git magic

The following, mostly accurate conversation (apologies for any liberties) should be a parable for the use of the git version control system: As I was about to leave work the other day...

tycho: I pushed today's work to our repository, have at, I'm headed out.

Coworker A: Awesome. I did too. (pause) wait. It's screwed up. I deleted a file I didn't mean to. (pastes link to diff into chatroom)

tycho: Oh, that's easy to fix. You can reset back to before the file, add all the changes that are in you're repository, except the deletion of the file, commit, and then "git reset --hard" and then publish that.

Coworker A: But your changes...

(as an aside, the original solution should still work, I think)

tycho: Oh. Hrm. Right. Well... Rebase to remove the bad commit and then add the file in question back on top of my changes.

Coworker A: Wait, what?

tycho: (looks at clook). Shit, I'll do it. (turns to Coworker P), have you pulled recently?

Coworker P: Nope I'll do that no--

tycho: Dont't!

Coworker P: Alright then!

tycho: (mumbles and works)


At this juncture, I pull out crazy git commands and rebase the repository, back a few commits to pull out a single changeset. And then recommit the file with the changes worth saving (which I had copied into ~/ before beginning this operation.)

One thing I've learned about using git rebase is that you always have to go back a commit or two before I think I need to, pick out the hash for the last good commit. Also when using "git rebase -i" I find that the commits are listed in the reverse order that I want them to be listed in.

Another great hint: Issue the following command if you're an emacs user and you don't want git to open rebase editing sessions in vim.

git config --global core.editor "emacsclient -t -a emacs -NOW"

The one issue here is that I had to rewrite the history of an already published series of changes. This is why I didn't want P to pull. When I was done, and the state of my repository was as it should have been, my next push (predictably failed), as it needed to be a "git push -f", which is something of a scary operation. It worked out, and when everyone pulled the next time everything was fine: I knew it would be for P because their local repository never knew about the first iteration of the history. I was less sure if A's would adjust so seamlessly, but it did.


tycho: Ok, done. Pull A.

Coworker A: All better! I have no clue what happened.

tycho: It's cool, don't sweat it. There's very little that isn't fixable. As long as you don't hard reset changes, and don't do crazy rebasing stuff, you should be ok.

Coworker A: Like what you just did?

tycho: Pretty much.


Here are the lessons:

  • "git push" and "git pull" would seem like parallel operations but they're not. Pull with abandon, it never hurts to pull. But if lots of people are pulling from the same repository, and you push a change that you don't mean to push, it's really hard to take that change back in a logical and productive way. So push with caution.
  • Rebasing is a tool that has great power shouldn't be feared even though theoretically you can screw stuff up with it. The git way says "commit your changes early and often," is great, but it can be sort of anti-social, as individual commits become sort of meaningless, and change logs can get hard to manage. Rebasing, though scary, can make it possible to both commit as often as you need to, and then rebase to be presentable.
  • Fear forced pushes.
  • Everything in git can be changed, so play with things, and then only publish changes when the repository is in a good working state.

Onward!

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