Git: Cherry-picking a PR/merge request

February 2, 2018 -
Tags: git, github, gitlab

GitLab offers a functionality for cherry picking a merge request (PR).

This functionality doesn’t exist in GitHub, and in Git, either; it is useful in some cases.

In this article I’ll explain some git fundamentals, and in the last section, how to cherry pick a PR/merge request.

Contents:

Disclaimer

Git offers many workflow, and there is disagreement about what is considered optimal.

The workflow(s) presented here are a sensitive subject for some. This post doesn’t make any judgment: all the ideas are exposed purely for informational purposes - to be used as option, for those who deem them appropriate, or discarded, by those who don’t.

What is a git merge

In git, a merge is the act of joining together two branches, so that both of their histories will become part of the new, joined, branch.

This is an example:

  ╭──C─────╮     topic
  │        │
──*──A──B──*──>  master

In this case, after the merge, master will include all the commits: B, B and C.

There are two notable commits in the history above, both displayed as *:

  • the merge base (on the left): the most recent common ancestor [commit] of the two branches
  • the merge commit (on the right): the commit that joins the two branches together

Rebasing onto a new base

Suppose two developers are working on a branch:

──A────────>   master
  │
  ╰──B──C──>   dev1   (origin)
     │
     ╰──D──>   dev2   (local)

Developer 1 performs a force push (!) and changes the commit B to B'. This will be the new history:

──A─────────>   master
  │
  ├──B'──C──>   dev1   (origin)
  │
  ╰──B───D──>   dev2   (local)

Now we have a problem. When developer 2 will merge (or rebase) dev1 with dev2, there will be a conflict; since B is still present in dev2, he will need to discard the changes of B, and retain the changes of B'.

In order to perform a conflict-free rebase, developer 2 needs to apply a different strategy, around the lines of “I’m only interested in my commit - D - just apply it on top of the new origin branch”.

What he wants is ultimately this:

──A────────────>   master
  │
  ╰──B'──C─────>   dev1   (origin)
         │
         ╰──D──>   dev2   (local)

In git, this strategy is executed via rebase --onto; the format is:

$ git rebase --onto <new_base> <new_branch_parent>

Going back to the pre-rebase history:

──A─────────>   master
  │
  ├──B'──C──>   dev1   (origin)
  │
  ╰──B───D──>   dev2   (local)

the references are:

  • new_base: C
  • new_branch_parent: B, otherwise referenceable as D~ (which means “parent of D”)

so the command is

$ git rebase --onto C B

or, using a relative reference:

$ git rebase --onto C D~

This will yield the desired history, without conflicts:

──A─────────────>   master
  │
  ╰──B'──C──────>   dev1   (origin)
         │
         ╰──D'──>   dev2   (local)

Note how D' is different from D. The change applied will be the same, but the commit as a whole is now different, since it’s parent of C, not parent of B anymore.

Finding the merge base

A functionality very common during rebases of any type is finding the merge base.

Suppose we have a feature branch:

──?──B──────>   master
  │
  ╰──C───D──>   feature

we want to know what’s the commit represented by ?, which is the most recent common ancestor [commit] of the two branches.

The git command format is merge-base branch1 branch2; in this case:

git merge-base B D

will return A.

Keep this in mind for later.

Cherry-picking a PR/merge request

What exactly do we mean with “cherry picking a PR/merge request”, and why we would want that?

Let’s see a case where we want to do this.

Suppose we have a master/release source control structure:

  • master represents the stable history of a project development
  • release represents the history of the releases

On our imaginary point in time, master is ahead of release, and needs to wait a certain day for being merged into release, and released.

A feature branch is branched from master, and it’s developed and completed, and a GitHub PR is created. For external reasons, it becomes high priority, and needs to be released urgently, before the release date. This is a representative history:

──A──B──C────────>   master
  │     │
  │     ╰──D──E      feature (origin, PR)
  │
  ╰──────────────>   release

Ultimately, we want A, D, and E to be in the release branch.

Let’s do the wrong thing, and merge, via command line, feature into release:

──A──B──C──────────>   master
  │     │
  │     ╰──D──E        feature (origin, PR)
  │            ╲
  ╰─────────────*──>   release

Yikes!! The release branch will now contain all the commits, including B and C.

This shows clearly what we want to do, and what cherry picking a merge is.

In informal terms, we want to “disconnect” a branch from mainline, and “attach” it on top of another, without carrying its previous history, in this case, feature on top of release.

More technically, we:

  • find the merge base between feature and master (C)
  • rebase feature onto release, starting from the above merge base

Lo and behold, this is our git command (with minor shell trickery), to be run from feature:

$ git rebase --onto release $(git merge-base master feature)

Note that:

  • $() is shell syntax for running a command in a subshell, and replacing the construct with the result
  • we can also use HEAD in place of feature, since we’re assuming to be running from feature

this is the new history:

──A──B───C───>   master
  │
  ├──D'──E'      feature (local)
  │
  ╰──────────>   release

Note how, as explained before, D and E turned into D' and E'.

Now we can safely merge (via command line):

──A──B───C────>   master
  │
  ├──D'──E'       feature (local)
  │       ╲
  ╰────────*──>   release

and release will contain A, D' and E' only!

The release branch can now be pushed to the origin, and deployed.

Merging the PR strategies

Now, there are a couple of things to close. How do we handle the fact that the local feature is now mismatching with the origin? And how do we handle master?

There are two strategies, depending on the git workflow used in the team

Maintaining the PR exactly as it is

In this case, we delete the local feature branch, and merge the PR via GitHub (and delete feature on origin); this will be the history:

──A──B──C────────>   master
  │     │
  │     ╰──D──E      feature (was origin, PR)
  │
  ├──D'──E'          feature (was local)
  │       ╲
  ╰────────*─────>   release

A few days later, master has a new commit:

──A──B──C────F──>   master

On release day, we merge, yielding the final history:

──A──B──C────F──*─────>   master
  │     │      ╱ ╲
  │     ╰──D──E   ╲       feature (was origin, PR)
  │                ⧹
  ├──D'──E'        │      feature (was local)
  │       ╲        │
  ╰────────*───────*──>   release

Inevitably, we’ll have both pairs D/E and D'/E' in release. Practically, this won’t be problem, since git recognizes that the commits have already been applied to release.

Changing the PR branch history

Alternatively, we force push the local feature branch, yielding:

──A──B───C────>   master
  │
  ├──D'──E'       feature (local and origin, PR)
  │       ╲
  ╰────────*──>   release

We merge the PR via GitHub (and delete the feature branch):

──A──B──C──*──>   master
  │       ╱
  ├──D'──E'       feature (local and origin, PR)
  │       ╲
  ╰────────*──>   release

Then a new commit, F, is added:

──A──B──C──*──F──>   master

And on release day, master is merged into release:

──A──B──C──*──F────>   master
  │       ╱    ╲
  ├──D'──E'     ⧹      feature (local and origin, PR)
  │       ╲     │
  ╰────────*────*──>   release

no more duplication, at the cost of having applied a force-push.

Conclusion

Git is cool, but handle with care.

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