Rebasing past reformats

December 12, 2024

TL;DR

Did you ever have to deal with a huge list of conflicts on rebase caused by automatic reformatting of an upstream codebase? If you got into a similar situation you might be able to automatically recreate your changes with git filter-branch --tree-filter and a git commit --allow-empty trick.

story mode

I have local fork of staging branch of nixpkgs git repository to do various tests against experimental upstream packages (like gcc from master branch) or experimental nix features (like ca-derivations). I have about 350 patches in the fork. I sync this forked branch about daily against the upstream nixpkgs/staging. Most of the time git pull --rebase is enough and no conflicts are there. Once a month there is one or two files to tweak. Not a big deal.

A few days ago nixpkgs landed a partial source code reformatting patch as PR#322537. It automatically re-indents ~21000 .nix files in the repository with a nixfmt tool. My git pull --rebase generated conflicts on first few patches against my branch. I aborted it with git rebase --abort. I would not be able to manually solve such a huge list of commits and I wondered if I could somehow regenerate my patches against the indented source. In theory rebasing past such change should be a mechanical operation: I have the source tree before the patch and after the patch. All I need to do is to autoformat both before and after trees and then diff them. I managed to do it with help of git commit --allow-empty and git filter-branch --tree-filter.

actual commands

Here is the step-by-step I did to rebase my local staging branch past the source reformatting 667d42c00d566e091e6b9a19b365099315d0e611 commit to avoid conflicts:

Create an empty commit (to absorb initial formatting later):

$ git commit --allow-empty -m "EMPTY commit: will absorb relevant formatting changes"

Move the last empty commit in the patch queue to the beginning of the patch queue:
```
$ git rebase -i --keep-base
```
In the edit menu move the "EMPTY commit: will absorb relevant formatting changes" entry from last line of the list to the first line.

Get files in the branch affected by the formatting change:

The formatting change is 667d42c00d566e091e6b9a19b365099315d0e611.

$ FORMATTED_FILES=$(git diff --name-only \
    667d42c00d566e091e6b9a19b365099315d0e611^..667d42c00d566e091e6b9a19b365099315d0e611 \
    -- $(git diff --name-only origin/staging...staging) | tr $'\n' ' ')

This will populate FORMATTED_FILES shell variable with affected files.

Reformat the $FORMATTED_FILES files:

$ FILTER_BRANCH_SQUELCH_WARNING=1 git filter-branch \
  --tree-filter "nixfmt $FORMATTED_FILES" -- $(git merge-base origin/staging staging)..
...
Rewrite 6fc0a951e9b7a7e3f80628ca0a6c4c9f54fd2dd6 (56/327) (65 seconds passed, remaining 314 predicted)
...
Rewrite c20df82da66da6521f355af508bfedc047cffa64 (326/326) (1183 seconds passed, remaining 0 predicted)
Ref 'refs/heads/staging' was rewritten

This command will populate our empty commit with reformatting changes and rebase the rest of commits against it without manual intervention.

Rebase past the formatting as usual:
```
$ git rebase -i
```
Here git rebase -i will tell you that the first commit became empty. You can either skip or commit an empty one. I skipped it with git rebase --skip.

Done!

Once I executed the above I got just one trivial conflict unrelated to reformatting.

parting words

git filter-branch --tree-filter is a great tool to mangle the repository! But before using it make sure you back you local tree: it’s very easy to get it to “destroy” all your work (git reflog will still be able to save your past commits).

It took git filter-branch --tree-filter about 5 minutes to rebase 326 commits that touch ~200 files. My understanding is that most time is spent on nixfmt utility itself and not on git operations. nixfmt is not very fast: it takes about a minute to reformat the whole of nixpkgs (~300MB of .nix files).

nixpkgs plans for reformat event more sources in future. I will likely be using this tip a few more times.

Have fun!