Experience on QA in the CernVM File System

<!--HTML-->The CernVM-File System (CVMFS) delivers experiment software installations to thousands of globally distributed nodes in the WLCG and beyond. In recent years it became a mission-critical component for offline data processing of the LHC experiments and many other collaborations. From a software engineering perspective, CVMFS is a medium-sized C++ system-level project. Following the growth of the project, we introduced a number of measures to improve the code quality, testability, and maintainability. In particular, we found very useful code reviews through github pull requests and automated unit- and integration testing. We are also transitioning to a test-driven development for new features and bug fixes. These processes are supported by a number of tools, such as Google Test, Jenkins, Docker, and others. We would like to share our experience on problems we encountered and on which processes and tools worked well for us.