jj/docs/conflicts.md

First-class conflicts

Introduction

Like Pijul and Darcs but 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 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.
• 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. Since the changes in a merge commit are displayed and rebased as expected, evil merges 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 and octopus merges 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.

Conflict markers

Conflicts are "materialized" using conflict markers in various contexts. For example, when you run 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). Here's an example of how Git can render a conflict using its "diff3" style:

<<<<<<< left
apple
grapefruit
orange
======= base
apple
grape
orange
||||||| right
APPLE
GRAPE
ORANGE
>>>>>>>

In this example, the left side changed "grape" to "grapefruit", and the right side made all lines uppercase. To resolve the conflict, we would presumably keep the right side (the third section) and replace "GRAPE" by "GRAPEFRUIT". This way of visually finding the changes between the base and one side and then applying them to the other side is a common way of resolving conflicts when using Git's "diff3" style.

Jujutsu helps you by combining the base and one side into a unified diff for you, making it easier to spot the differences to apply to the other side. Here's how that would look for the same example as above:

<<<<<<<
%%%%%%%
 apple
-grape
+grapefruit
 orange
+++++++
APPLE
GRAPE
ORANGE
>>>>>>>

As in Git, the <<<<<<< and >>>>>>> lines mark the start and end of the conflict. The %%%%%%% line indicates the start of a diff. The +++++++ line indicates the start of a snapshot (not a diff).

There is another reason for this format (in addition to helping you spot the differences): The format supports more complex conflicts involving more than 3 inputs. Such conflicts can arise when you merge more than 2 commits. They would typically be rendered as a single snapshot (as above) but with more than one unified diffs. The process for resolving them is similar: Manually apply each diff onto the snapshot.