evolve extension for Mercurial

Changeset Evolution with Mercurial

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Changeset Evolution with Mercurial

evolve is a Mercurial extension for faster and safer mutable history. It implements the changeset evolution concept for Mercurial.

Installation and setup

We recommend you subscribe to the evolve-testers mailing list to stay up to date with the latest news and announcement.

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.

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.

Resources

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