How do you get git bisect to ignore merged branches?

Question

I\'m aware that git bisect is branch-aware by design, so that if between good commit, G, and bad commit, B, you merged in a branch, it needs to take those changes into consi

Answer 1

So, the assumption that the first parent of a merge commit is always the same branch isn't always correct. For instance, if you are off on a topic branch and merge master to it to get up to date (so for this merge commit, the first parent is the topic branch) and then checkout master and merge topic back to it, you get a fast forward merge, which just moves master to the merge commit that had first parent as your topic branch. This may seem contrived, but its actually a pretty normal workflow - I always merge master into my branch so that my merge back to master will be a trivial merge (i.e., fast forward-able) (Sorry James, always forget to rebase it).

There is one way that I've found to help figure out which parent is your branch - the merge commit comment itself. By default, git composes a merge commit comment that says which branch was merged and you can use this to deduce which parent is the branch you are interested in, so long as the person doing the merge commit didn't overwrite this merge commit comment.

So I tried this out and it seems to work for me. I wrote a Python script to help do this on github. If you run this script, it will try to trace backwards and follow your branch and emit a list of commit ids that are the tips of the branches that are merged into your branch. With this list, you can give these to "git bisect good" and bisect will then omit all of the commits on the merged branches from your bisection, achieving the desired result.