On the road to libfprint and fprintd 2.0, we've been fixing some long-standing bugs, including one that required porting our PAM module from dbus-glib to sd-bus, systemd's D-Bus library implementation.
As you can imagine, I have confidence in my ability to write bug-free code at the first attempt, but the foresight to know that this code will be buggy if it's not tested (and to know there's probably a bug in the tests if they run successfully the first time around). So we will have to test that PAM module, thoroughly, before and after the port.
First, to make it easier to run and instrument, we needed to replace fprintd itself. For this, we used dbusmock, which is both a convenience Python library and way to write instrumentable D-Bus services, and wrote a template. There are a number of existing templates for a lot of session and system services, in case you want to test the integration of your code with NetworkManager, low-memory-monitor, or any number of other services.
We then used this to write tests for the command-line utilities, so we can both test our new template and test the command-line utilities themselves.
Now that we've got a way to replace fprintd and a physical fingerprint reader, we should write some tests for the (old) PAM module to replace sudo, gdm, or the login authentication services.
Co-workers Andreas Schneier and Jakub Hrozek worked on pam_wrapper, an LD_PRELOAD library to mock the PAM library, and Python helpers to write simple PAM services. This LWN article explains how to test PAM applications, and PAM modules.
After fixing a few bugs in pam_wrapper, and combining with the fprintd dbusmock work above, we could wrap and test the fprintd PAM module like it never was before.
Porting to sd-bus
Finally, porting the PAM module to sd-bus was pretty trivial, a loop of 1) writing tests that work against the old PAM module, 2) porting a section of the code (like the fingerprint reader enumeration, or the timeout support), and 3) testing against the new sd-bus based code. The result was no regressions that we could test for.
Both dbusmock, and pam_wrapper are useful tools in your arsenal to write tests, and given those (fairly) easy to use CIs in GNOME and FreeDesktop.org's GitLabs, it would be a shame not to.
You might also be interested in umockdev, to mock a number of device types, and mocklibc (which combined with dbusmock powers polkit's unattended CI)