Migrating open source code repositories from CVS to GitHub

Removal Helpers

© altomedia, 123RF

© altomedia, 123RF

Article from Issue 106/2009

GitHub makes it easier for programmers to contribute to open source projects by simplifying and accelerating communications between project maintainers and people willing to contribute.

If you have ever tried sending a patch to an open source project, you will be familiar with the obstacles that can put off even the most motivated of developers. For example, you need to discover the project maintainer's email address and negotiate the various levels of moderation. If somebody in a position to make decisions actually finds the time to take a look, the patch format might be wrong, or the patch might collide with another, unpublished change.

Reducing the PITA Factor

GitHub's aim is to reduce the PITA (pain in the ass) factor [2]. Public code repositories that are hosted on GitHub use the branch- and merge-friendly Git revision control system that allows the open source community to apply changes, test locally, and – if successful – flow the code into the original project with little pain.

Creating a fork – spawning a copy of an open source project – is not a sneaky move on github.com. Here, forks are not a means of taking control of a project, but the recommended way of developing and testing new features and, finally, asking the project maintainer to add them to the main branch of the project.

GitHub hosts open source projects free of charge for public access, offering 300MB of disc space per developer. If you want to use the service for a non-public project, GitHub has a number of commercial variants that provide the small startup with a handful of developers a steady source of income.

From CVS to Git

Listing 1, cvs2github, helps convert CVS repositories to Git and prepare them for publication on github.com. Git with the git-cvs add-on package already includes an import function. All cvs2github needs to do is use rsync to download the CVS repository and the CVSROOT directory from wherever it is being hosted to the local machine and then call git-cvsimport.

For this to happen, line 16 of the script creates a temporary directory in which rsync stores the local copy of the server's CVS repository files. The CVS server name and account information is stored in line 24. The directory in which the Git repository finally lands is set in line 20 with the $git_dir variable. Because developers create entries in the CVS repository with their Unix IDs but use a different ID on GitHub, lines 30 through 32 map the old Unix usernames to new GitHub IDs with an email address, using an author conversion file.

Listing 1



Developer Names

Here, I am mapping three different IDs (mschill, perlmeis, mikeschilli) to a single new GitHub ID mschilli. If multiple developers have worked on a project, you need to convert all of their IDs to new GitHub IDs. The blurt function from the CPAN Sysadm::Install module stores the lines in a temporary file, which git-cvsimport accepts for mapping via the -A option. Because cvs2github is intended for Perl modules, line 38 converts project names to lowercase letters and adds a -perl suffix; hence, Log-Log4perl becomes log-log4perl-perl; this is in line with the Debian name schema, keeps the namespace on GitHub clean, and helps avoid clashes between original projects and their Perl ports or wrappers. Line 52 calls the rsync command to copy the server's CVS repository and then calls it again for the CVSROOT metadata directory to the local machine because git-cvsroot requires them both to make the transformation to Git. The RSYNC_RSH environmental variable gets set to the SSH client program on the local machine (ssh) because the client communicates via SSH with the server-side repository. A similar approach exists for SourceForge projects [3]. Importing the CVS repositories in line 60 creates a Git repository in the directory specified by line 20 ($HOME/DEV/log-log4perl-perl in this case). The command called in line 63, git remote add, points Git's default remote branch origin to the GitHub project to be created. Git uses this later to synchronize the local copy and the GitHub server version via push and pull.

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