Julia Evans

How git cherry-pick and revert use 3-way merge

Hello! I was trying to explain to someone how git cherry-pick works the other day, and I found myself getting confused.

What went wrong was: I thought that git cherry-pick was basically applying a patch, but when I tried to actually do it that way, it didn’t work!

Let’s talk about what I thought cherry-pick did (applying a patch), why that’s not quite true, and what it actually does instead (a “3-way merge”).

This post is extremely in the weeds and you definitely don’t need to understand this stuff to use git effectively. But if you (like me) are curious about git’s internals, let’s talk about it!

cherry-pick isn’t applying a patch

The way I previously understood git cherry-pick COMMIT_ID is:

  • calculate the diff for COMMIT_ID, like git show COMMIT_ID --patch > out.patch
  • Apply the patch to the current branch, like git apply out.patch

Before we get into this – I want to be clear that this model is mostly right, and if that’s your mental model that’s fine. But it’s wrong in some subtle ways and I think that’s kind of interesting, so let’s see how it works.

If I try to do the “calculate the diff and apply the patch” thing in a case where there’s a merge conflict, here’s what happens:

$ git show 10e96e46 --patch > out.patch
$ git apply out.patch
error: patch failed: content/post/2023-07-28-why-is-dns-still-hard-to-learn-.markdown:17
error: content/post/2023-07-28-why-is-dns-still-hard-to-learn-.markdown: patch does not apply

This just fails – it doesn’t give me any way to resolve the conflict or figure out how to solve the problem.

This is quite different from what actually happens when run git cherry-pick, which is that I get a merge conflict:

$ git cherry-pick 10e96e46
error: could not apply 10e96e46... wip
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".

So it seems like the “git is applying a patch” model isn’t quite right. But the error message literally does say “could not apply 10e96e46”, so it’s not quite wrong either. What’s going on?

so what is cherry-pick doing?

I went digging through git’s source code to see how cherry-pick works, and ended up at this line of code:

res = do_recursive_merge(r, base, next, base_label, next_label, &head, &msgbuf, opts);

So a cherry-pick is a… merge? What? How? What is it even merging? And how does merging even work in the first place?

I realized that I didn’t really know how git’s merge worked, so I googled it and found out that git does a thing called “3-way merge”. What’s that?

how git merges files: the 3-way merge

Let’s say I want to merge these 2 files. We’ll call them v1.py and v2.py.

def greet():
    greeting = "hello"
    name = "julia"
    return greeting + " " + name
def say_hello():
    greeting = "hello"
    name = "aanya"
    return greeting + " " + name

There are two lines that differ: we have

  • def greet() and def say_hello
  • name = "aanya" and name = "julia"

How do we know what to pick? It seems impossible!

But what if I told you that the original function was this (base.py)?

def say_hello():
    greeting = "hello"
    name = "julia"
    return greeting + " " + name

Suddenly it seems a lot clearer! v1 changed the function’s name to greet and v2 set name = "aanya". So to merge, we should make both those changes:

def greet():
    greeting = "hello"
    name = "aanya"
    return greeting + " " + name

We can ask git to do this merge with git merge-file, and it gives us exactly the result we expected: it picks def greet() and name = "aanya".

$ git merge-file v1.py base.py v2.py -p
def greet():
    greeting = "hello"
    name = "aanya"
    return greeting + " " + name⏎

This way of merging where you merge 2 files + their original version is called a 3-way merge.

If you want to try it out yourself in a browser, I made a little playground at jvns.ca/3-way-merge/. I made it very quickly so it’s not mobile friendly.

git merges changes, not files

The way I think about the 3-way merge is – git merges changes, not files. We have an original file and 2 possible changes to it, and git tries to combine both of those changes in a reasonable way. Sometimes it can’t (for example if both changes change the same line), and then you get a merge conflict.

Git can also merge more than 2 possible changes: you can have an original file and 8 possible changes, and it can try to reconcile all of them. That’s called an octopus merge but I don’t know much more than that, I’ve never done one.

how git uses 3-way merge to apply a patch

Now let’s get a little weird! When we talk about git “applying a patch” (as you do in a rebase or revert or cherry-pick), it’s not actually creating a patch file and applying it. Instead, it’s doing a 3-way merge.

Here’s how applying commit X as a patch to your current commit corresponds to this v1, v2, and base setup from before:

  1. The version of the file in your current commit is v1.
  2. The version of the file before commit X is base
  3. The version of the file in commit X. Call that v2
  4. Run git merge-file v1 base v2 to combine them (technically git does not actually run git merge-file, it runs a C function that does it)

Together, you can think of base and v2 as being the “patch”: the diff between them is the change that you want to apply to v1.

how cherry-pick works

Let’s say we have this commit graph, and we want to cherry-pick Y on to main:

A - B (main)
   X - Y - Z

How do we turn that into a 3-way merge? Here’s how it translates into our v1, v2 and base from earlier:

  • B is v1
  • X is the base, Y is v2

So together X and Y are the “patch”.

And git rebase is just like git cherry-pick, but repeated a bunch of times.

how revert works

Now let’s say we want to run git revert Y on this commit graph

X - Y - Z - A - B
  • B is v1
  • Y is the base, X is v2

This is exactly like a cherry-pick, but with X and Y reversed. We have to flip them because we want to apply a “reverse patch”.

Revert and cherry-pick are so closely related in git that they’re actually implemented in the same file: revert.c.

this “3-way patch” is a really cool trick

This trick of using a 3-way merge to apply a commit as a patch seems really clever and cool and I’m surprised that I’d never heard of it before! I don’t know of a name for it, but I kind of want to call it a “3-way patch”.

The idea is that with a 3-way patch, you specify the patch as 2 files: the file before the patch and after (base and v2 in our language in this post).

So there are 3 files involved: 1 for the original and 2 for the patch.

The point is that the 3-way patch is a much better way to patch than a normal patch, because you have a lot more context for merging when you have both full files.

Here’s more or less what a normal patch for our example looks like:

@@ -1,1 +1,1 @@:
- def greet():
+ def say_hello():
    greeting = "hello"

and a 3-way patch. This “3-way patch” is not a real file format, it’s just something I made up.

BEFORE: (the full file)
def greet():
    greeting = "hello"
    name = "julia"
    return greeting + " " + name
AFTER: (the full file)
def say_hello():
    greeting = "hello"
    name = "julia"
    return greeting + " " + name

“Building Git” talks about this

The book Building Git by James Coglan is the only place I could find other than the git source code explaining how git cherry-pick actually uses 3-way merge under the hood (I thought Pro Git might talk about it, but it didn’t seem to as far as I could tell).

I actually went to buy it and it turned out that I’d already bought it in 2019 so it was a good reference to have here :)

merging is actually much more complicated than this

There’s more to merging in git than the 3-way merge – there’s something called a “recursive merge” that I don’t understand, and there are a bunch of details about how to deal with handling file deletions and moves, and there are also multiple merge algorithms.

My best idea for where to learn more about this stuff is Building Git, though I haven’t read the whole thing.

so what does git apply do?

I also went looking through git’s source to find out what git apply does, and it seems to (unsurprisingly) be in apply.c. That code parses a patch file, and then hunts through the target file to figure out where to apply it. The core logic seems to be around here: I think the idea is to start at the line number that the patch suggested and then hunt forwards and backwards from there to try to find it:

	 * There's probably some smart way to do this, but I'll leave
	 * that to the smart and beautiful people. I'm simple and stupid.
	backwards = current;
	backwards_lno = line;
	forwards = current;
	forwards_lno = line;
	current_lno = line;
  for (i = 0; ; i++) {

That all seems pretty intuitive and about what I’d naively expect.

how git apply --3way works

git apply also has a --3way flag that does a 3-way merge. So we actually could have more or less implemented git cherry-pick with git apply like this:

$ git show 10e96e46 --patch > out.patch
$ git apply out.patch --3way
Applied patch to 'content/post/2023-07-28-why-is-dns-still-hard-to-learn-.markdown' with conflicts.
U content/post/2023-07-28-why-is-dns-still-hard-to-learn-.markdown

--3way doesn’t just use the contents of the patch file though! The patch file starts with:

index d63ade04..65778fc0 100644

d63ade04 and 65778fc0 are the IDs of the old/new versions of that file in git’s object database, so git can retrieve them to do a 3-way patch application. This won’t work if someone emails you a patch and you don’t have the files for the new/old versions of the file though: if you’re missing the blobs you’ll get this error:

$ git apply out.patch
error: repository lacks the necessary blob to perform 3-way merge.

3-way merge is old

A couple of people pointed out that 3-way merge is much older than git, it’s from the late 70s or something. Here’s a paper from 2007 talking about it

that’s all!

I was pretty surprised to learn that I didn’t actually understand the core way that git applies patches internally – it was really cool to learn about!

I have lots of issues with git’s UI but I think this particular thing is not one of them. The 3-way merge seems like a nice unified way to solve a bunch of different problems, it’s pretty intuitive for people (the idea of “applying a patch” is one that a lot of programmers are used to thinking about, and the fact that it’s implemented as a 3-way merge under the hood is an implementation detail that nobody actually ever needs to think about).

Also a very quick plug: I’m working on writing a zine about git, if you’re interested in getting an email when it comes out you can sign up to my very infrequent announcements mailing list.

git rebase: what can go wrong? Some notes on nix flakes