June 02, 2015
Announcing dbus-deviation, a small tool and set of libraries for automatically checking whether a D-Bus interface has broken API between two releases of a piece of software, developed as part of my work at Collabora.
If you have a large software project, worked on by multiple developers, it might not be clear when D-Bus interfaces change. For example, they might be pulled in from another repository, or might be accidentally changed without anyone noticing.
Breaks in the D-Bus API of a project (when it’s supposed to be stable) are potentially worse than breaks in its C API, because they can only be detected at runtime — when client applications suddenly error out half-way through an operation because they’ve called a D-Bus method with the wrong argument type. At least with C API breaks, the compiler will catch the break.
(In this respect, I guess D-Bus APIs are actually a form of ABI — a runtime interface, rather than a compile-time interface.)
dbus-deviation provides a utility called dbus-interface-diff, plus some GNU Make glue to plug it into your build system. It only works with git: for each tagged release of your project, it uses git-notes to store copies of all the D-Bus interfaces you care about, in their state at the time of that release. They’re stored as introspection XML; if you have that XML committed into the repository anyway, the git-note becomes a ref to the existing file blob, and takes up virtually no space at all. The dbus-interface-diff tool then does a diff between two XML files (for example, one stored for the most recent release, and the one currently in your working tree), and flags up any forwards- or backwards-incompatibilities.
A backwards-incompatibility, as far as dbus-deviation is concerned, is one where existing clients will not work against new versions of the D-Bus service, for example because a method they use has been removed.
A forwards-incompatibility is one where new versions of clients may not work against old versions of the D-Bus service, for example because they use a method which has been added in a new version of the API.
Traditionally, projects care about preventing backwards-incompatible API changes, and don’t care so much about forwards-incompatibilities. dbus-deviation lets you set your desired stability policy.
dbus-deviation has a spartan website, a git repository, and bugs are stored using Bugs Everywhere in git; contact me in the comments or by e-mail if you want to report something.API documentation is available for the Python libraries underpinning it, which provide an AST and diff methods for D-Bus APIs.
To get using it, follow the instructions in the README file!
All feedback is very much welcome. One area I feel is still a little awkward is how dbus-deviation integrates with make dist — it forces use of a pre-push git hook to update the remote git-notes for the API signatures of newly pushed release tags. That needs to be set up by each developer who releases a project (using make dist) — any suggestions for improving this are welcome.
API stability checking for GIR APIs, perhaps? This one needs some more work.
PipeWire continues to evolve with the recent integration of libcamera, a library to support complex cameras. In this blog post, I'll explain…
A high-level introduction of the Linux graphics stack, how it is used within ChromeOS, and the work done to improve software rendering (while…
Last year, a (controversial) feature was added to the Linux kernel to support optimized case-insensitive file name lookups in the Ext4 filesystem.…
We have now integrated Mali GPU hardware counters supported by Panfrost with Perfetto's tracing SDK, unlocking all-in-one graphics-aware…
Key performance improvements and fixes to GStreamer's RTP stack have landed in GStreamer 1.18, due in the coming months. The latest enhancements…
Following our recent presentation at OSSummit, many showed interest in learning more about solving real-world problems with computer vision.…