Wednesday, May 29, 2013

Annotating Git History

As is well known, git history is sacred. There are good reasons for it, but it does make it difficult sometimes to correct errors and integrate git into larger processes.

For example, it is very common to include references to issue tracking systems in git commit comments. It would be nice if one could mine this information from git in order to update the issues automatically, for example at build time or at deploy time.

Unfortunately, we're all human and make mistakes. Even if you have validation hooks and do the best possible effort, invalid references or forgotten references make it difficult to rely on the commit comments.

It would be nice if it was easier to fix old git commit comments, but history being immutable is one of the core concepts baked into git, and that's a good thing. So I went for a different solution.

How about attaching annotated git tags to commits? That would preserve the history while adding annotations with newer information. The only trouble with this is that the current git commands aren't very good at revealing this information to casual git users.

The convention I currently use is quite simple: attach a tag whose name begins with CANCEL to the commit to be updated.

Let's say we wish to correct this commit:
% git log -1 5a434a
commit 5a434ac808d9a5c10437b45b06693cf03227f6b3
Author: Christian Goetze <cgoetze@miaow.com>
Date:   Wed May 29 12:57:57 2013 -0700

    TECHOPS-218 Render superceded messages.
Attach the tag, like this:
% git tag -m "new commit message" CANCEL_5a434a 5a434a
Now, the build and deploy scripts can mine this information. They start by listing all the tags attached to the commit:
% git show-ref --tags -d \
     | grep '^'5a434a \
     | sed -e 's,.* refs/tags/,,' -e 's/\^{}//'
Then, for every tag, we check whether it's an annotated tag:
% git describe --exact-match CANCEL_5a434a
CANCEL_5a434a
Note that this command fails if it's a "lightweight" tag. Once you know it's an annotated tag, you can run:
% git cat-file tag CANCEL_5a434a
object 5a434ac808d9a5c10437b45b06693cf03227f6b3
type commit
tag CANCEL_5a434a
tagger Christian Goetze <cgoetze@miaow.com> 1369865583

new commit message
This is easy to parse, and can be used as a replacement for the commit comment.

And what if there is another error in the annotation? Simply add another tag with the correction. The number in red above is the unix timestamp expressed as seconds since 1970, so you can sort the annotations by time and just take the latest.

By the way, if anyone knows a nicer way to show all tags attached to a specific commit, please let me know.

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