git remote helper to interact with mercurial repositories
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README.md

git-cinnabar 0.5

When you update, please read this file again, it may contain important notes.

cinnabar is the common natural form in which mercury can be found on Earth. It contains mercury sulfide and its powder is used to make the vermillion pigment.

git-cinnabar is a git remote helper to interact with mercurial repositories. Contrary to other such helpers*, it doesn't use a local mercurial clone under the hood, although it currently does require mercurial to be installed for some of its libraries.

* This applies to the following tools:

The main focus at the moment is to make it work with mozilla-central and related mercurial repositories and support Mozilla workflows (try server, etc.).

Repositories last used with versions lower than 0.3.0 are not supported. Please run git cinnabar fsck with version 0.3.0 first.

Requirements:

  • Git (any version should work)
  • Mercurial version 1.9 or newer

Setup:

  • Add this directory to your PATH. If you have another git-remote-hg project in your PATH already, make sure the git-cinnabar path comes before.

  • A native helper is used for faster operations. You can download a prebuilt binary with the following command (assuming one is available for your system):

    $ git cinnabar download
    

    Alternatively, you can build it:

    $ make
    

    If you want to build git along the helper, you can run make git.

    If you have a non-standard Python installation location (for example if you are on macOS and have installed it using homebrew) you need to pass --with-python=/path/to/python to the configure script or set the PYTHON_PATH environment variable to your Python installation path when using make to build the helper.

Usage:

$ git clone hg::<mercurial repo>

where <mercurial repo> can be a path to a local directory containing a mercurial repository, or a http, https or ssh url.

Essentially, use git like you would for a git repository, but use a hg:: url where you would use a git:// url.

Mercurial bookmarks are exposed as refs/heads/bookmarks/$bookmark remote refs. If you want to interact exclusively with mercurial with bookmarks, you can use a refspec like refs/heads/bookmarks/*:refs/remotes/$remote/*.

Mercurial branches are exposed as namespaces under refs/heads/branches/. As mercurial branches can have multiple heads, each head is exposed as refs/heads/branches/$branch/$head, where $head is the mercurial sha1 of the head changeset. There is however an exception to that pattern, for the tip changeset of the branch, which is exposed as refs/heads/branches/$branch/tip. If you only care about the tip changeset of each branch, you can use a refspec like refs/heads/branches/*/tip:ref/remotes/$remote/*.

See https://github.com/glandium/git-cinnabar/wiki/Mozilla:-A-git-workflow-for-Gecko-development for an example workflow for Mozilla repositories.

Tags:

Because mercurial stores tags in a file in the repository, it is not possible for git-cinnabar to know them when git asks for them, except when the repository has already been updated. Until version 0.4.0, git-cinnabar would try to get tags in a best effort way.

Furthermore, the way tags are tracked across branches in mercurial can make it awkward when pulling from multiple mercurial repositories. For example, pulling tags from mozilla-release, mozilla-beta, and mozilla-esr* repositories is messy.

So, as of 0.5.0, tags are not associated with mercurial remotes anymore, and one needs to setup a separate remote that consolidates all mercurial tags tracked by git-cinnabar. That remote can be set like the following:

$ git remote add tags hg::tags:

And tags can be updated with, e.g.:

$ git fetch tags

Fetching a specific mercurial changeset:

It can sometimes be useful to fetch a specific mercurial changeset from a remote server, without fetching the entire repository. This can be done with a command line such as:

$ git cinnabar fetch hg::<mercurial repo> <changeset sha1>

Translating git commits to mercurial changesets and vice-versa:

When dealing with a remote repository that doesn't use the same identifiers, things can easily get complicated. Git-cinnabar comes with commands to know the mercurial changeset a git commit represents and the other way around.

The following command will give you the git commit corresponding to the given mercurial changeset sha1:

$ git cinnabar hg2git <changeset>

The following command will give you the mercurial changeset corresponding to the given git commit sha1:

$ git cinnabar git2hg <commit>

Both commands allow abbreviated forms, as long as they are unambiguous (no need for all the 40 hex digits of the sha1).

Avoiding metadata:

In some cases, it is not desirable to have git-cinnabar create metadata for all pushed commits. Notably, for volatile commits such as those used on the Mozilla try repository.

By default, git-cinnabar doesn't store metadata when pushing to non-publishing repositories. It does otherwise.

This behavior can be changed per-remote with a remote.$remote.cinnabar-data preference with one of the following values:

  • always
  • never
  • phase

phase is the default described above. always and never are self-explanatory.

Limitations:

At the moment, push is limited to non-merge commits.

There is no support for the following mercurial features:

  • obsolescence markers
  • phases
  • namespaces

Checking corruptions:

Git-cinnabar is still in early infancy, and its metadata might get corrupted for some reason.

The following command allows to detect various types of metadata corruption:

git cinnabar fsck

This command will fix the corruptions it can, as well as adjust some of the metadata that contains items that became unnecessary in newer versions.

The --manifests and --files options may be added for additional validation on manifests and files. Using either or both adds a significant amount of work, and the command can take more than half an hour on repositories the size of mozilla-central.

hg:// urls:

The msys shell (not msys2) doesn't keep hg::url intact when crossing the msys/native boundary, so when running cinnabar in a msys shell with a native git, the url is munged as hg;;proto;\host\path\, which git doesn't understand and doesn't even start redirecting to git-remote-hg.

To allow such setups to still work, hg:// urls are supported. But since mercurial can be either on many different protocols, we abuse the port in the given url to pass the protocol.

A hg:// url thus looks like:

hg://<host>[:[<port>.]<protocol>]/<path>

The default protocol is https, and the port can be omitted.

  • hg::https://hg.mozilla.org/mozilla-central becomes hg://hg.mozilla.org/mozilla-central

  • hg::http://hg.mozilla.org/mozilla-central becomes hg://hg.mozilla.org:http/mozilla-central

  • hg::ssh://hg.mozilla.org/mozilla-central becomes hg://hg.mozilla.org:ssh/mozilla-central

  • hg::file:///some/path becomes (awkward) hg://:file/some/path

  • hg::http://localhost:8080/foo becomes hg://localhost:8080.http/foo

  • hg::tags: becomes hg://:tags

Experimental features:

Git-cinnabar has a set of experimental features that can be enabled independently. You can set the cinnabar.experiments git configuration to a comma-separated list of those features to enable the selected ones, or to all to enable them all. The available features are:

  • wire

    In order to talk to Mercurial repositories, git-cinnabar normally uses mercurial python modules. This experimental feature allows to access Mercurial repositories without using the mercurial python modules. It then relies on git-cinnabar-helper to connect to the repository through the mercurial wire protocol. Please note the mercurial python modules are still needed for mercurial bundle v2 support.

    The feature is automatically enabled when Mercurial is not installed.

  • merge

    Git-cinnabar currently doesn’t allow to push merge commits. The main reason for this is that generating the correct mercurial data for those merges is tricky, and needs to be gotten right.

    The main caveat with this experimental support for pushing merges is that it currently doesn’t handle the case where a file was moved on one of the branches the same way mercurial would (i.e. the information would be lost to mercurial users).

  • git-clone

    For large repositories, an initial clone can take a large amount of time. This experimental feature allows to get an initial clone (including git-cinnabar metadata) from a git repository. This requires an extension on the mercurial server (see mercurial/cinnabarclone.py), and to push a fresh refs/cinnabar/metadata to some git repository.

    It can also be used without the extension on the mercurial server, by setting the cinnabar.clone git configuration item to the url of the git cinnabar metadata repository as it would be set up in the mercurial server configuration.

  • store-manifest

    Importing mercurial manifests is usually the longest operation when getting mercurial data. It currently happens with battle tested python code. This experimental feature makes it use newer native code from the cinnabar-helper, which at the moment is only slightly faster. Future versions will have an even faster, albeit more risky, implementation.