jj/docs/conflicts.md

# First-class conflicts


## Introduction

Conflicts happen when Jujutsu can't figure out how to merge different changes
made to the same file. For instance, this can happen if two people are working
on the same file and make different changes to the same part of the file, and
then their commits are merged together with `jj new` (or one is rebased onto the
other with `jj rebase`).

Unlike most other VCSs, Jujutsu can record conflicted states in commits. For
example, if you rebase a commit and it results in a conflict, the conflict will
be recorded in the rebased commit and the rebase operation will succeed. You can
then resolve the conflict whenever you want. Conflicted states can be further
rebased, merged, or backed out. Note that what's stored in the commit is a
logical representation of the conflict, not conflict *markers*; rebasing a
conflict doesn't result in a nested conflict markers (see
[technical doc](technical/conflicts.md) for how this works).


## Advantages

The deeper understanding of conflicts has many advantages:

* Removes the need for things like
  `git rebase/merge/cherry-pick/etc --continue`. Instead, you get a single
  workflow for resolving conflicts: check out the conflicted commit, resolve
  conflicts, and amend.
* Enables the "auto-rebase" feature, where descendants of rewritten commits
  automatically get rewritten. This feature mostly replaces Mercurial's
  [Changeset Evolution](https://www.mercurial-scm.org/wiki/ChangesetEvolution).
* Lets us define the change in a merge commit as being compared to the merged
  parents. That way, we can rebase merge commits correctly (unlike both Git and
  Mercurial). That includes conflict resolutions done in the merge commit,
  addressing a common use case for
  [git rerere](https://git-scm.com/docs/git-rerere).
  Since the changes in a merge commit are displayed and rebased as expected,
  [evil merges](https://git-scm.com/docs/gitglossary/2.22.0#Documentation/gitglossary.txt-aiddefevilmergeaevilmerge)
  are arguably not as evil anymore.
* Allows you to postpone conflict resolution until you're ready for it. You
  can easily keep all your work-in-progress commits rebased onto upstream's head
  if you like.
* [Criss-cross merges](https://stackoverflow.com/questions/26370185/how-do-criss-cross-merges-arise-in-git)
  and [octopus merges](https://git-scm.com/docs/git-merge#Documentation/git-merge.txt-octopus)
  become trivial (implementation-wise); some cases that Git can't currently
  handle, or that would result in nested conflict markers, can be automatically
  resolved.
* Enables collaborative conflict resolution. (This assumes that you can share
  the conflicts with others, which you probably shouldn't do if some people
  interact with your project using Git.)

For information about how conflicts are handled in the working copy, see
[here](working-copy.md#conflicts).


## Conflict markers

Conflicts are "materialized" using *conflict markers* in various contexts. For
example, when you run `jj new` or `jj edit` on a commit with a conflict, it will
be materialized in the working copy. Conflicts are also materialized when they
are part of diff output (e.g. `jj show` on a commit that introduces or resolves
a conflict).

As an example, imagine that you have a file which contains the following text,
all in lowercase:

```
apple
grape
orange
```

One person replaces the word "grape" with "grapefruit" in commit A, while
another person changes every line to uppercase in commit B. If you merge the
changes together with `jj new A B`, the resulting commit will have a conflict
since Jujutsu can't figure out how to combine these changes. Therefore, Jujutsu
will materialize the conflict in the working copy using conflict markers, which
would look like this:

```
<<<<<<< Conflict 1 of 1
%%%%%%% Changes from base to side #1
 apple
-grape
+grapefruit
 orange
+++++++ Contents of side #2
APPLE
GRAPE
ORANGE
>>>>>>> Conflict 1 of 1 ends
```

The markers `<<<<<<<` and `>>>>>>>` indicate the start and end of a conflict
respectively. The marker `+++++++` indicates the start of a snapshot, while the
marker `%%%%%%%` indicates the start of a diff to apply to the snapshot.
Therefore, to resolve this conflict, you would apply the diff (changing "grape"
to "grapefruit") to the snapshot (the side with every line in uppercase),
editing the file to look like this:

```
APPLE
GRAPEFRUIT
ORANGE
```

In practice, conflicts are usually 2-sided, meaning that there's only 2
conflicting changes being merged together at a time, but Jujutsu supports
conflicts with arbitrarily many sides, which can happen when merging 3 or more
commits at once. In that case, you would see a single snapshot section and
multiple diff sections.

Compared to just showing the content of each side of the conflict, the main
benefit of Jujutsu's style of conflict markers is that you don't need to spend
time manually comparing the sides to spot the differences between them. This is
especially beneficial for many-sided conflicts, since resolving them just
requires applying each diff to the snapshot one-by-one.

## Alternative conflict marker styles

If you prefer to just see the contents of each side of the conflict without the
diff, Jujutsu also supports a "snapshot" style, which can be enabled by setting
the `ui.conflict-marker-style` config option to "snapshot":

```
<<<<<<< Conflict 1 of 1
+++++++ Contents of side #1
apple
grapefruit
orange
------- Contents of base
apple
grape
orange
+++++++ Contents of side #2
APPLE
GRAPE
ORANGE
>>>>>>> Conflict 1 of 1 ends
```

Some tools expect Git-style conflict markers, so Jujutsu also supports [Git's
"diff3" style](https://git-scm.com/docs/git-merge#_how_conflicts_are_presented)
conflict markers by setting the `ui.conflict-marker-style` config option to
"git":

```
<<<<<<< Side #1 (Conflict 1 of 1)
apple
grapefruit
orange
||||||| Base
apple
grape
orange
=======
APPLE
GRAPE
ORANGE
>>>>>>> Side #2 (Conflict 1 of 1 ends)
```

This conflict marker style only supports 2-sided conflicts though, so it falls
back to the similar "snapshot" conflict markers if there are more than 2 sides
to the conflict.