Google Summer of Code 2009

For the last few years, Google has offered a fantastic opportunity for students to help out Open Source software projects in the summer while getting paid for it. It's called Google Summer of Code™, and it provides free software projects a great way of attracting development effort while providing software developers who are still in university with some interesting and useful experiences. Find out more about the Summer of Code (SoC) from [http://code.google.com/summerofcode.html their site].

Mercurial is a Distributed Version Control System (DVCS), and we are one of the mentoring organizations for 2008's Summer of Code (see also http://code.google.com/soc/2008/hg/about.html). In recent years (notably since the Linux kernel project started looking for a new VCS), DVCSs have rapidly started to gain traction. For Open Source projects, especially, the use of a distributed system can be a great enabler, in that it's much easier to keep track of your own changes and publish them back to the community in a structured way. With centralized systems, you have a central repository that only a happy few have (write) access to, and people spend a lot of time mailing around diffs. With distributed systems, everyone has their own branch, and it's very easy to publish it on the web, compare the changes to those on some other branch or merge two branches back together.

We believe distributed version control is the future, and it looks like many other technologists believe the same. There's some healthy competition between Open Source VCS systems. In one corner, there's the old CVS and the better, but still centralized and complex Subversion, and in the other corner, there are three main contenders in the distributed VCS space: git, Bazaar-NG and Mercurial. The competition for users is fierce and stimulates fast-paced development in all three systems. It's therefore important (and good for the competition) that projects like ours gain more developers, to keep up with the race for robustness, performance and features. We view the Summer of Code as a great opportunity to help us in this competition.

Written largely in Python (with the exceptions of a few performance-critical core modules), it is easy to develop for and allows for good code organization and clean abstractions. Moreover, it has been optimized from the start for good performance, using an append-only datastore, minimizing random disk access and smart uses of compression. Finally, it has extensive features for sharing changesets, over HTTP, SSH and in files over email and has an innovative extension (Mercurial Queues) to help building and organizing changesets into a thoroughly useful source history.

TableOfContents

1. Project Ideas

Here are a bunch of project ideas you might like to apply for. Of course, if you have a different idea of something in Mercurial that badly needs fixing or some feature you think would make a difference, go ahead and apply with it! Some more project ideas can be found via NewFeatureDiscussions, CategoryNewFeatures and NewIdeas.

1.1. Instantaneous status on Windows, OS X

The InotifyExtension makes a huge difference to performance on moderate to large [:Repository:repositories] on Linux. Since Windows and Mac OS X provide file status notification APIs, it should be possible to port the inotify extension to one or other (or both) of these platforms, providing the same kinds of speed improvements as on Linux.

1.2. Partial cloning

Currently, it's only possible to clone one whole repository at a time. PartialClone and TrimmingHistory could help make cloning more efficient by limiting the cloning process in either of two dimensions: time or space. For time, we could maybe clone the last few [:ChangeSet:changesets] and lazily fetch the rest as needed. For space, it would be nice if it was possible to clone just a subtree of any repositories. For these features, any number of thorny issues can arise because of current assumptions in Mercurial code.

1.3. Lightweight copies/renames

Copies and renames currently are not light-weight. Mercurial copies the copied/renamed source file to the new initial revision of the target file in its internal history store. For renames, this is especially counter-intuitive, as renaming a large file grows the store by the file's size. It would be better if Mercurial had some way of referring to the existing revision from the new file, while preserving its bounded I/O guarantees for retrieving revisions.

1.4. Interactive patch selection for commit/Mercurial Queues/record/import

Being able to select parts of the existing changes, with hunk or greater granularity, in an interactive way can improve a lot the use of commands and extensions that take changes, as commit, mercurial queues, record or import. Record currently allows patch hunk selection, but sometimes a better granularity is desired, as when a set of adjacent function definitions should go in different commits. This feature could be added as an --interactive mode for some commands.

1.5. Subrepositories

The ForestExtension implements a solution for repositories that want to include a number of subrepositories (similar to svn:externals). For large projects and because DVCS systems in general advocate smaller repositories, it can be helpful to implement a coherent set of repositories. A [NestedRepositories subrepositories implementation] is in the works but it needs further polish, and integration into the mercurial core, as well as extended functionality, as the capability to use other VCS sources.

1.6. TortoiseHg

TortoiseHg is a GUI front-end, similar to TortoiseCVS and TortoiseSVN. For many people, this makes interacting with Mercurial much easier. There's a lot of room for improvement, adding graphical UI to interesting extensions like pbranch and others. The project keeps a [http://bitbucket.org/tortoisehg/crew/wiki/TODO TODO list] of suitable tasks for a Summer of Code.

1.7. GUI integration

Mercurial is currently lacking features to support integration into a GUI, like TortoiseHg or Mercurial Eclipse. One example of a feature that would benefit GUI is [http://www.nabble.com/Progress-bar-on-network-actions-tt20596124.html#a20596124 a progress indicator] on operations that may take some time

1.8. Conversion tools

Mercurial is a relatively new entrant in the VCS market, and many projects are still using older VCSs such as CVS and SVN. While we currently have some tools to help migrate to Mercurial in the form of the ConvertExtension, these tools could certainly use more improvements. Specifically, enabling the use of Mercurial as a client for SVN or even git and/or Bazaar-NG repositories would be very nice, as it enables developers to make their own choice regarding the use of their VCS client, thereby drastically enlarging our userbase.

The hgsubversion extension already works quite well, but better integration with the existing mercurial user interface would be nice.

1.9. C version of some lower-level operations

It should be possible to extract a large performace improvement from writing a C version of some basic operations. The main candidate for this would be the code that handles the index of a [:RevlogNG:revlog]; other parts of the revlog implementation (like the heads operation that is used in the WireProtocol and when calculating tags) could also benefit quite a bit.

Some work has already been done at http://sharesource.org/project/hgc/

2. Notes on applying

Here are some tips on what you might want to include in your application.

Tell us something about yourself. We don't know you, so it helps if you give an outline of your background and what your prior experiences are (e.g. Open Source development, using Python, general software engineering experience or education). But don't hesitate to apply just because this would be your first time working in Open Source or with Python; at some point this was new for each of us.
Make it clear that you've thought through your application. Don't use a project idea from this page verbatim. Instead, come up with your own proposal, or expand on a proposal from this page. Explore the code a little bit (it's Python, quite easy to read), read CategoryContributing to get a feel for how the community works. Make something of a schedule with some intermediate milestones that progress toward your project's goal.
Communicate why you care, what you like about DVCSs in general and Mercurial specifically, what things you think could be improved and how they could be improved. We like working on this project because this project makes our work more effective, and we hope you'll like it, too. Showing your motivation helps.
Get feedback on your proposal from the community. The MailingLists and IRC (#mercurial on irc.freenode.net) are quite responsive. Ask around and get to know some people, see if they think your project is feasible or how you should change the scope to better fit the timeline and the project. (If you are new to Open Source development, reading [http://producingoss.com/en/communications.html#you-are-what-you-write this part of "Producing OSS"] may help you find the right tone.)

3. Getting things done

Some idea of how we would expect the project to be carried out.

Working on Mercurial in the summer should be your main activity. Having a vacation for one or two weeks is fine, of course, but we want you to take the project and the time mentors put into it seriously. This also means that we want you to set some intermediate milestones to be able to keep track of your progress.
We want you to work in the open, with our community. Get on the MailingLists, both to ask for help and to provide it to other users, spend some time on IRC (#mercurial on irc.freenode.net) discussing your work with other developers and explaining Mercurial to all the new users coming in with questions. Set up a public repository with an MQ containing your patches against the [:CrewRepository:crew repository] (on [http://freehg.org/ freehg.org], for instance; see also MercurialHosting).
We're not going to just compare what you did at the end to what you stated you'd be doing in the beginning. We want you to put effort into the project, to think about the feature you're doing, to communicate with the community and integrate your code with the project. If you end up implementing some other cool feature or fixing some annoyance, that is great as well.

4. Mentors

[wiki:mpm MattMackall] (mpm on IRC; creator of Mercurial)
Alexis S. L. Carvalho (asak on IRC)
PeterArrenbrecht (parren on IRC)
DirkjanOchtman (djc on IRC)
AugieFackler (durin42 on IRC)

SummerOfCode