python-peps/pep-0385.txt

PEP: 385
Title: Migrating from svn to Mercurial
Version: $Revision$
Last-Modified: $Date$
Author: Dirkjan Ochtman <dirkjan@ochtman.nl>
Status: Active
Type: Process
Content-Type: text/x-rst
Created: 25-May-2009

.. warning::
   This PEP is in the draft stages.


Motivation
==========

After having decided to switch to the Mercurial DVCS, the actual migration
still has to be performed. In the case of an important piece of
infrastructure like the version control system for a large, distributed
project like Python, this is a significant effort. This PEP is an attempt
to describe the steps that must be taken for further discussion. It's
somewhat similar to `PEP 347`_, which discussed the migration to SVN.

To make the most of hg, I (Dirkjan) would like to make a high-fidelity
conversion, such that (a) as much of the svn metadata as possible is
retained, and (b) all metadata is converted to formats that are common in
Mercurial. This way, tools written for Mercurial can be optimally used. In
order to do this, I want to use the `hgsubversion`_ software to do an initial
conversion. This hg extension is focused on providing high-quality conversion
from Subversion to Mercurial for use in two-way correspondence, meaning it
doesn't throw away as much available metadata as other solutions.

Such a conversion also seems like a good time to reconsider the contents of
the repository and determine if some things are still valuable. In this spirit,
the following sections also propose discarding some of the older metadata.

.. _PEP 347: http://www.python.org/dev/peps/pep-0347/
.. _hgsubversion: http://bitbucket.org/durin42/hgsubversion/


Timeline
========

TBD; needs fully working hgsubversion and consensus on this document.


Transition plan
===============

Branch strategy
---------------

Mercurial has two basic ways of using branches: cloned branches, where each
branch is kept in a separate repository, and named branches, where each revision
keeps metadata to note on which branch it belongs. The former makes it easier
to distinguish branches, at the expense of requiring more disk space on the
client. The latter makes it a little easier to switch between branches, but
often has somewhat unintuitive results for people (though this has been
getting better in recent versions of Mercurial).

I'm still a bit on the fence about whether Python should adopt cloned
branches and named branches. Since it usually makes more sense to tag releases
on the maintenance branch, for example, mainline history would not contain
release tags if we used cloned branches. Also, Mercurial 1.2 and 1.3 have the
necessary tools to make named branches less painful (because they can be
properly closed and closed heads are no longer considered in relevant cases).

A disadvantage might be that the used clones will be a good bit larger (since
they essentially contain all other branches as well). This can me mitigated by
keeping non-release (feature) branches in separate clones. Also note that it's
still possible to clone a single named branch from a combined clone, by
specifying the branch as in hg clone http://hg.python.org/main/#2.6-maint.
Keeping the py3k history in a separate clone problably also makes sense.

XXX To do: size comparison for selected separation scenarios.

Converting branches
-------------------

There are quite a lot of branches in SVN's branches directory. I propose to
clean this up a bit, by employing the following the strategy:

* Keep all release (maintenance) branches
* Discard branches that haven't been touched in 18 months, unless somone
  indicates there's still interest in such a branch
* Keep branches that have been touched in the last 18 months, unless someone
  indicates the branch can be deprecated

Converting tags
---------------

The SVN tags directory contains a lot of old stuff. Some of these are not, in
fact, full tags, but contain only a smaller subset of the repository. I think
we should keep all release tags, and consider other tags for inclusion based
on requests from the developer community. I'd like to consider unifying the
release tag naming scheme to make some things more consistent, if people feel
that won't create too many problems. For example, Mercurial itself just uses
'1.2.1' as a tag, where CPython would currently use r121.

Author map
----------

In order to provide user names the way they are common in hg (in the 'First Last
<user@example.org>' format), we need an author map to map cvs and svn user
names to real names and their email addresses. I have a complete version of such
a map in my `migration tools repository`_. The email addresses in it might be
out of date; that's bound to happen, although it would be nice to try and
have as many people as possible review it for addresses that are out of date.
The current version also still seems to contain some encoding problems.

.. _migration tools repository: http://hg.xavamedia.nl/cpython/pymigr/

Generating .hgignore
--------------------

The .hgignore file can be used in Mercurial repositories to help ignore files
that are not eligible for version control. It does this by employing several
possible forms of pattern matching. The current Python repository already
includes a rudimentary .hgignore file to help with using the hg mirrors.

It might be useful to have the .hgignore be generated automatically from
svn:ignore properties. This would make sure all historic revisions also have
useful ignore information (though one could argue ignoring isn't really
relevant to just checking out an old revision).

Revlog reordering
-----------------

As an optional optimization technique, we should consider trying a reordering
pass on the revlogs (internal Mercurial files) resulting from the conversion.
In some cases this results in dramatic decreases in on-disk repository size.

Other repositories
------------------

Richard Tew has indicated that he'd like the Stackless repository to also be
converted. What other projects in the svn.python.org repository should be
converted? Do we want to convert the peps repository? distutils? others?


Infrastructure
==============

hg-ssh
------

Developers should access the repositories through ssh, similar to the current
setup. Public keys can be used to grant people access to a shared hg@ account.
A hgwebdir instance should also be set up for easy browsing and read-only
access. If we're using ssh, developers should trivially be able to start new
clones (for longer-term features that profit from a separate branch).

Hooks
-----

A number of hooks is currently in use. The hg equivalents for these should be
developed and deployed. The following hooks are being used:

* check whitespace: a hook to reject commits in case the whitespace doesn't
  match the rules for the Python codebase. Should be straightforward to
  re-implement from the current version. We can also offer a whitespace hook
  for use with client-side repositories that people can use; it could either
  warn about whitespace issues and/or truncate trailing whitespace from changed
  lines. Open issue: do we check only the tip after each push, or do we check
  every commit in a changegroup?

* commit mails: we can leverage the notify extension for this

* buildbots: both the regular and the community build masters must be notified.
  Fortunately buildbot includes support for hg. I've also implemented this for
  Mercurial itself, so I don't expect problems here.

* check contributors: in the current setup, all changesets bear the username of
  committers, who must have signed the contributor agreement. In a DVCS, the
  committers are not necessarily the same people who push, and so we can't
  check if the committer is a contributor. We could use a hook to check if the
  committer is a contributor if we keep a list of registered contributors.

hgwebdir
--------

A more or less stock hgwebdir installation should be set up. We might want to
come up with a style to match the Python website. It may also be useful to
build a quick extension to augment the URL rev parser so that it can also take
r[0-9]+ args and come up with the matching hg revision.


After migration
===============

Where to get code
-----------------

It needs to be decided where the hg repositories will live. I'd like to
propose to keep the hgwebdir instance at hg.python.org. This is an accepted
standard for many organizations, and an easy parallel to svn.python.org.
The 2.7 (trunk) repo might live at http://hg.python.org/main/, for example,
with py3k at http://hg.python.org/py3k/. For write access, developers will
have to use ssh, which could be ssh://hg@hg.python.org/main/. A demo
installation will be set up with a preliminary conversion so people can
experiment and review; it can live at http://hg.python.org/example/.

code.python.org was also proposed as the hostname. Personally, I think that
using the VCS name in the hostname is good because it prevents confusion: it
should be clear that you can't use svn or bzr for hg.python.org.

hgwebdir can already provide tarballs for every changeset. I think this
obviates the need for daily snapshots; we can just point users to tip.tar.gz
instead, meaning they will get the latest. If desired, we could even use
buildbot results to point to the last good changeset.

Python-specific documentation
-----------------------------

hg comes with good built-in documentation (available through hg help) and a
`wiki`_ that's full of useful information and recipes. In addition to that,
the `parts of the developer FAQ`_ concerning version control will gain a
section on using hg for Python development. Some of the text will be dependent
on the outcome of debate about this PEP (for example, the branching strategy).

 .. _wiki: http://www.selenic.com/mercurial/wiki/
 .. _parts of the developer FAQ: http://www.python.org/dev/faq/#version-control

Think first, commit later?
--------------------------

In recent history, old versions of Python have been maintained by a select
group of people backporting patches from trunk to release branches. While
this may not scale so well as the development pace grows, it also runs into
some problems with the current crop of distributed versioning tools. These
tools (I believe similar problems would exist for either git, bzr, or hg,
though some may cope better than others) are based on the idea of a Directed
Acyclic Graph (or DAG), meaning they keep track of relations of changesets.

Mercurial itself has a stable branch which is a ''strict'' subset of the
unstable branch. This means that generally all fixes for the stable branch
get committed against the tip of the stable branch, then they get merged into
the unstable branch (which already contains the parent of the new cset). This
provides a largely frictionless environment for moving changes from stable to
unstable branches. Mistakes, where a change that should go on stable goes on
unstable first, do happen, but they're usually easy to fix. That can be done by
copying the change over to the stable branch, then trivial-merging with
unstable -- meaning the merge in fact ignores the parent from the stable
branch).

This strategy means a little more work for regular committers, because they
have to think about whether their change should go on stable or unstable; they
may even have to ask someone else (the RM) before committing. But it also
relieves a dedicated group of committers of regular backporting duty, in
addition to making it easier to work with the tool.

Now would be a good time to consider changing strategies in this regard,
although it would be relatively easy to switch to such a model later on.

The future of Subversion
------------------------

What happens to the Subversion repositories after the migration? Since the svn
server contains a bunch of repositories, not just the CPython one, it will
probably live on for a bit as not every project may want to migrate or it
takes longer for other projects to migrate. To prevent people from staying
behind, we may want to remove migrated projects from the repository.

Build identification
--------------------

Python currently provides the sys.subversion tuple to allow Python code to
find out exactly what version of Python it's running against. The current
version looks something like this:

* ('CPython', 'tags/r262', '71600')
* ('CPython', 'trunk', '73128M')

Another value is returned from Py_GetBuildInfo() in the C API, and available
to Python code as part of sys.version:

* 'r262:71600, Jun  2 2009, 09:58:33'
* 'trunk:73128M, Jun  2 2009, 01:24:14'

I propose that the revision identifier will be the short version of hg's
revision hash, for example 'dd3ebf81af43', augmented with '+' (instead of 'M')
if the working directory from which it was built was modified. This mirrors
the output of the hg id command, which is intended for this kind of usage.

For the tag/branch identifier, I propose that hg will check for tags on the
currently checked out revision, use the tag if there is one ('tip' doesn't
count), and uses the branch name otherwise. sys.subversion becomes

* ('CPython', '2.6.2', 'dd3ebf81af43')
* ('CPython', 'default', 'af694c6a888c+')

and the build info string becomes

* '2.6.2:dd3ebf81af43, Jun  2 2009, 09:58:33'
* 'default:af694c6a888c+, Jun  2 2009, 01:24:14'

This reflects that the default branch in hg is called 'default' instead of
Subversion's 'trunk', and reflects the proposed new tag format.