git-filter-branch-msgs

Update mentions of hashes in commits messages — for git-filter-branch. In Scala.

Goal

Many commit messages mention other commits by hash. However, when using git-filter-branch, all hashes are changed completely, so such references are broken. git-filter-branch-msgs updates these references to point to the rewritten commits.

Request for comments

Do you think this could be useful? If so, let me know, open issues when something does not work. There are a few known limitations, and I'll be happy to work on them (or accept contributions) if there's interest.

Status

This is an early beta, written in a day and a half. It worked well for me on two splits in one project, but that's the testing it got.

Moreover, git-filter-branch is a dangerous beast anyway, so you're supposed to know what you're doing.

Limitations/bugs

• No command-line options for tuning settings.
• Recognizing commit IDs requires heuristics, and they cannot be perfect. Right now, instances of [0-9a-f]* are rewritten if:
  • They're at least 5 chars long (this should be configurable).
  • They are valid commit IDs, as identified by git itself.
  • git-filter-branch already rewrote them to a commit with the same title.
• No system testing whatsoever. Testing this requires quite some automation and I'm not sure how to best test it.
• There's no "real" deployment yet.
• The tool uses git command-line tools instead of any git binding (in particular, JGit). But to use this tool, you need an installation of command-line git tools, so I am not sure switching to JGit would be an improvement (there might be a performance improvement, but I'm not sure it's high enough).

Installation/usage instruction

I think what's below should list all the steps needed, but you should not do them blindly; details and caveats are probably missing.

1. Install nailgun 0.9.1. You'll just need the C client, not the Java part. Precompiled binaries can be stolen from zinc, in particular for Windows.

2. Allow the client being invoked simply as git-filter-branch-msgs:

   $ ln -s $(which ng) ~/bin/git-filter-branch-msgs

   If you do not do that (for instance because it's inconvenient, or you are on Windows), you'll need to invoke the client as ng git-filter-branch-msgs instead.

3. Run sbt stage in this directory to compile this program and generate launcher scripts for it. This requires SBT 0.13 or later; all other dependencies will be automatically downloaded and cached.

4. Run the server with ./target/universal/stage/bin/git-filter-branch-msgs

5. Now you can use it with git-filter-branch. The intended usage pattern is through

git-filter-branch
  --msg-filter 'git-filter-branch-msgs' $your_other_filters_here \
  -- --date-order $other_git_rev_list_params

However, it turned out that currently --date-order must be added directly inside `git-filter-branch`. We provide a fixed version of git-filter-branch, incorporating this option directly, in`scripts/git-filter-branch`.

6. After running git-filter-branch, you can kill the server with Ctrl-C.
7. The server will create a log, named echo-translate.log, where it is running. Check it for anything amiss.

It should be possible in theory to use this program without nailgun, but that will start a JVM for each commit, so it is not recommended for any non-trivial project. Plus, I don't provide complete docs for it (you need to change mainClass in build.sbt to filterbranch.EchoTranslate and redo sbt stage, then you'll need git to use the produced script). I could do it if there's interest.