Koji is an RPM-based build system. The Fedora Project uses Koji for their build system, as do several other projects.
Koji's goal is to provide a flexible, secure, and reproducible way to build software.
If you have found a bug or would like to request a new feature, please report an issue in Pagure.
The koji source code can be downloaded with git via:
git clone https://pagure.io/koji.git
You may browse code at https://pagure.io/koji
Archived releases can be found at https://fedorahosted.org/released/koji/