Changesets Evolution

Changeset Evolution is a set of feature to gracefully handle history rewriting operation. It offers a **safe** and **simple** way to refines changesets. Results of your local history rewriting operations can be propagated to other clone in a *solid* way. It is even possible for multiple people to rewrite the same part of the history in a distributed way.

The changeset evolution concept is still being implemented. Most of it is disabled by default.

The functionality is currently available from an external repository: mutable-history repo.

1. Rewriting history

Mercurial offers multiple commands to rewrite history

hg commit --amend:: can adds more changes into a commit hg rebase:: can move changesets around in your graph (requires the RebaseExtension) hg histedit:: can performs rewrite operation on some of your changesets (require the HisteditExtension)

The Experimental EvolveExtension adds more commands that will eventualy moved into core

hg uncommit:: can removes changes from a commit a put them back in your working directory hg fold:: can squash multiple change set together as a single new commit hg prune:: can removes changesets from your history.

All those 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. Reads about 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Obsolescence markers make it possible to mark changesets that have been deleted or superseded in a new version of the changeset.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, by stripping 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 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 an 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 is three kind of **troubled changesets**

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

In some other situation 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 "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 2.5.1)

Most history rewritting commands can now create obsolescences 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, thoses commandes are allowed to rewrite an arbitrary part of the history leaving untouched descendant behind as unstable.

log --graph will display 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 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/