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How to use git bisect

git bisect is a useful debugging tool. For context, git bisect is a git command that can be used to track down the specific commit which a bug was introduced into the codebase.

If you can find a commit where the code works properly and a commit with the offending bug, you don’t have to trace down the buggy commit by hand. The git bisect command, via binary search, will help you find the offending commit. For example, the Git documentation has a handy git bisect guide that shows two ways to use it.

How to git bisect in projects using Bundler

A few things that may not be obvious are needed for git bisect to work in a project that uses Bundler.

  1. The Gemfile.lock needs to be in the git repo, so that each commit will load the same dependencies every time.
  2. Each step during the bisect needs to run bundle install first, so that the correct dependencies are installed and available to be loaded.
  3. After determining if the commit is good or bad, each step needs to git reset. If bundle install or running the test can cause changes on the file system, which would prevent git checkout of the next commit to test if they are not reset.

Here’s a minimal example script that runs the rake task spec:

bundle install
bin/rake spec
git reset --hard HEAD
exit $status

See also the discussion at bundler/bundler#3726.