Is there a way to retroactively flatten merge commits?

Question

I have a git repo with a combination of commits and merges containing multiple commits. I am trying to use git bisect to determine where a bug was introduced, but when the resulting commit points to a merge commit with a dozen different commits, this is not as helpful.

Is there a way to retroactively flatten a merge commit into its individual commits? Or is there some other method of performing a git bisect that will allow you to continue into the individual commits of a merge commit?

What is a merge commit with different commits if this is not a squash ? — Ôrel, Apr 04 '23 at 16:55
Can you use `git bisect --first-parent` and then do a second-level (second round?) bisect on the second parent of the offending merge commit? — Guildenstern, Apr 04 '23 at 17:00
Could the bug have been introduced via conflict resolution and not in one of the commits? — Schleis, Apr 04 '23 at 18:27
I think git bisect --first-parent would be the best approach. But I suppose this would still fail to catch issues due to resolution? — user21113865, Apr 04 '23 at 22:05

score 1 · Answer 1 · answered Apr 04 '23 at 22:49

Yes. The key factor here is to recognize that when using git-bisect to track down a bug you do not necessarily have to run along the original branch, you can also create a copy branch that you slightly modify.

The modifications I think of are things like for instance

Temporary add some more files to .gitignore as the first commit in the bisect branch to make git status output clean.
Fix commits that does not compile for trivial things like missing include statements etc.
Disable unit test for parts of the system that are not affected by the bug you are trying to find (this can be a huge time saver).
Split a large commit into multiple commits.
Etc

Now of course any change might mask/fix errors/add new errors, but if you keep the changes small/unrelated to what you are suspecting this should normally be fine. In worst case this will not work, but most likely it can identify the problem in a commit on the copy branch which you then need to correlate with the same commit on the original branch.

Now for the case you describe I assume the you have something like the following

and you want to bisect between main and Test-A. To flatten merge commit you can use interactive rebase which by default flattens the history¹. So with

git branch Test-A.bisect Test-A
git checkout Test-A.bisect
git rebase --interactive main       # Just saving and exiting the editor here.
git diff Test-A.bisect Test-A  # Verify that there are no unexpected differences.

then you end up with a Test-A.bisect branch without merges, which you then can run bisect on (with the caveat that it is not exactly the same as the original branch, but this very seldomly is a significant problem).

¹ You need to specify --rebase-merges for it to keep merge commits. I do include it by default in my ri alias for interactive rebase.

Is there a way to retroactively flatten merge commits?

1 Answers1