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

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Changeset Evolution with Mercurial

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

`evolve`_ is an experimental Mercurial extension for safe mutable history.

.. _`evolve`: http://mercurial.selenic.com/wiki/EvolveExtension

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—they are carved in stone for all

eternity.

For years, Mercurial has included various extensions that allow

history modification: ``rebase``, ``mq``, ``histedit``, and so forth.

These are useful and popular extensions, and in fact history

modification is one of the big reasons DVCSes (distributed version

control systems) like Mercurial took off.

But 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 made invisible.

``evolve`` makes things better in a couple of ways:

  * It changes the behaviour of most existing history modification

    extensions (``rebase``, ``histedit``, etc.) so they use a safer

    mechanism (*changeset obsolescence*, covered below) rather than

    the older, less safe *strip* operation.

  * It provides a new way of modifying history that is roughly

    equivalent to ``mq`` (but much nicer and safer).

It helps to understand that ``evolve`` builds on infrastructure

already in core Mercurial:

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

    mutable and immutable changesets. We'll cover phases early in the

    user guide, since understanding phases is essential to

    understanding ``evolve``.

  * *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 an

    invisible feature until you start using ``evolve``, so we'll cover

    it in the user guide too.

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 existing

    features of core Mercurial, ``evolve`` 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.

``evolve`` is experimental!

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

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

However, it is not yet stable enough for that. In particular:

  * The UI is unstable: ``evolve``'s command names and command options

    are not completely nailed down yet. They are subject to occasional

    backwards-incompatible changes. If you write scripts that use

    evolve commands, a future release could break your scripts.

  * There are still some corner cases that aren't handled yet. If you

    think you have found such a case, please check if it's already

    described in the Mercurial bug tracker (http://bz.selenic.com).

    Bugs in ``evolve`` are files under component "evolution": use

    `this query`_ to view open bugs in ``evolve``.

.. _`this query`: http://bz.selenic.com/buglist.cgi?component=evolution&bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=NEED_EXAMPLE

Installation and setup

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

To use ``evolve``, you must:

  #. Clone the ``evolve`` repository::

       cd ~/src

       hg clone https://bitbucket.org/marmoute/mutable-history

  #. Configure the extension, either locally ::

       hg config --local

     or for all your repositories ::

       hg config --edit

     Then add ::

       evolve=~/src/mutable-history/hgext/evolve.py

     in the ``[extensions]`` section (adding the section if necessary). Use

     the directory that you actually cloned to, of course.

Next steps:

-----------

  * 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

.. _`concepts`: concepts.html

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