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Checking D-Bus API stability

Posted on 02/06/2015 by Philip Withnall

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.

Why?

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.)

How?

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.

What?

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.

Where?

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.

What next?

API stability checking for GIR APIs, perhaps? This one needs some more work.

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