.. Copyright © 2014 Greg Ward <greg@gerg.ca>

==================================

Changeset Evolution with Mercurial

==================================

`evolve`_ is a Mercurial extension for faster and safer mutable history. It

implements the `changeset evolution`_ concept for `Mercurial`_.

* It offers a safe and simple way to refine changesets locally and propagate

  those changes to other repositories.

* It can automatically detect and handle the complex issues that can arise from

  exchanging draft changesets.

* It even makes it possible for multiple developers to safely rewrite the same

  parts of history in a distributed way.

* It fully respects the Phases concept so users will only be able to rewrite

  parts of the history that are safe to change. Phases have been part of

  Mercurial since early 2012.

.. _`evolve`: https://www.mercurial-scm.org/wiki/EvolveExtension

.. _`Mercurial`: https://www.mercurial-scm.org/

Installation and setup

----------------------

We recommend you subscribe to the `evolve-testers`_ mailing list to stay up

to date with the latest news and announcement.

.. _`evolve-testers`: https://www.mercurial-scm.org/mailman/listinfo/evolve-testers

Using pip::

    pip install --user hg-evolve

Then add in your `hgrc` config::

   [extensions]

   evolve=

You can easily edit the `hgrc` of a repository using `hg config --local`.

Alternatively, you can edit your user configuration with `hg config --edit`.

Table of Contents

-----------------

.. toctree::

   :maxdepth: 2

   index

   user-guide

   sharing

   concepts

   from-mq

.. _`changeset evolution`:

What is Changeset Evolution?

----------------------------

With core Mercurial, changesets are permanent and immutable. You can

commit new changesets to modify your source code, but you cannot

modify or remove old changesets.

For years, Mercurial has included various commands that allow

history modification: ``rebase``, ``histedit``, ``commit --amend`` and so forth.

However, there's a catch: until now, Mercurial's various mechanisms for

modifying history have been *unsafe*, in that changesets were

destroyed (“stripped”) rather than simply hidden and still easy to recover.

``evolve`` makes things better by changing the behaviour of most existing

history modification commands so they use a safer mechanism (*changeset

obsolescence*, covered below) rather than the older, less safe *strip*

operation.

``evolve`` is built on infrastructure in core Mercurial:

  * *Phases* (starting in Mercurial 2.1) allow you to distinguish

    mutable and immutable changesets.

  * *Changeset obsolescence* (starting in Mercurial 2.3) is how

    Mercurial knows how history has been modified, specifically when

    one changeset replaces another. In the obsolescence model, a

    changeset is neither removed nor modified, but is instead marked

    *obsolete* and typically replaced by a *successor*. Obsolete

    changesets usually become *hidden* as well. Obsolescence is a

    disabled feature in Mercurial until you start using ``evolve``.

Some of the things you can do with ``evolve`` are:

  * Fix a mistake immediately: “Oops! I just committed a changeset

    with a syntax error—I'll fix that and amend the changeset so no

    one sees my mistake.” (While this is possible using default

    features of core Mercurial, changeset evolution makes it safer.)

  * Fix a mistake a little bit later: “Oops! I broke the tests three

    commits back, but only noticed it now—I'll just update back to the

    bad changeset, fix my mistake, amend the changeset, and evolve

    history to update the affected changesets.”

  * Remove unwanted changes: “I hacked in some debug output two

    commits back; everything is working now, so I'll just prune that

    unwanted changeset and evolve history before pushing.”

  * Share mutable history with yourself: say you do most of your

    programming work locally, but need to test on a big remote server

    somewhere before you know everything is good. You can use

    ``evolve`` to share mutable history between two computers, pushing

    finely polished changesets to a public repository only after

    testing on the test server.

  * Share mutable history for code review: you don't want to publish

    unreviewed changesets, but you can't block every commit waiting

    for code review. The solution is to share mutable history with

    your reviewer, amending each changeset until it passes review.

  * Explore and audit the rewrite history of a changeset. Since Mercurial is

    tracking the edits you make to a changeset, you can look at the history of

    these edits. This is similar to Mercurial tracking the history of file

    edits, but at the changeset level.

Why the `evolve` extension?

---------------------------

Mercurial core already has some support for `changeset evolution`_ so why have a

dedicated extension?

The long-term plan for ``evolve`` is to add it to core Mercurial. However,

having the extension helps us experiment with various user experience

approaches and technical prototypes. Having a dedicated extension helps current

users deploy the latest changes quickly and provides developers with low latency

feedback.

Whenever we are happy with a experimental direction in the extension, the

relevant code can go upstream into Core Mercurial.

Development status

------------------

While well underway, the full implementation of the `changeset evolution`_

concept is still a work in progress. Core Mercurial already supports many of the

associated features, but for now they are still disabled by default. The current

implementation has been usable for multiple years already, and some parts of it

are used in production by multiple projects and companies (including the

Mercurial project itself, Facebook, Google, etc…).

However, there are still some areas were the current implementation has gaps.

This means some use cases or performance issues are not handled as well as they

currently are without evolution. Mercurial has been around for a long time and

is strongly committed to backward compatibility. Therefore turning evolution on

by default today could regress the experience of some of our current users. The

features will only be enabled by default at the point where users who do not use

or care about the new features added by evolution won't be negatively impacted

by the new default.

.. # .. _`this query`: https://bz.mercurial-scm.org/buglist.cgi?component=evolution&bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=NEED_EXAMPLE

Resources

---------

  * For a practical guide to using ``evolve`` in a single repository,

    see the `user guide`_.

  * For more advanced tricks, see `sharing mutable history`_.

  * To learn about the concepts underlying ``evolve``, see `concepts`_

    (incomplete).

  * If you're coming from MQ, see the `MQ migration guide`_ (incomplete).

.. _`user guide`: user-guide.html

.. _`sharing mutable history`: sharing.html

.. _`concepts`: concepts.html

.. _`MQ migration guide`: from-mq.html