Obsolete experimentation

Safe Mutable History

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Safe Mutable History

Here are various materials on planned improvement to Mercurial regarding rewriting history.

First, read about what challenges arise while rewriting history and how we plan to solve them once and for all.

The effort is split in two parts:

  • The obsolescence marker concept aims to provide an alternative to strip to get rid of changesets. This concept has been partially implemented since Mercurial 2.3.
  • The evolve Mercurial extension rewrites history using obsolete marker under the hood.

The first and most important step is by far the obsolescence marker. However most users will never be directly exposed to the concept. For this reason this manual starts with changeset evolution.

Evolve: A robust alternative to MQ

Evolve is an experimental history rewriting extension that uses obsolete markers. It is inspired by MQ and pbranch but has multiple advantages over them:

  • Focus on your current work.

    You can focus your work on a single changeset and take care of adapting descendent changesets later.

  • Handle non-linear history with branches and merges

  • Rely internally on Mercurial’s robust merge mechanism.

    Simple conflicts are handled by real merge tools using the appropriate ancestor. Conflicts are much rarer and much more user friendly.

  • Mutable history fully available all the time.

Always use ‘hg update’ and forget about (un)applying patches to access the mutable part of your history.
  • Use only plain changesets and forget about patches. Evolve will create and exchange real changesets. Mutable history can be used in all usual operations: pull, push, log, diff, etc.
  • Allow sharing and collaboration mutable history without fear of duplicates (thanks to obsolete marker).
  • Cover all MQ usage but guard.

Warning

The evolve extension and obsolete markers are at an experimental stage. While using obsolete you willl likely be exposed to complex implications of the obsolete marker concept. I do not recommend non-power users to test this at this stage.

While numbered 1.0.0, the command line API of this version should not be regarded as stable: command behavior, name and options may change in future releases or once integrated into Mercurial. It is still an immature extension; a lot of features are still missing but there is low risk of repository corruption.

Production-ready version should hide such details from normal users.

The evolve extension requires Mercurial 2.5 (older versions supports down to 2.2)

To enable the evolve extension use:

$ hg clone https://bitbucket.org/marmoute/mutable-history -u stable
$ echo '[extensions]\nevolve=$PWD/mutable-history/hgext/evolve.py' >> ~/.hgrc

You will probably want to use hgview to visualize obsolescence. Version 1.7.1 or later is required.

For more information see the documents below:

Smart changeset deletion: Obsolete Marker

The obsolete marker is a powerful concept that allows Mercurial to safely handle history rewriting operations. It is a new type of relation between Mercurial changesets that tracks the result of history rewriting operations.

This concept is simple to define and provides a very solid base for:

  • very fast history rewriting operations
  • auditable and reversible history rewriting process
  • clean final history
  • share and collaborate on mutable parts of the history
  • gracefully handle history rewriting conflicts
  • allow various history rewriting UI to collaborate with a underlying common API

For more information, see the documents below

Known limitations and bugs

Here is a list of known issues that will be fixed later:

  • You need to provide to graft –continue -O if you started you graft using -O.

    you to manually specify target all the time.

  • Trying to exchange obsolete marker with a static http repo will crash.

  • Extinct changesets are hidden using the hidden feature of mercurial only supported by a few commands.

    Only hg log, hgview and hg glog support it. Neither hg heads nor other visual viewers do.

  • hg heads shows extinct changesets.

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