Git compatibility - Jujutsu docs

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# Git compatibility

Jujutsu has two backends for storing commits. One of them uses a regular Git repo, which means that you can collaborate with Git users without them even knowing that you're not using the `git` CLI.

See `jj help git` for help about the `jj git` family of commands, and e.g. `jj help git push` for help about a specific command (use `jj git push -h` for briefer help).

## Supported features

The following list describes which Git features Jujutsu is compatible with. For a comparison with Git, including how workflows are different, see the Git-comparison doc.

- Configuration: Partial. The only configuration from Git (e.g. in `~/.gitconfig`) that's respected is the following. Feel free to file a bug if you miss any particular configuration options.
  - The configuration of remotes (`[remote "<name>"]`). Only the names and URLs are respected (refspecs are not respected, and only the last pushurl is respected).
  - `core.excludesFile`
- Authentication: Partial. Only `ssh-agent`, a password-less key ( only `~/.ssh/id_rsa`, `~/.ssh/id_ed25519` or `~/.ssh/id_ed25519_sk`), or a `credential.helper`.
- Branches: Yes. You can read more about how branches work in Jujutsu and how they interoperate with Git.
- Tags: Partial. You can check out tagged commits by name (pointed to be either annotated or lightweight tags), but you cannot create new tags.
- .gitignore: Yes. Patterns in `.gitignore` files are supported. So are ignores in `.git/info/exclude` or configured via Git's `core.excludesfile` config. Since working-copy files are snapshotted by every `jj` command, you might need to run `jj file untrack` to exclude newly ignored files from the working-copy commit. It's recommended to set up the ignore patterns earlier. The `.gitignore` support uses a native implementation, so please report a bug if you notice any difference compared to `git`.
- .gitattributes: No. There's #53 about adding support for at least the `eol` attribute.
- Hooks: No. There's #405 specifically for providing the checks from https://pre-commit.com.
- Merge commits: Yes. Octopus merges (i.e. with more than 2 parents) are also supported.
- Detached HEAD: Yes. Jujutsu supports anonymous branches, so this is a natural state.
- Orphan branch: Yes. Jujutsu has a virtual root commit that appears as parent of all commits Git would call "root commits".
- Staging area: Kind of. The staging area will be ignored. For example, `jj diff` will show a diff from the Git HEAD to the working copy. There are ways of fulfilling your use cases without a staging area.
- Garbage collection: Yes. It should be safe to run `git gc` in the Git repo, but it's not tested, so it's probably a good idea to make a backup of the whole workspace first. There's no garbage collection and repacking of Jujutsu's own data structures yet, however.
- Bare repositories: Yes. You can use `jj git init --git-repo=<path>` to create a repo backed by a bare Git repo.
- Submodules: No. They will not show up in the working copy, but they will not be lost either.
- Partial clones: No. We use the libgit2 library, which doesn't have support for partial clones.
- Shallow clones: Kind of. Shallow commits all have the virtual root commit as their parent. However, deepening or fully unshallowing a repository is currently not yet supported and will cause issues.
- git-worktree: No. However, there's native support for multiple working copies backed by a single repo. See the `jj workspace` family of commands.
- Sparse checkouts: No. However, there's native support for sparse checkouts. See the `jj sparse` command.
- Signed commits: Partial. So far only by configuration, later perhaps a command.
- Git LFS: No. (#80)

## Creating an empty repo

To create an empty repo using the Git backend, use `jj init --git <name>`. Since the command creates a Jujutsu repo, it will have a `.jj/` directory. The underlying Git repo will be inside of that directory (currently in `.jj/repo/store/git/`).

## Creating a repo backed by an existing Git repo

To create a Jujutsu repo backed by a Git repo you already have on disk, use `jj git init --git-repo=<path to Git repo> <name>`. The repo will work similar to a Git worktree, meaning that the working copies files and the record of the working-copy commit will be separate, but the commits will be accessible in both repos. Use `jj git import` to update the Jujutsu repo with changes made in the Git repo. Use `jj git export` to update the Git repo with changes made in the Jujutsu repo.

## Creating a repo by cloning a Git repo

To create a Jujutsu repo from a remote Git URL, use `jj git clone <URL> [<destination>]`. For example, `jj git clone https://github.com/octocat/Hello-World` will clone GitHub's "Hello-World" repo into a directory by the same name.

By default, the remote repository will be named `origin`. You can use a name of your choice by adding `--remote <remote name>` to the `jj git clone` command.

## Co-located Jujutsu/Git repos

A "co-located" Jujutsu repo is a hybrid Jujutsu/Git repo. These can be created if you initialize the Jujutsu repo in an existing Git repo by running `jj git init --colocate` or with `jj git clone --colocate`. The Git repo and the Jujutsu repo then share the same working copy. Jujutsu will import and export from and to the Git repo on every `jj` command automatically.

This mode is very convenient when tools (e.g. build tools) expect a Git repo to be present.

It is allowed to mix `jj` and `git` commands in such a repo in any order. However, it may be easier to keep track of what is going on if you mostly use read-only `git` commands and use `jj` to make changes to the repo. One reason for this (see below for more) is that `jj` commands will usually put the git repo in a "detached HEAD" state, since in `jj` there is not concept of a "currently tracked branch". Before doing mutating Git commands, you may need to tell Git what the current branch should be with a `git switch` command.

You can undo the results of mutating `git` commands using `jj undo` and `jj op restore`. Inside `jj op log`, changes by `git` will be represented as an "import git refs" operation.

There are a few downsides to this mode of operation. Generally, using co-located repos may require you to deal with more involved Jujutsu and Git concepts.

- Interleaving `jj` and `git` commands increases the chance of confusing branch conflicts or conflicted (AKA divergent) change ids. These never lose data, but can be annoying.

  Such interleaving can happen unknowingly. For example, some IDEs can cause it because they automatically run `git fetch` in the background from time to time.

- In co-located repos with a very large number of branches or other refs, `jj` commands can get noticeably slower because of the automatic `jj git import` executed on each command. This can be mitigated by occasionally running `jj util gc` to speed up the import (that command includes packing the Git refs).

- Git tools will have trouble with revisions that contain conflicted files. While `jj` renders these files with conflict markers in the working copy, they are stored in a non-human-readable fashion inside the repo. Git tools will often see this non-human-readable representation.

- When a `jj` branch is conflicted, the position of the branch in the Git repo will disagree with one or more of the conflicted positions. The state of that branch in git will be labeled as though it belongs to a remote named "git", e.g. `branch@git`.

- Jujutsu will ignore Git's staging area. It will not understand merge conflicts as Git represents them, unfinished `git rebase` states, as well as other less common states a Git repository can be in.

- Colocated repositories are less resilient to concurrency issues if you share the repo using an NFS filesystem or Dropbox. In general, such use of Jujutsu is not currently thoroughly tested.

- There may still be bugs when interleaving mutating `jj` and `git` commands, usually having to do with a branch pointer ending up in the wrong place. We are working on the known ones, and are not aware of any major ones. Please report any new ones you find, or if any of the known bugs are less minor than they appear.

### Converting a repo into a co-located repo

A Jujutsu repo backed by a Git repo has a full Git repo inside, so it is technically possible (though not officially supported) to convert it into a co-located repo like so:

    # Ignore the .jj directory in Git
    echo '/*' > .jj/.gitignore
    # Move the Git repo
    mv .jj/repo/store/git .git
    # Tell jj where to find it
    echo -n '../../../.git' > .jj/repo/store/git_target
    # Make the Git repository non-bare and set HEAD
    git config --unset core.bare
    # Convince jj to update .git/HEAD to point to the working-copy commit's parent
    jj new && jj undo

We may officially support this in the future. If you try this, we would appreciate feedback and bug reports.

## Branches

TODO: Describe how branches are mapped

## Format mapping details

Paths are assumed to be UTF-8. I have no current plans to support paths with other encodings.

Commits created by `jj` have a ref starting with `refs/jj/` to prevent GC.

Commit metadata that cannot be represented in Git commits (such as the Change ID and information about conflicts) is stored outside of the Git repo (currently in `.jj/store/extra/`).

Commits with conflicts cannot be represented in Git. They appear in the Git commit as as root directories called`.jjconflict-base-*/` and `.jjconflict-side-*/`. Note that the purpose of this representation is only to prevent GC of the relevant trees; the authoritative information is in the Git-external storage mentioned in the paragraph above. As long as you use `jj` commands to work with them, you won't notice those paths. If, on the other hand, you use e.g. `git switch` to check one of them out, you will see those directories in your working copy. If you then run e.g. `jj status`, the resulting snapshot will contain those directories, making it look like they replaced all the other paths in your repo. You will probably want to run `jj abandon` to get back to the state with the unresolved conflicts.