Abstract

This PEP proposes setting up a new PSF provided resource, forge.python.org, as a location for maintaining various supporting repositories (such as the repository for Python Enhancement Proposals) in a way that is more accessible to new contributors, and easier to manage for core developers.

This PEP does not propose any changes to the core development workflow for CPython itself (see PEP 462 in relation to that).

PEP Withdrawal

This PEP has been withdrawn by the author in favour of the GitLab based proposal in PEP 507.

If anyone else would like to take over championing this PEP, contact the core-workflow mailing list

Proposal

This PEP proposes that an instance of the self-hosted Kallithea code repository management system be deployed as "forge.python.org".

Individual repositories (such as the developer guide or the PEPs repository) may then be migrated from the existing hg.python.org infrastructure to the new forge.python.org infrastructure on a case by case basis. Each migration will need to decide whether to retain a read-only mirror on hg.python.org, or whether to just migrate wholesale to the new location.

In addition to supporting read-only mirrors on hg.python.org, forge.python.org will also aim to support hosting mirrors on popular proprietary hosting sites like GitHub and BitBucket. The aim will be to allow users familiar with these sites to submit and discuss pull requests using their preferred workflow, with forge.python.org automatically bringing those contributions over to the master repository.

Given the availability and popularity of commercially backed "free for open source projects" repository hosting services, this would not be a general purpose hosting site for arbitrary Python projects. The initial focus will be specifically on CPython and other repositories currently hosted on hg.python.org. In the future, this could potentially be expanded to consolidating other PSF managed repositories that are currently externally hosted to gain access to a pull request based workflow, such as the repository for the python.org Django application. As with the initial migrations, any such future migrations would be considered on a case-by-case basis, taking into account the preferences of the primary users of each repository.

Rationale

Currently, hg.python.org hosts more than just the core CPython repository, it also hosts other repositories such as those for the CPython developer guide and for Python Enhancement Proposals, along with various "sandbox" repositories for core developer experimentation.

While the simple "pull request" style workflow made popular by code hosting sites like GitHub and BitBucket isn't adequate for the complex branching model needed for parallel maintenance and development of the various CPython releases, it's a good fit for several of the ancillary projects that surround CPython that we don't wish to move to a proprietary hosting site.

The key requirements proposed for a PSF provided software forge are:

• MUST support simple "pull request" style workflows
• MUST support online editing for simple changes
• MUST be backed by an active development organisation (community or commercial)
• MUST support self-hosting of the master repository on PSF infrastructure without ongoing fees

Additional recommended requirements that are satisfied by this proposal, but may be negotiable if a sufficiently compelling alternative is presented:

• SHOULD be a fully open source application written in Python
• SHOULD support Mercurial (for consistency with existing tooling)
• SHOULD support Git (to provide that option to users that prefer it)
• SHOULD allow users of git and Mercurial clients to transparently collaborate on the same repository
• SHOULD allow users of GitHub and BitBucket to submit proposed changes using the standard pull request workflows offered by those tools
• SHOULD be open to customisation to meet the needs of CPython core development, including providing a potential path forward for the proposed migration to a core reviewer model in PEP 462

The preference for self-hosting without ongoing fees rules out the free-as-in-beer providers like GitHub and BitBucket, in addition to the various proprietary source code management offerings.

The preference for Mercurial support not only rules out GitHub, but also other Git-only solutions like GitLab and Gitorious.

The hard requirement for online editing support rules out the Apache Allura/HgForge combination.

The preference for a fully open source solution rules out RhodeCode.

Of the various options considered by the author of this proposal, that leaves Kallithea SCM as the proposed foundation for a forge.python.org service.

Kallithea is a full GPLv3 application (derived from the clearly and unambiguously GPLv3 licensed components of RhodeCode) that is being developed under the auspices of the Software Freedom Conservancy. The Conservancy has affirmed that the Kallithea codebase is completely and validly licensed under GPLv3. In addition to their role in building the initial Kallithea community, the Conservancy is also the legal home of both the Mercurial and Git projects. Other SFC member projects that may be familiar to Python users include Twisted, Gevent, BuildBot and PyPy.

Intended Benefits

The primary benefit of deploying Kallithea as forge.python.org is that supporting repositories such as the developer guide and the PEP repo could potentially be managed using pull requests and online editing. This would be much simpler than the current workflow which requires PEP editors and other core developers to act as intermediaries to apply updates suggested by other users.

The richer administrative functionality would also make it substantially easier to grant users access to particular repositories for collaboration purposes, without having to grant them general access to the entire installation. This helps lower barriers to entry, as trust can more readily be granted and earned incrementally, rather than being an all-or-nothing decision around granting core developer access.

Sustaining Engineering Considerations

Even with its current workflow, CPython itself remains one of the largest open source projects in the world (in the top 2% of projects tracked on OpenHub). Unfortunately, we have been significantly less effective at encouraging contributions to the projects that make up CPython's workflow infrastructure, including ensuring that our installations track upstream, and that wherever feasible, our own customisations are contributed back to the original project.

As such, a core component of this proposal is to actively engage with the upstream Kallithea community to lower the barriers to working with and on the Kallithea SCM, as well as with the PSF Infrastructure team to ensure the forge.python.org service integrates cleanly with the PSF's infrastructure automation.

This approach aims to provide a number of key benefits:

• allowing those of us contributing to maintenance of this service to be as productive as possible in the time we have available
• offering a compelling professional development opportunity to those volunteers that choose to participate in maintenance of this service
• making the Kallithea project itself more attractive to other potential users by making it as easy as possible to adopt, deploy and manage
• as a result of the above benefits, attracting sufficient contributors both in the upstream Kallithea community, and within the CPython infrastructure community, to allow the forge.python.org service to evolve effectively to meet changing developer expectations

Some initial steps have already been taken to address these sustaining engineering concerns:

• Tymoteusz Jankowski has been working with Donald Stufft to work out what would be involved in deploying Kallithea using the PSF's Salt based infrastructure automation.
• Graham Dumpleton and I have been working on making it easy to deploy demonstration Kallithea instances to the free tier of Red Hat's open source hosting service, OpenShift Online. (See the comments on that post, or the quickstart issue tracker for links to Graham's follow on work)

The next major step to be undertaken is to come up with a local development workflow that allows contributors on Windows, Mac OS X and Linux to run the Kallithea tests locally, without interfering with the operation of their own system. The currently planned approach for this is to focus on Vagrant, which is a popular automated virtual machine management system specifically aimed at developers running local VMs for testing purposes. The Vagrant based development guidelines for OpenShift Origin provide an extended example of the kind of workflow this approach enables. It's also worth noting that Vagrant is one of the options for working with a local build of the main python.org website.

If these workflow proposals end up working well for Kallithea, they may also be worth proposing for use by the upstream projects backing other PSF and CPython infrastructure services, including Roundup, BuildBot, and the main python.org web site.

Personal Motivation

As of July 2015, I now work for Red Hat as a software development workflow designer and process architect, focusing on the upstream developer experience in Fedora. Two of the key pieces of that experience will be familiar to many web service developers: Docker for local container management, and Vagrant for cross-platform local development VM management. Spending time applying these technologies in multiple upstream contexts helps provide additional insight into what works well and what still needs further improvement to provide a good software development experience that is well integrated on Fedora, but also readily available on other Linux distributions, Windows, Mac OS X.

In relation to code review workflows in particular, the primary code review workflow management tools I've used in my career are Gerrit (for multi-step code review with fine-grained access control), GitHub and BitBucket (for basic pull request based workflows), and Rietveld (for CPython's optional pre-commit reviews).

Kallithea is interesting as a base project to build, as it's currently a combined repo hosting and code review management platform, but doesn't directly integrate the two by offering online merges. This creates the opportunity to blend the low barrier to entry benefits of the GitHub/BitBucket pull request model with the mentoring and task hand-off benefits of Gerrit in defining an online code merging model for Kallithea in collaboration with the upstream Kallithea developers.

Technical Concerns and Challenges

Introducing a new service into the CPython infrastructure presents a number of interesting technical concerns and challenges. This section covers several of the most significant ones.

Pilot Objectives and Timeline

[TODO: Update this section for Brett's revised timeline, which aims to have a CPython demo repository online by October 31st, to get a better indication of future capabilities once CPython itself migrates over to the new system, rather than just the support repos]

This proposal is part of Brett Cannon's current evaluation of improvement proposals for various aspects of the CPython development workflow. Key dates in that timeline are:

• Feb 1: Draft proposal published (for Kallithea, this PEP)
• Apr 8: Discussion of final proposals at Python Language Summit
• May 1: Brett's decision on which proposal to accept
• Sep 13: Python 3.5 released, adopting new workflows for Python 3.6

If this proposal is selected for further development, it is proposed to start with the rollout of the following pilot deployment:

• a reference implementation operational at kallithea-pilot.python.org, containing at least the developer guide and PEP repositories. This will be a "throwaway" instance, allowing core developers and other contributors to experiment freely without worrying about the long term consequences for the repository history.
• read-only live mirrors of the Kallithea hosted repositories on GitHub and BitBucket. As with the pilot service itself, these would be temporary repos, to be discarded after the pilot period ends.
• clear documentation on using those mirrors to create pull requests against Kallithea hosted Mercurial repositories (for the pilot, this will likely not include using the native pull request workflows of those hosted services)
• automatic linking of issue references in code review comments and commit messages to the corresponding issues on bugs.python.org
• draft updates to PEP 1 explaining the Kallithea based PEP editing and submission workflow

The following items would be needed for a production migration, but there doesn't appear to be an obvious way to trial an updated implementation as part of the pilot:

• adjusting the PEP publication process and the developer guide publication process to be based on the relocated Mercurial repos

The following items would be objectives of the overall workflow improvement process, but are considered "desirable, but not essential" for the initial adoption of the new service in September (if this proposal is the one selected and the proposed pilot deployment is successful):

• allowing the use of python-social-auth to authenticate against the PSF hosted Kallithea instance
• allowing the use of the GitHub and BitBucket pull request workflows to submit pull requests to the main Kallithea repo
• allowing easy triggering of forced BuildBot runs based on Kallithea hosted repos and pull requests (prior to the implementation of PEP 462, this would be intended for use with sandbox repos rather than the main CPython repo)

Future Implications for CPython Core Development

The workflow requirements for the main CPython development repository are significantly more complex than those for the repositories being discussed in this PEP. These concerns are covered in more detail in PEP 462.

Given Guido's recommendation to replace Rietveld with a more actively maintained code review system, my current plan is to rewrite that PEP to use Kallithea as the proposed glue layer, with enhanced Kallithea pull requests eventually replacing the current practice of uploading patche files directly to the issue tracker.

I've also started working with Pierre Yves-David on a custom Mercurial extension that automates some aspects of the CPython core development workflow.

Copyright

This document has been placed in the public domain.