The Uncherrypick Identity Crisis

Page last updated 2 June 2026

This was a scenario presented to me at the Mercurial Sprint days that poses a subtle problem to VCSes like JJ and GitButler that choose to constantly replay history.

We start with a small commit graph that looks like:

* B
| * C
|/
* A

Let’s imagine they have the file contents as follows:

A: a
B: ab
C: abc

We can imagine this scenario as having some bug fix in $B$, while $C$ contains both the same bug fix and an additional feature.

In our scenario, we then perform two operations:

• Rebase $C$ on top of $B$, to get $C'$
• Rebase $C'$ back on top of $A$, to get $C''$

After the first step, we achieve a history $A \larr B \larr C'$, where the contents of $C'$ are abc as expected.

The problem appears when we cherry-pick $C'$ back onto $A$. In that operation, the merge base is still $B$: one side removes b to get back to $A$, while the other side adds only c to get from $B$ to $C'$. The three-way merge combines those changes and produces ac, not abc.

That is surprising because the two rebases look like they should cancel out. We started with $C$, moved it onto $B$, then moved it back onto $A$; but the result is no longer equal to $C$.

This example shows that when performing the cherry-pick in a VCS that makes use of three-way merges, merge(l=A, base=B, r=merge(l=B, base=A, r=C)) does not always equal $C$.

If we go back to imagining that the content b was some sort of bug fix, then we have potentially introduced a regression by reverting what should have otherwise been an identity operation.

This is something that affects both Git and Mercurial, but is not so much of a concern since users rarely do this by hand. However, in a VCS like JJ or GitButler we encourage users to drag and drop changes about the place with the promise of some notion of consistency and reversibility in these operations.

As a result, there is really an onus on us to take scenarios like this seriously and to try and define better primitives that can avoid pitfalls like this.