# Mercurial and Heptapod short tutorial

Mercurial is a free, distributed source control management tool. We use it for PyPy, not only because it is written in Python, but because its branch model fits our development process better than Git's branch model.

Mercurial couples very well with the programs TortoiseHG and Meld (if you can, just install them, especially Meld).

There are a lot of tutorials and documentations about Mercurial (for example the official Mercurial tutorial). This page is meant to show you what you need to get going with PyPy development.

Heptapod is a friendly fork of GitLab Community Edition supporting Mercurial.

## Installation

### With TortoiseHG (simple for Windows)

Download the installer from https://tortoisehg.bitbucket.io/. (but note you might need the evolve extension)

### With conda (cross-platform, recommended for Linux and macOS)

On Windows, macOS and Linux, one can use `conda` (installed with miniconda) to install Mercurial with few extensions (hg-evolve and hg-git). On Windows, these commands have to be run in the Anaconda Prompt. First, we need to install conda-app in the base conda environment:

    conda activate base
    python -mpip install conda-app

Then, with the conda-forge channel added (`conda config --add channels conda-forge`), one just needs to run:

    conda-app install mercurial

Open a new terminal and the Mercurial command `hg` should be available.

Note

If you don't use TortoiseHG, you should really install the visual diff and merge tool Meld!

## Set-up Mercurial

You need to create a file `~/.hgrc`. For a good starting point, you can use the command:

    hg config --edit

An example of configuration file:

    [ui]
    username=myusername <email@adress.org>
    editor=emacs -nw
    tweakdefaults = True

    [extensions]
    hgext.extdiff =
    # only to use Mercurial with GitHub and Gitlab
    hggit =
    # more advanced extensions (really useful for PyPy dev)
    churn =
    shelve =
    rebase =
    absorb =
    evolve =
    topic =

    [extdiff]
    cmd.meld =

The line starting with hggit is optional and enables the extension hg-git. This extension is useful to work on projects using Git, for example hosted on Github and Gitlab.

The extensions churn, shelve, rebase, absorb, evolve and topic are very useful for more advanced users. Note that evolve and topic comes from the package hg-evolve.

Note

For occasional contribution to PyPy, the evolve and topic extensions have to be installed and activated since we use topic branches for short-term development

## Get help

To get help on Mercurial, one can start with:

    hg help

or for a specific command (here `clone`):

    hg help clone

## Simple workflow

To make a copy of an existing repository:

    hg clone https://foss.heptapod.net/pypy/pypy

To get a summary of the working directory state:

    hg summary

or just `hg sum`.

Before you begin work, open a new topic branch:

    hg topic my_branch

To show changed files in the working directory:

    hg status

or just `hg st`.

If you add new files or if you deleted files:

    hg add name_of_the_file

    hg remove name_of_the_file

Each time you do some consistent changes:

    hg commit -m "A message explaining the commit"

After a commit command `hg st` to check that you did what you wanted to do. If you are unhappy with the commit, you can amend it with another commit with:

    hg commit --amend

To push the state of your working repository to your repository on the web:

    hg push

The inverse command (pull all commits from the remote repository) is:

    hg pull

### Get the last version of a code

First pull all the changesets from the remote repository:

    hg pull

Then update the code to the tip:

    hg update

or just `hg up`. You can also directly do:

    hg pull -u

### Read the history

You can get a list of the changesets with:

    hg log --graph

or just `hg log -G`. With the `--graph` or `-G` option, the revisions are shown as an ASCII art.

### Update the code to an old revision

Use `hg up 220` to update to the revision 220. We can use a tag, bookmark, topic name or branch name instead of a number. To get a clean copy, add the option `-C` (beware).

### Create a repository from a directory

Create a new repository in the given directory by doing:

    hg init

## Merge-Request based workflow with hg-evolve

We now use a Merge-Request (MR) based workflow

Note

GitLab's "merge requests" are equivalent to GitHub's "pull requests".

Note

In contrast to the standard workflow in Github, Gitlab and Bitbucket, you don't need to fork the repository to create Merge Requests.

Instead, you need to become a "developer" of the project. The developers have the permission to push changesets (i.e. "commits") in a topic in the main repository (for example https://foss.heptapod.net/pypy/pypy). To acquire the "developer" role, please send a message in an issue or if needed, create a dedicated issue.

Topics are used in Mercurial for "lightweight branches" (like Git branches). The principle is that you first create a topic (with `hg topic`). Once a topic is activated, the changesets created belong to this topic. The new changesets gathered in a topic can be pushed in the main repository. Even after having been pushed to the main repository, they stay in the `draft` phase (which means they can be modified, as opposed to `public` changesets. Run `hg help phases` for more info).

To list the topics:

    hg topic

To activate a topic already created:

    hg up the_name_of_the_topic

To deactivate the topic and come back to the tip of the default branch:

    hg up default

To get the list of the changesets in the active topic (very useful):

    hg stack

Developers have to create Merge Requests (MR) to get things merged in the targeted branch (at the time of writing: `default` for Python2.7 or RPython changes, `py3.6` for Python 3.6, `py3.7` for Python 3.7). Let's present an example. A developer can do (here, we use ssh but you can also use https):

    hg clone ssh://hg@foss.heptapod.net/pypy/pypy
    hg up default
    hg topic fix_something
    hg commit -m "Fix a bug related to ..."
    hg push

Mercurial is going to print an URL to create the associated MR. Once created, the MR should then be reviewed by a contributor with the "maintainer" or higher role. Only maintainers have the permissions to merge a MR, i.e. to publish changesets. The maintainer can tell you how to modify your MR and can also directly modify the changesets of the MR.

We strongly advice to install and activate the evolve, rebase and absorb extensions locally (see the example of `.hgrc` above). This gives a very nice user experience for the MRs, with the ability to modify a MR with `hg absorb` and safe history editing.

Tip

`hg absorb` is very useful during code review. Let say that a developer submitted a MR containing few commits. As explained in this blog post, `hg absorb` is a mechanism to automatically and intelligently incorporate uncommitted changes into prior commits. Edit the files to take into account the remarks of the code review and just run:

    hg absorb
    hg push

and the MR is updated!

Tip

If you are asked to "rebase" your MR, it should work with the following commands:

    hg pull
    hg up name_of_my_topic
    hg rebase
    hg push

## Working with hggit and Github

To clone a git repository using hg:

    hg clone git+ssh://git@github.com/numpy/numpy

or just:

    hg clone https://github.com/numpy/numpy

Git branches are represented as Mercurial bookmarks so such commands can be useful:

    hg log --graph

    hg up master

    hg help bookmarks

    # list the bookmarks
    hg bookmarks

    # put the bookmark master where you are
    hg book master

    # deactivate the active bookmark (-i like --inactive)
    hg book -i

Note

`bookmarks`, `bookmark` and `book` correspond to the same mercurial command.

Warning

If a bookmark is active, `hg pull -u` or `hg up` will move the bookmark to the tip of the active branch. You may not want that so it is important to always deactivate an unused bookmark with `hg book -i` or with `hg up master`.

Do not forget to place the bookmark `master` as wanted.

### Delete a bookmark in a remote repository (close a remote Git branch)

With Mercurial, we can do:

    hg bookmark --delete <bookmark name>
    hg push --bookmark <bookmark name>

Unfortunately, it does not work for a remote Git repository (with hg-git). We have to use a Git client, clone the repository with Git and do something like:

    # this deletes the branch locally
    git branch --delete <branch name>
    # this deletes the branch in the remote repository
    git push origin --delete <branch name>