 30 2020, 8:50AM

A problem has happened due to some offending code landing on your main, production branch. You use git and your best bet is to keep rolling back commits until the system finds itself in a steady state. You come late into this picture and you're unsure how far back you need to go.

Firstly, you ought to be using something that alleviates the need for running through out an entire CI pipeline in order to produce a deploy. I've talked a bit about this in the past on my screencasts about setting up a CI regarding the distinction between a deployment and a release. If you have something like this, rolling back a fair few number of releases is probably trivial to attempt.

However, if you don't have this in place or you really do need to roll through an entire CI pipeline, then you can still using something like git bisect to find the first offending commit.

git bisect runs a binary search across a span of commits. The general framework for running a git bisect is the following:

  1. git bisect start FROM_COMMIT TO_COMMIT
  2. Test the commit, determine if it is good or bad, and tell git with git bisect good or git bisect bad. You can also skip commits with git bisect skip.

The trick to finding the first offending commit isn't to run the same steps your CI pipeline would; you should have all those builds available for review and they will tell you whether or not a build truly succeeded, unless you can't trust your CI and, in that case, you have other issues on your hand. Crafting your own test and running it each time in (2) will help guide you in the decision to making a choice for whether or not the commit is good or bad in light of what you are trying to find.

You can alleviate the tedium of (2) by using git bisect run and supplying a program. If the script fails or you ever want to abandon your search midway, you can always run git bisect reset and start over again. There are some tricks to how you can craft the exit codes from the script you write for git bisect run that really make this process a lot faster. To give a sense of the range of use for git bisect as a general search tool, let's call our test script predicate.

#!/bin/sh -eux

# NB.
# exiting with 125 tells `git bisect run` to skip this commit.
# exiting with 0 means the commit is `good'.
# exiting with 1 means the commit is `bad'.

cargo build || exit 125 # skip failed builds.
target/debug/program > /tmp/program.out
[ ! diff /tmp/program.out /tmp/program.snapshot ] && exit 1

You'll need to place this script somewhere outside of the current git repository as it will mess up checkouts between commits, and, as always, ensure it is executable. Another pitfall that can hurt is how you structure your git history; if you use merge styled commits, as is the default for GitHub, then you will probably not care if the commits in between the merge commits fail. You can do one of two things: output the list of all merge commits that match a particular pattern, e.g., the way GitHub does it, or you could also, if your history is clean enough, use git show --no-patch --format="%P" <commit hash> to determine if a commit has more than one parent; you'll see more than one hash noted in the output. You can find quick version I hacked together filtering out commits with the GitHub styled subject lines you can tweak at this gist.

In the above example I show testing against a snapshot given some program output, but really the predicate could be anything. Using git bisect to drive things like textual search has better alternatives like the "pickaxe" with -S in git log, but if you want to find the first commit where something happened and it isn't part of the data that git saves, such as program behavior, then git bisect will let you find it far faster. I've also used this in the past to whip up quick, minimal tests that I can inject after the checkout and run some test suite against. git bisect run takes any binary, too, meaning you don't have to use a shell script like I have in the example above. The real aim is not to think of the predicate script or program as something that has to be about failures; you can easily use it to discover first instances of any kind of particular behavior a program may exhibit, as long as it is reproducible locally.

Granted, a system may be so complex in it's operation that there is no way for you to locally verify the offending commit. Mitigating or "stopping the bleeding" is something that needs to happen quick. With that said, git bisect might be a better tool for analysis later, when the pressure is low and you can better craft a test or predicate to find where the fault first occurred, but if you haven't spent a lot of time with release engineering or you are in a place where it could use some improvements, running git bisect in this matter might help save you precious time, and even if you do have good release engineering in place, it might help save you a pulling out a lot of hair finding the place where code has effectively broken down.

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