Changeset Evolution

Changeset Evolution is a set of features to gracefully handle history rework operations.

• It offers a safe and simple way to refine changesets locally and propagate that changes to other repository.

• It can automatically detect and handles the complex issues that can arise from exchanging draft changeset.

• It makes is even possible for multiple developer to rewrite the same part of the history in a distributed way.

• It fully respect the Phases concept to unsure user will only rewrite part of the history that are meant to be. Phases have been part of Mercurial since early 2012.

You can read the documentation or watch the talk at FOSDEM 2013 to learn more. (Slides).

1. Status

While well on the way, the full implementation of the changeset evolution concept is still in progress. Core Mercurial already support many of the associated feature, but for now they are still disabled by default. The current implementation have been usable for multiple years already, and some part of it are used in production in multiple project and company (including the Mercurial project itself, Facebook, Google, etc…).

However, there is still some area were the current implementation have gaps. This means some use case or performance are not handled as well as they current are without evolution. Mercurial have been around for a long time and is strongly committed to backward compatibility, turning evolution on by default nowaday could regress the experience of some of our current users. The feature will be enabled by default at the point where user who do not use or care about the new feature added by evolution won't be impacted by it.

2. Using Evolution

Using evolution is safe and no data loss/corruption is to be expected. Once you turn evolution on, all commands from core Mercurial will use it. In addition, one can enable evolution locally and still use an "old" server, you won't "poison" the server (but won't be able to use the new feature with that server).

Testing the concept earlier is useful for us, core developper. It provides us with data and use case that we need to iron out before enabling it by default. This is especially True if you can test evolve in a context that involve client/server/distributed workflow.

Here are the recommended step to try evolution:

1. Subscribe to the evolve beta tester mailing list,

2. Install and use the EvolveExtension

3. Enjoy changeset evolution

The evolve extensions will take care of enabling all the appropriate features in core and add a small layer with the latest state of the new commands and algorithm. The extension is developed by core Mercurial developers and its code collaborate closely with Mercurial core internal. It leave outside of core to support older versions of Mercurial (extending the tester base) and gain flexibility when experimenting new algorithm.

As the implementation is still in progress, some command behavior might change. Subscribe to the evolve beta tester mailing list to make sure you stay aware of new release and the change they might introduce.

3. Overview

You can also check the evolve documentation.

3.1. Rewriting history

Mercurial offers multiple commands to rewrite history:

• hg commit --amend: can add more changes into a commit

• hg rebase: can move changesets around in your graph (requires the RebaseExtension)

• hg histedit: can perform rewrite operation on some of your changesets (requires the HisteditExtension)

The experimental EvolveExtension adds more commands, which will eventually moved into core:

• hg uncommit: can remove changes from a commit and put them back in your working directory

• hg fold: can squash multiple changesets together as a single new commit

• hg prune: can remove changesets from your history

• hg split: can remove changesets from your history

All these operations are very safe to use, even for Mercurial rookies. Mercurial will actively prevent you from rewriting parts of history which are not safe to rewrite. Read about the Phases concept for details.

3.2. Tracking and sharing rewriting

Obsolescence markers make it possible to mark changesets that have been deleted or superseded by a new version of the changeset.

Unlike the previous way of handling such changes (which stripped the old changesets from the repository), obsolescence markers can be propagated between repositories. This allows for a safe and simple way of exchanging mutable history and altering it after the fact. Changeset phases are respected, such that only draft and secret changesets can be altered (see hg phases for details).

Obsolescence is tracked using "obsolescence markers", a piece of metadata that tracks which changesets have been made obsolete, potential successors for a given changeset, the moment the changeset was marked as obsolete, and the user who performed the rewriting operation. The markers are stored separately from standard changeset data and can be exchanged without any of the precursor changesets, preventing unnecessary exchange of obsolescence data.

The complete set of obsolescence markers describes a history of changeset modifications that is orthogonal to the repository history of file modifications. This changeset history allows for detection and automatic resolution of edge cases arising from multiple users rewriting the same part of history concurrently.

3.3. Automatic detection and resolution arising troubles

Exchanging mutable changesets has inherent troubles that we must be prepare to deal with. Most people will never run into them but Mercurial is able to detect and solve them automatically.

There are three kinds of troubled changesets

In some situations you may have non-obsolete changesets descending from obsolete changesets. Such changesets are said to be "unstable".

In some other situations you may have successors for changesets which are now immutable. In such case the obsolescence marker does not apply and the unlucky successors are said to be "bumped".

Finally when multiple changesets claim to be the successors of changesets they are said to be "divergent".

When Mercurial detect such troubles it will warn the user and prevent push by default. You can use the hg evolve command to automatically resolve them.

This command is partially implemented in the EvolveExtension.

4. Current implementation state

As of Janurary 2017, the following area are covered:

• All commands know how to creates, store and read obsolescence information,
• obsolete changesets are properly excluded the user view and not exchanged between repository,
• the obsolescence markers are properly exchanged between repository, propagating relevant obsolescence information alongside changesets,
• hg log and summary will display some information related to evolution,

• command like hg update, hg pull and hg rebase can use obsolescence information to take smarter decision,

• all evolution troubles are properly detected and diagnosed,

• the evolve command can automatically resolves the most common cases of such troubles.

And the following area need improvement: (Get on the tester mailing list if get stuck into one of them)

• the most rare/advance case of evolution troubles are not automatically handled yet,
• we need more advance barrier to prevent the user to shoot himself in the foot in some corner cases
• raw obsolescence data are available to the user, but we lack good tool to visualise the obsolescence history,
• bringing an obsolete changesets to life is harder that it should be,
• support for other advance maintance operation (like bundling or strippig obsmarker") is missing,
• during exchange, the discovery protocol for markers can be very slow in some case (we have a second iteration to be implemented),
• we need more scaling work for large/old installation, for update and bookmarks.

5. Older materials

• Initial presentation at Copenhagen: http://public.octopoid.net/talk.pdf

• First tutorial, written by Peter Arrenbrecht: http://arrenbrecht.ch/mercurial/evolution/

• Blog post by Arne Babenhauserheide (Test of the hg evolve extension for easier upstreaming): http://draketo.de/light/english/mercurial/hg-evolve-2013-01-12

6. Related Developer pages

• list of easy bug

• ChangesetEvolutionDevel

• ChangesetEvolutionDevelUI

• ChangesetEvolutionDevelRebase

• ChangesetEvolutionDevelObsmarkersExchange