Lost in Technopolis

by John Wiegley

The saga of rebase versus merge

Posted by John Wiegley on February 26, 2009 with labels: immutable, merge, rebase

Following on my last entry, where I built a better pre-commit hook to ensure every commit in my Git history passed make check with flying colors, I realized something today about my good friend, rebase.

git rebase is a brilliant tool for editing history you haven’t pushed yet. If I have a set of ten commits, and realize the 3rd commit has an oversight I’d like to smooth out, I can make a new commit to fix the problem and then merge it with that old commit, resulting in a nice, clean set of commits for pushing.

However, using rebase at any time invalidates the work of my pre-commit hook. Why? Because any time I use rebase, I throw away whatever I’ve confirmed in the past about those commits. A rewritten commit is really a new commit, which means it hasn’t been tested, may never have existed as a working tree, and certainly isn’t the same as the previous commit, however similar its diff output may be.

What this goes to show is that immutability is a requirement of sane integration. Not only does code go into a commit, plus a date and a description, but also the work that went into verifying that commit. All of these details are bound up in the immutable state of that commit. If the state is change, and the immutability guarantee broken, all bets are off.

Thus the only way I could use rebase in confidence would be to run the same pre-commit again on every changed commit during the rebase operation – which is something Git doesn’t do. It thinks that if you rewrite a series of commits, the final HEAD will have the same contents as the previous HEAD, which is true. But the rebased commits leading up to that head, especially if their order was changed, now represent a fictitious history behind that new HEAD.

It makes me think more and more about the virtues of merging.


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