Merge from master overrides a revert commit

First time I encounter a Git behavior which, on first look, seems to defy common sense.

Scenario:
1. create a feature branch off master
2. perform some changes and commit (in this example – add a new line to foo.c)
3. merge the changes to master using a pull request with squash
4. on the feature branch, revert the previous commit (in this example – remove the line from foo.c)
5. merge master to the feature branch

One would expect that the reverted content will remain or that at least a manual merge conflict will occur. But surprise – the merge overrides them with the content on master (the content before the revert).

So why it happens? because how three-way merge works when there is a single common ancestor.

In the diagram, you can see that C1 is the common ancestor of C3 and C4. When doing the merge, git compares C4 to C1 and C3 to C1.
Since C1 and C4 are identical, it causes git to “understand” that the feature branch did not change the file at all, so it should take the change introduced in C3 and apply it as the merge result.

How it differs from a regular (non squashed) merge?

In this case, the common ancestor changes. It is now C2. So git compares C2 and C3, find that they are identical, hence selecting the change between C2 and C4 as the “interesting” change to apply as the merge result.

 

Conclusion: if you merge with squash, do not use the same branch to introduce additional changes…

 

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Git in Visual Studio: Prevent Fast-Forward Merge

A useful feature of git merge is the –no-ff flag. What it does is always generate a merge commit, even if it was a fast-forward (trivial) merge.

The main point of using this feature is avoid losing information about the historical existence of a certain branch, and (to some degree) being able to associate commits with the branches they were created on.

Turns out, this feature existed in Team Explorer for quite some time, but you would never guessed it if you didn’t stumble upon this User Voice entry.

By unchecking the “commit changes after merging” option, you’ll prevent a fast-forward merge and have the ability supply a custom commit message.

Why not just call it ‘prevent fast-forward merge’? 🙂