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push to and pull from a Git repository using Mercurial

Project description

Hg-Git Mercurial Plugin

Homepage:
https://hg-git.github.io/
Repository:
https://foss.heptapod.net/mercurial/hg-git
Mailing list:
hg-git@googlegroups.com (Google Group)

This is the Hg-Git plugin for Mercurial, adding the ability to push and pull to/from a Git server repository from Hg. This means you can collaborate on Git based projects from Hg, or use a Git server as a collaboration point for a team with developers using both Git and Hg.

The Hg-Git plugin can convert commits/changesets losslessly from one system to another, so you can push via a Mercurial repository and another Hg client can pull it and their changeset node ids will be identical - Mercurial data does not get lost in translation. It is intended that Hg users may wish to use this to collaborate even if no Git users are involved in the project, and it may even provide some advantages if you’re using Bookmarks (see below).

Dependencies

This plugin is implemented entirely in Python — there are no Git binary dependencies, and you do not need to have Git installed on your system. The only dependencies are Mercurial 4.4 or later and a relatively recent Dulwich.

Installing

A common way to install Mercurial extensions is from their Mercurial repository. I.e. clone this repository somewhere and make the [extensions] section in your ~/.hgrc:

[extensions]
hggit = [path-to]/hg-git/hggit

That will enable the Hg-Git extension for you.

Alternatively, you can install the plugin using your favourite package manager, e.g. pip3 install hg-git, but note that you must make sure to use the version of python that is used by mercurial. Thus, the safest way to do this with pip is:

PYTHON="$(hg debuginstall -T'{pythonexe}')"  # Point PYTHON to the version used by hg
"$PYTHON" -m pip install --user hg-git       # Drop the --user if you want a system install

With hg-git visible to mercurial, it can simply be enabled in your ~/.hgrc with:

[extensions]
hggit =

Note

hg-git is an extension and can be used with versions of mercurial already installed on your system, but, as mentioned above, needs to be installed so that mercurial can find it. The version of python mercurial uses is listed by:

hg debuginstall -T'{pythonexe}'

This is the reason for setting PYTHON above. This will work, for example, if you are using a version of mercurial installed by the system, which might depends on Python 2.7. Keep in mind that python 2 reached its end of life in April 2020 and will not be supported with versions of hg-git 0.11 and higher (see issue #349.

Perhaps better, install a more recent version of Mercurial along with hg-git in your working python environment using something like:

python3 -m pip install mercurial hg-git hg-evolve

This will also ensure that the evolve extension is installed, allowing you to use topics as outlined in the Heptapod workflow:

[extensions]
hggit =
evolve =
topics =

Contributing

The primary development location for Hg-Git is Heptapod, and you can follow their guide on how to contribute patches.

Alternatively, you can follow the guide on how to contribute to Mercurial itself, and send patches to the list.

Usage

You can clone a Git repository from Mercurial by running hg clone <url> [dest]. For example, if you were to run:

$ hg clone git://github.com/hg-git/hg-git.git

Hg-Git would clone the repository and convert it to a Mercurial repository for you. Other protocols are also supported, see hg help git for details.

If you are starting from an existing Mercurial repository, you have to set up a Git repository somewhere that you have push access to, add a path entry for it in your .hg/hgrc file, and then run hg push [name] from within your repository. For example:

$ cd hg-git # (a Mercurial repository)
$ # edit .hg/hgrc and add the target git url in the paths section
$ hg push

This will convert all your Mercurial data into Git objects and push them to the Git server.

Now that you have a Mercurial repository that can push/pull to/from a Git repository, you can fetch updates with hg pull:

$ hg pull

That will pull down any commits that have been pushed to the server in the meantime and give you a new head that you can merge in.

Hg-Git pushes your bookmarks up to the Git server as branches and will pull Git branches down and set them up as bookmarks.

Hg-Git can also be used to convert a Mercurial repository to Git. You can use a local repository or a remote repository accessed via SSH, HTTP or HTTPS. Use the following commands to convert the repository, it assumes you’re running this in $HOME:

$ mkdir git-repo; cd git-repo; git init; cd ..
$ cd hg-repo
$ hg bookmarks hg
$ hg push ../git-repo

The hg bookmark is necessary to prevent problems as otherwise hg-git pushes to the currently checked out branch, confusing Git. The snippet above will create a branch named hg in the Git repository. To get the changes in master use the following command (only necessary in the first run, later just use git merge or git rebase).

$ cd git-repo
$ git checkout -b master hg

To import new changesets into the Git repository just rerun the hg push command and then use git merge or git rebase in your Git repository.

.gitignore and .hgignore

If present, .gitignore will be taken into account provided that there is no .hgignore. In the latter case, the rules from .hgignore apply, regardless of what .gitignore prescribes.

This has been so since version 0.5.0, released in 2013.

Further reading

See hg help -e hggit.

Alternatives

Since version 5.4, Mercurial includes an extension called git. It interacts with a Git repository directly, avoiding the intermediate conversion. This has certain advantages:

  • Each commit only has one node ID, which is the Git hash.
  • Data is stored only once, so the on-disk footprint is much lower.

The extension has certain drawbacks, however:

  • It cannot handle all Git repositories. In particular, it cannot handle octopus merges, i.e. merge commits with more than two parents. If any such commit is included in the history, conversion will fail.
  • You cannot interact with Mercurial repositories.
  • Experimental status.

Another extension packaged with Mercurial, the convert extension, also has Git support.

Other alternatives exist for Git users wanting to access Mercurial repositories, such as git-remote-hg.

Configuration

git.authors

Git uses a strict convention for “author names” when representing changesets, using the form [realname] [email address]. Mercurial encourages this convention as well but is not as strict, so it’s not uncommon for a Mercurial repository to have authors listed as, for example, simple usernames. hg-git by default will attempt to translate Mercurial usernames using the following rules:

  • If the Mercurial username fits the pattern NAME <EMAIL>, the Git name will be set to NAME and the email to EMAIL.
  • If the Mercurial username looks like an email (if it contains an @), the Git name and email will both be set to that email.
  • If the Mercurial username consists of only a name, the email will be set to none@none.
  • Illegal characters (stray <s or >s) will be stripped out, and for NAME <EMAIL> usernames, any content after the right-bracket (for example, a second >) will be turned into a url-encoded sigil like ext:(%3E) in the Git author name.

Since these default behaviors may not be what you want (none@none, for example, shows up unpleasantly on GitHub as “illegal email address”), the git.authors option provides for an “authors translation file” that will be used during outgoing transfers from Mercurial to Git only, by modifying hgrc as such:

[git]
authors = authors.txt

Where authors.txt is the name of a text file containing author name translations, one per each line, using the following format:

johnny = John Smith <jsmith@foo.com>
dougie = Doug Johnson <dougiej@bar.com>

Empty lines and lines starting with a “#” are ignored.

It should be noted that this translation is in the Mercurial to Git direction only. Changesets coming from Git back to Mercurial will not translate back into Mercurial usernames, so it’s best that the same username/email combination be used on both the Mercurial and Git sides; the author file is mostly useful for translating legacy changesets.

git.blockdotgit

Blocks exporting revisions to Git that contain a directory named .git or any letter-case variation thereof. This prevents creating repositories that newer versions of Git and many Git hosting services block due to security concerns. Defaults to True.

git.blockdothg

Blocks importing revisions from Git that contain a directory named .hg. Defaults to True.

git.branch_bookmark_suffix

Hg-Git does not convert between Mercurial named branches and git branches as the two are conceptually different; instead, it uses Mercurial bookmarks to represent the concept of a Git branch. Therefore, when translating a Mercurial repository over to Git, you typically need to create bookmarks to mirror all the named branches that you’d like to see transferred over to Git. The major caveat with this is that you can’t use the same name for your bookmark as that of the named branch, and furthermore there’s no feasible way to rename a branch in Mercurial. For the use case where one would like to transfer a Mercurial repository over to Git, and maintain the same named branches as are present on the hg side, the branch_bookmark_suffix might be all that’s needed. This presents a string “suffix” that will be recognized on each bookmark name, and stripped off as the bookmark is translated to a Git branch:

[git]
branch_bookmark_suffix=_bookmark

Above, if a Mercurial repository had a named branch called release_6_maintenance, you could then link it to a bookmark called release_6_maintenance_bookmark. hg-git will then strip off the _bookmark suffix from this bookmark name, and create a Git branch called release_6_maintenance. When pulling back from Git to hg, the _bookmark suffix is then applied back, if and only if a Mercurial named branch of that name exists. E.g., when changes to the release_6_maintenance branch are checked into Git, these will be placed into the release_6_maintenance_bookmark bookmark on hg. But if a new branch called release_7_maintenance were pulled over to hg, and there was not a release_7_maintenance named branch already, the bookmark will be named release_7_maintenance with no usage of the suffix.

The branch_bookmark_suffix option is, like the authors option, intended for migrating legacy hg named branches. Going forward, a Mercurial repository that is to be linked with a Git repository should only use bookmarks for named branching.

git.findcopiesharder

Whether to consider unmodified files as copy sources. This is a very expensive operation for large projects, so use it with caution. Similar to git diff’s –find-copies-harder option.

git.intree

Hg-Git keeps a Git repository clone for reading and updating. By default, the Git clone is the subdirectory git in your local Mercurial repository. If you would like this Git clone to be at the same level of your Mercurial repository instead (named .git), add the following to your hgrc:

[git]
intree = True

git.mindate

If set, branches where the latest commit’s commit time is older than this will not be imported. Accepts any date formats that Mercurial does – see hg help dates for more.

git.public

A list of Git branches that should be considered “published”, and therefore converted to Mercurial in the ‘public’ phase. This is only used if hggit.usephases is set.

git.renamelimit

The number of files to consider when performing the copy/rename detection. Detection is disabled if the number of files modified in a commit is above the limit. Detection is O(N^2) in the number of files modified, so be sure not to set the limit too high. Similar to Git’s diff.renameLimit config. The default is “400”, the same as Git.

git.similarity

Specify how similar files modified in a Git commit must be to be imported as Mercurial renames or copies, as a percentage between “0” (disabled) and “100” (files must be identical). For example, “90” means that a delete/add pair will be imported as a rename if more than 90% of the file has stayed the same. The default is “0” (disabled).

hggit.mapsavefrequency

Controls how often the mapping between Git and Mercurial commit hashes gets saved when importing or exporting changesets. Set this to a number greater than 0 to save the mapping after converting that many commits. This can help when the conversion encounters an error partway through a large batch of changes. Defaults to 0, so that the mapping is saved once at the end.

Please note that this is meaningless for an initial clone, as any error or interruption will delete the destination. So instead of cloning a large Git repository, you might want to pull instead:

$ hg init linux
$ cd linux
$ echo "[paths]\ndefault = https://github.com/torvalds/linux" > .hg/hgrc
$ hg pull

…and be extremely patient. Please note that converting very large repositories may take days rather than mere hours, and may run into issues with available memory for very long running clones. Even any small, undiscovered leak will build up when processing hundreds of thousands of files and commits. Cloning the Linux kernel is likely a pathological case, but other storied repositories such as CPython do work well, even if the initial clone requires a some patience.

hggit.usephases

When converting Git revisions to Mercurial, place them in the ‘public’ phase as appropriate. Namely, revisions that are reachable from the remote Git repository’s HEAD will be marked public. For most repositories, this means the remote master branch will be converted as public. Publishing commits prevents their modification, and speeds up many local Mercurial operations, such as hg shelve.

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