The Network UPS Tools project will soon be a quarter of a century old and, being the first open multi-vendor power hardware monitoring solution for UPS, ePDU, Solar and such, it became the de facto standard in this area (de jure with RFC 9271 recently published). Over the years it was embedded in all sorts of server and desktop operating systems, from embedded to mainframe and NAS, and built with a wide array of toolkits and third-party dependencies. As evolution marches on, warnings are eradicated and features are added, the codebase is still expected to run on any platform released over the past couple of decades. If the machines and their OSes still run, modern NUT should too.
With one of my first goals as an active community member and eventually project maintainer, I set out to address the hundreds of compiler warnings emitted by different implementations and versions of toolkits during builds -- since they did raise valid concerns, and presence of those reports obscured visibility of new bugs introduced by contributions. In fact, several large and useful changes sat in the PR queue for years because nobody including authors had a good grasp on their reliability. This "fightwarn" effort took a couple of years with an in-house Jenkins farm for a project using NUT, and Travis CI (free for FOSS at that time) as well as one team members' BuildBot instance handling several versions of Linux for a few CPU architectures to cover different bitness and endianness, as well as a mix of GCC and LLVM CLANG. Quite a few issues were only relevant to certain generations of toolkits, C standard revisions, Make or Shell implementations, Autotools versions... and some fixes for one platform caused bugs for another. So to keep NUT working everywhere its iterations had to be regularly built everywhere.
Eventually the reign of free Travis CI ended, and NUT got VMs for a CI farm sponsored by Fosshost.org to proceed with the multi-platform testing. Using a custom Jenkins instance to take care of the project's codebase builds and make use of numerous operating systems in other VMs (as SSH Build Agents), as well as build agents contributed by community (e.g. Swarm agents), was a natural choice: by that time work had started on the https://github.com/networkupstools/jenkins-dynamatrix project. It is a Jenkins shared library which constructs a huge Map of parallel stages similar to a standard matrix build.
However in this case, it is not the matrix definition fully dictating what should be built, but rather the build agents are expected to report capabilities with their labels -- such as which platform they run and what versions and implementations of toolkits are available to test with, and whether "everything" can be built or just certain profiles (not all OS distributions offer all of the dependencies that NUT can build against). The library also helps determine which C or C++ language revisions can be built according to GCC or CLANG version installed. Whenever a build starts, the NUT Jenkinsfile (consumer of jenkins-dynamatrix library) can evaluate what it can build today -- based on currently known population of build agents, and so constructs the matrix for testing.
This arrangement allows NUT to be built on a multitude of platforms such that common free CI platforms do not offer. It is possible to find ways to build FOSS on recent Linux, MacOS and Windows, and sometimes even on non-x86 CPUs. It is nearly impossible to find builders for FreeBSD, OpenBSD or Solaris/illumos, to name a few, or on Linux distributions older than half a dozen years.
Every iteration of NUT is now tested with GNU, BSD and Sun Make implementations, with Shell helpers tested by Bash, Dash, Ksh, Busybox and others, with GCC starting from early 4.x releases and Clang from 3.x up to the dozenth releases out recently, across a dozen hardware platforms (some in QEMU), and several revisions of C/C++ standard with and without GNU extensions. All in all, with recently regularly connected builders about 150 stages are prepared (some running several build scenarios as scripted inside, so roughly a thousand builds happen). Sometimes a nuance warning causes one of these scenarios to complain during PR builds -- on this or that OS, this or that compiler. It keeps us confident both that such mechanism remains useful, and that any OS the community cares about enough to contribute a builder for remains fully supported.
With the hundreds of builds succeeding without warnings -- reliably, repeatably, every time -- the floodgates for ingestion of long-standing PRs were open. Largest beasts included the dual-citizenship with libusb-0.1 and libusb-1.x support (queued for 5 years) and revival of NUT for Windows effort (queued for 9 years), confidently merged without introducing warnings and regressions. The overwhelming majority of these builds happen on the custom "NUT CI" farm with Jenkins driving them. A few scenarios run on CircleCI and Appveyor to take advantage of some free FOSS builds on MacOS and Windows respectively.
Of notable interactions with the Jenkins community, this and related efforts led to improvements in the git-client-plugin to extend the use of reference repositories, instant-messaging-plugin and ircbot-plugin for querying Jenkins server status via IRC servers, lockable-resources plugin for many itches scratched here and there, and pipeline-groovy-lib-plugin for easier use of test library versions along with test pipeline branches. And very many lessons learned and friends made :)