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PEP 347 - Migrating the Python CVS to Subversion

PEP: 347
Title: Migrating the Python CVS to Subversion
Author: Martin von Löwis <martin at v.loewis.de>
Discussions-To: < python-dev at python.org >
Status: Final
Type: Process
Created: 14-Jul-2004
Post-History: 14-Jul-2004

Abstract

The Python source code is currently managed in a CVS repository on sourceforge.net. This PEP proposes to move it to a Subversion repository on svn.python.org.

Rationale

This change has two aspects: moving from CVS to Subversion, and moving from SourceForge to python.org. For each, a rationale will be given.

Moving to Subversion

CVS has a number of limitations that have been eliminated by Subversion. For the development of Python, the most notable improvements are:

  • the ability to rename files and directories, and to remove directories, while keeping the history of these files.
  • support for change sets (sets of correlated changes to multiple files) through global revision numbers. Change sets are transactional.
  • atomic, fast tagging: a cvs tag might take many minutes; a Subversion tag (svn cp) will complete quickly, and atomically. Likewise, branches are very efficient.
  • support for offline diffs, which is useful when creating patches.

Moving to python.org

SourceForge has kindly provided an important infrastructure for the past years. Unfortunately, the attention that SF received has also caused repeated overload situations in the past, to which the SF operators could not always respond in a timely manner. In particular, for CVS, they had to reduce the load on the primary CVS server by introducing a second, read-only CVS server for anonymous access. This server is regularly synchronized, but lags behind the read-write CVS repository between synchronizations. As a result, users without commit access can see recent changes to the repository only after a delay.

On python.org, it would be possible to make the repository accessible for anonymous access.

Migration Procedure

To move the Python CVS repository, the following steps need to be executed. The steps are elaborated upon in the following sections.

  1. Collect SSH keys for all current committers, along with usernames to appear in commit messages.
  2. At the beginning of the migration, announce that the repository on SourceForge closed.
  3. 24 hours after the last commit, download the CVS repository.
  4. Convert the CVS repository into a Subversion repository.
  5. Publish the repository with write access for committers, and read-only anonymous access.
  6. Disable CVS access on SF.

Collect SSH keys

After some discussion, svn+ssh was selected as the best method for write access to the repository. Developers can continue to use their SSH keys, but they must be installed on python.org.

In order to avoid having to create a new Unix user for each developer, a single account should be used, with command= attributes in the authorized_keys files.

The lines in the authorized_keys file should read like this (wrapped for better readability):

command="/usr/bin/svnserve --root=/svnroot -t
--tunnel-user='<username>'",no-port-forwarding,
no-X11-forwarding,no-agent-forwarding,no-pty
ssh-dss <key> <comment>

As the usernames, the real names should be used instead of the SF account names, so that people can be better identified in log messages.

Administrator Access

Administrator access to the pythondev account should be granted to all current admins of the Python SF project. To distinguish between shell login and svnserve login, admins need to maintain two keys. Using OpenSSH, the following procedure can be used to create a second key:

cd .ssh
ssh-keygen -t DSA -f pythondev -C <user>@pythondev
vi config

In the config file, the following lines need to be added:

Host pythondev
  Hostname dinsdale.python.org
  User pythondev
  IdentityFile ~/.ssh/pythondev

Then, shell login becomes possible through "ssh pythondev".

Downloading the CVS Repository

The CVS repository can be downloaded from

http://cvs.sourceforge.net/cvstarballs/python-cvsroot.tar.bz2

Since this tarball is generated only once a day, some time must pass after the repository freeze before the tarball can be picked up. It should be verified that the last commit, as recorded on the python-commits mailing list, is indeed included in the tarball.

After the conversion, the converted CVS tarball should be kept forever on www.python.org/archive/python-cvsroot-<date>.tar.bz2

Converting the CVS Repository

The Python CVS repository contains two modules: distutils and python. The python module is further structured into dist and nondist, where dist only contains src (the python code proper). nondist contains various subdirectories.

These should be reorganized in the Subversion repository to get shorter URLs, following the <project>/{trunk,tags,branches} structure. A project will be created for each nondist directory, plus for src (called python), plus distutils. Reorganizing the repository is best done in the CVS tree, as shown below.

The fsfs backend should be used as the repository format (which requires Subversion 1.1). The fsfs backend has the advantage of being more backup-friendly, as it allows incremental repository backups, without requiring any dump commands to be run.

The conversion should be done using the cvs2svn utility, available e.g. in the cvs2svn Debian package. As cvs2svn does not currently support the project/trunk structure, each project needs to be converted separately. To get each conversion result into a separate directory in the target repository, svnadmin load must be used.

Subversion has a different view on binary-vs-text files than CVS. To correctly carry the CVS semantics forward, svn:eol-style should be set to native on all files that are not marked binary in the CVS.

In summary, the conversion script is:

#!/bin/sh
rm cvs2svn-*
rm -rf python py.new
tar xjf python-cvsroot.tar.bz2
rm -rf python/CVSROOT
svnadmin create --fs-type fsfs py.new
mv python/python python/orig
mv python/orig/dist/src python/python
mv python/orig/nondist/* python
# nondist/nondist is empty
rmdir python/nondist
rm -rf python/orig
for a in python/*
do
  b=`basename $a`
  cvs2svn -q --dump-only --encoding=latin1 --force-branch=cnri-16-start \
  --force-branch=descr-branch --force-branch=release152p1-patches \
  --force-tag=r16b1 $a
  svn mkdir -m"Conversion to SVN" file:///`pwd`/py.new/$b
  svnadmin load -q --parent-dir $b py.new < cvs2svn-dump
  rm cvs2svn-dump
done

Sample results of this conversion are available at

http://www.dcl.hpi.uni-potsdam.de/pysvn/

Publish the Repository

The repository should be published at http://svn.python.org/projects . Read-write access should be granted to all current SF committers through svn+ssh://pythondev@svn.python.org/; read-only anonymous access through WebDAV should also be granted.

As an option, websvn (available e.g. from the Debian websvn package) could be provided. Unfortunately, in the test installation, websvn breaks because it runs out of memory.

The current SF project admins should get write access to the authorized_keys2 file of the pythondev account.

Disable CVS

It appears that CVS cannot be disabled entirely. Only the user interface can be removed from the project page; the repository itself remains available. If desired, write access to the python and distutils modules can be disabled through a CVS commitinfo entry.

Discussion

Several alternatives had been suggested to the procedure above. The rejected alternatives are shortly discussed here:

  • create multiple repositories, one for python and one for distutils. This would have allowed even shorter URLs, but was rejected because a single repository supports moving code across projects.

  • Several people suggested to create the project/trunk structure through standard cvs2svn, followed by renames. This would have the disadvantage that old revisions use different path names than recent revisions; the suggested approach through dump files works without renames.

  • Several people also expressed concern about the administrative overhead that hosting the repository on python.org would cause to pydotorg admins. As a specific alternative, BerliOS has been suggested. The pydotorg admins themselves haven't objected to the additional workload; migrating the repository again if they get overworked is an option.

  • Different authentication strategies were discussed. As alternatives to svn+ssh were suggested

    • Subversion over WebDAV, using SSL and basic authentication, with pydotorg-generated passwords mailed to the user. People did not like that approach, since they would need to store the password on disk (because they can't remember it); this is a security risk.
    • Subversion over WebDAV, using SSL client certificates. This would work, but would require us to administer a certificate authority.
  • Instead of hosting this on python.org, people suggested hosting it elsewhere. One issue is whether this alternative should be free or commercial; several people suggested it should better be commercial, to reduce the load on the volunteers. In particular:

    • Greg Stein suggested http://www.wush.net/subversion.php . They offer 5 GB for $90/month, with 200 GB download/month. The data is on a RAID drive and fully backed up. Anonymous access and email commit notifications are supported. wush.net elaborated the following details:

      • The machine would be a Virtuozzo Virtual Private Server (VPS), hosted at PowerVPS.
      • The default repository URL would be http://python.wush.net/svn/projectname/ , but anything else could be arranged
      • we would get SSH login to the machine, with sudo capabilities.
      • They have a Web interface for management of the various SVN repositories that we want to host, and to manage user accounts. While svn+ssh would be supported, the user interface does not yet support it.
      • For offsite mirroring/backup, they suggest to use rsync instead of download of repository tarballs.

      Bob Ippolito reported that they had used wush.net for a commercial project for about 6 months, after which time they left wush.net, because the service was down for three days, with nobody reachable, and no explanation when it came back.

Source: https://hg.python.org/peps/file/tip/pep-0347.txt