Rebase and move tags

Warning

This is generally a bad idea, and I only did this out of ignorance of best practices for maintaining a pre-commit mirror. Was this the right thing to do? I don’t know! In the Conda Forge ecosystem I would never do this.

If you know, please share. I have a comments section now!

I created a pre-commit mirror to enable Alex.js to be used with pre-commit. Everything went smoothly until I needed to make changes to the pre-commit hook metadata; for example, a correction to the description field that applies to every version.

Background

Alex.js is an inclusive language checker. It scans your prose to detect and suggest alternatives to uninclusive language, for example profanity or condescending language. pre-commit is an tool for automating other tools to run before making a Git commit, so you can detect issues before they make it to your repository.

There are some pre-commit checks, like check-added-large-files that I recommend for every repository!

To create my pre-commit mirror for Alex.js, I used pre-commit-mirror-maker to generate a repository with one commit (plus tag) for each version of Alex.js. I only needed to do a couple of steps, and everything worked great! 🎉

The rebase

I can’t stress enough that I don’t know what I’m doing and I may have caused pain for users by doing this! I’m sharing this because I learned something new.

Another Alex.js user generously contributed to this mirror repository by adding a description I had neglected to populate to the pre-commit metadata. ❤️ Thanks! Neither of us had experience maintaining this type of repository, and it’s a new repository with likely a small number of users, so we decided to try applying this metadata change to every commit. The only problem was moving the tags.

git rebase, probably for good reason, doesn’t have a --tags flag you can pass to tell it to move tags while it’s re-writing history. Thanks to Git’s amazing hook system, implementing this functionality by myself was not as hard as I expected!

Important

Git’s hook system is not to be confused with pre-commit. pre-comit is a tool built on top of Git hooks. It adds the ability to share project-level Git hooks and automates the installation of those hooks and their respective software environments.

I implemented a post-rewrite hook. This type of hook will run after git rebase and git commit --amend, which create new commits from old commits. Git will pass the rewrites as old-new commit pairs, e.g.:

aaf4c61ddcc5e8a2dabede0f3b482cd9aea9434d a3457012afa04a5d109ba19650d1ebb909dc9a94
2ced3ee86f82bf91c15cc30605df6d3ddf0769ff 0beec7b5ea3f0fdbc95d0dd47f3c5bc275da8a33

I wrote my hook code in Python:

${MY_REPO}/.git/hooks/post-rewrite

#!/usr/bin/env python
"""Move tags from old commit to new commit on every re-written commit.

WARNING: This is generally not a good idea.

Full: https://gist.github.com/mfisher87/094ddc4619ba0f4c428ea66236212ce6
"""
import subprocess
import sys


def git(*args: list[str]) -> str:
    """Run a git command with `args`."""
    args = ['git', *args]
    subp = subprocess.run(args, check=True, capture_output=True)
    return subp.stdout.decode('utf-8').strip()


def retag(old_commit: str, new_commit: str) -> None:
    """For each old/new commit pair, move tags from old commit to new."""
3    tags = git('tag', '--points-at', old_commit).split('\n')
    if tags == ['']:
        tags = []

4    for tag in tags:
        git('tag', '--force', tag, new_commit)


def main(lines) -> None:
1    rewrites: list[tuple[str, str]] = []
    for line in lines:
        old, new = line.strip().split(' ')
        rewrites.append((old, new))

2    for old, new in rewrites:
        retag(old, new)


if __name__ == "__main__":
    main(sys.stdin)

1

Convert the text Git is passing into an easier-to-work-with list of 2-tuples (old, new).

2

Perform a retag operation for each pair of old and new commit identifiers.

3

git tag --points-at {old_commit_id} will yield all tags on the old commit, separated by newlines. We .split("\n") to once again make the data easier to work with.

4

For each tag on the old commit, we move it to the new commit with git tag --force {tag} {new_commit}

🎉 We’ve committed a horrible sin!