One really nice thing about Chromium is its source code is open and released under the BSD license. This allows people to reuse code, extend the browser, or fully fork the project. Each of those are probably worthy of a blog post on its own, but I will focus only on the last one.
Taking Chromium and forking it is fairly easy process, just clone the repository. Make all the changes you would like to do - add missing features, include enhancements, create a totally new UI - it is only limited by one’s imagination. Building the binary from the source code is a little bit laborious, though not too hard. It does take beefy hardware and some time. Once it is built, publishing it is deceptively easy. However, what comes next?
Software in today’s world is not static. As a colleague of mine likes to say - it is almost like a living organism and continuously evolves. There is no shipping it as it was the norm in the ‘90s. The web is in a constant release mode and its model of development has trickled to client side software - be it desktop or mobile apps. Chromium has adopted this model from its initial release and is updating on a very short cycle - currently averaging six weeks between stable releases and two weeks between intermediate stable updates. It is this constant change that makes forking it a bit more challenging. However, there are few steps that one can take to ensure a smoother ride.
With constantly changing codebase, having a continuous build system is a must for project as big as Chromium and is very useful even for much smaller projects. Setting one up from the get go will be tremendously useful if there is more than one developer working on the code. Its value is even higher if the project needs to build on more than one platform.
What is more important and I would argue it is a must - using a continuous integration system. Running tests on each commit (or thereabout) to ensure there are no breaking changes. It is a requirement for any software project that needs to be in a position to release a new version at any point in time.
The system used in the Chromium project - buildbot - is actually open source and can be adapted to most projects.
The most important action one can take when forking Chromium is to study the design of the browser before diving in and making any changes. There are multiple components and layers involved, which interact through well defined interfaces. Understanding the architecture and the patterns used will pay off tremendously in the long run.
Chromium has two main component layers - content and chrome. The former is what implements the barebones of a browser engine - networking stack, rendering engine, browser kernel, multiprocess support, navigation and session history, etc. The chrome layer is built on top of content to implement the browser UI, extensions system, and everything else visible to the user that is not web content.
Each layer communicates with the upper ones through two main patterns - observer and delegate interfaces. Using those interfaces should be the preferred way of extending the browser and building on top of it. Whenever this is not possible, changes to the core are needed. I would strongly suggest preferring to upstream those, if possible of course. It will make maintaining the fork much easier by reduing the burden of keeping up with changes and also shares the improvements with the whole community!
Finally, do yourself a favor to keep you sane in the long run - write tests for all the features you are adding or changes made. It is the only way to ensure that long term the regressions and bug rate is manageable. It will save your sanity!
Keep it moving
The Chromium codebase changes constantly and gets around 100 commits each day. The sane way to keep up with the rate of change is to rebase (or merge) your code on tip-of-tree (ToT) daily or at most weekly. Letting more time lapse makes resolving conflicts a lot harder.
Updating the install base is key to long term success. The update cient used in Chrome on Windows, called Omaha, is also open source. The server side code is not available, though, since it depends heavily on how Google’s internal infrastructure is setup. However the protocol used to communicate between the client and the server is publicly documented.
Development for Chromium relies quite a bit on mailing lists. Subscribing to the two main ones - firstname.lastname@example.org and email@example.com - is very helpful. It is place where major changes are announced, discussion on development happens, and questions about Chromium development are answered. The security team has a dedicated list for discussions - firstname.lastname@example.org.
Keep it secure
Security is one of the core tenets of Chromium. Keeping up with security fixes can be a challenging task, which is best solved by keeping your code always rebased on tip-of-tree. If this is not possible, it is best to subscribe to the email@example.com list. It is the communication mechanism the security team uses to keep external projects based on Chromium up-to-date with all the security bugfixes happening in the project.
The web is moving more and more to a world without plugins. For me, this is a very exciting time, as plugins usually tend to weaken the browser security. There are two plugins bundled with Chromium to produce Chrome - Adobe Flash Player and a PDF viewer. The latter is now an open source project of its own - PDFium. It can be built and packaged with Chromium, though the same care should be taken as with the browser itself - keep it up-to-date.
Overall, maintaining a fork of Chromium isn’t trivial, but it isn’t impossible either. There are a bunch of examples, including the successful migration of the Opera browser from their own rendering engine to building on top of the Chromium content module.
Last, but not least - feel free to reach out and ask questions or advice.