Changeset Evolution

Changeset Evolution is a set of features to gracefully handle history rewriting operations. It offers a safe and simple way to refine changesets. Results of your local history rewriting operations can be propagated to other clones in a solid way. It is even possible for multiple people to rewrite the same part of the history in a distributed way. You can watch the explanation of the concept at FOSDEM 2013 to learn more. Slides for the presentation are also available.

The changeset evolution concept is still being implemented. It is disabled by default.

The functionality is currently available with an external extension: EvolveExtension.

If you are using this extension, please subscribe to the evolve beta tester mailing list.

Using this EvolveExtension will alter some of the core commands and behaviors.

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

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

2. Tracking and sharing rewriting

Obsolescence markers make it possible to mark changesets that have been deleted or superseded in a new version of the changeset.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 drafts 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. 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 (Mercurial 2.9)

Most history rewritting commands can now create obsolescence markers instead of stripping:

• commit --amend

• rebase

• histedit

This behavior is disabled by default. You need to explicitly enable obsolescence support using EvolveExtension to get this behavior. When using obsolescence markers, those commands are allowed to rewrite an arbitrary part of the history leaving untouched descendant behind as unstable.

log --graph will use 'x' instead of 'o' to display obsolete changesets.

Obsolete changesets with no non-obsolete descendants are said to be "extinct" and are hidden to all mercurial commands and changesets exchanged. A --hidden switch is added globally to have them accessible again.

Obsolescence markers will be exchanged between repositories that explicitly assert support for the obsolescence feature (this can currently only be done via an extension).

Successors of a changeset are seen as valid destinations 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 Development page

• ObsolescenceMarkersExchange