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Sun, 04 Mar 2012

Why can't Git resolve all conflicted merges?
I like to be prepared ahead of time for questions, and one such question is why Git can't resolve all merge conflicts automatically. People do show up on IRC asking this from time to time. If you're a sophisticated user the answer is obvious, but I've made a pretty good living teaching classes to people who don't find such things obvious.

What we need is a nice example. In the past my example was sort of silly. You have a file that contains the instruction:

           Pay potato tax every April 15

One branch adds an exception:

           Pay potato tax every April 15
             (Except in years of potato blight.)

While another branch broadens the original instruction:

           Pay all tax due every April 15

What's the correct resolution here? It's easy to understand that mashing together the two changes is a recipe for potential catastrophe:

           Pay all tax due every April 15
             (Except in years of potato blight.)

You get fined for tax evasion after the next potato blight. And it's similarly easy to construct scenarios in which the correct resolution is to leave the whole thing in place including the modifier, change the thing to something else completely, delete the whole thing, or to refer the matter to Legal and shut down the whole system until you hear back. Clearly it's outside Git's scope to recognize when to call in the lawyers, much less to predict what their answer will be.

But a few months ago I ran into a somewhat less silly example. At work we had two seprate projects, "Moonpig" and "Stick", each in its own repository. Moonpig contained a subsystem, "Collections", which we decided would make more sense as part of Stick. I did this work, removing the Collections code from the Moonpig project and integrating it into the Stick project. From the point of view of the Moonpig repository, the Collections system was deleted entirely.

Meanwhile, on a parallel branch of Moonpig, R.J.B. Signes made some changes that included bug fixes to the Collections. After I removed the collections, he tried to merge his changes into the master branch, and got a merge conflict, because some of the files to which he was making bug fixes were no longer there.

The correct resolution was to perform the rest of the merge without the bug fixes, which Git could conceivably have done. But then the unapplied bug fixes needed to be applied to the Collections module that was now in the completely separate Stick project, and there is no way Git could have done this, or even to have known it should be done. Human intervention was the only answer.

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