rpm-mock-build job is made to be attached to a dist-git repository. But when build_from_source is set then the job can handle that the project source repository is also a dist-git repository (source + included specfile).

Here the idea is to handle a build_from_source where the source is elsewhere.

https://pagure.io/zuul-distro-jobs/blob/master/f/zuul.d/jobs.yaml#_3

To achieve that in addition to 'build_from_source' var we could have another var 'source'.
'source' is an object that describe the external source such as:
{
'clone_target_dir': path where the source land on the testnode,
'clone_url': url to clone the source
}

'clone_url' if specified will tell the job to clone 'clone_url' into 'clone_target_dir'
if not set, it means the job must have a required-project set (the source) and 'clone_target_dir'
set to the directory where zuul has cloned the external source.

We will then support cases where the external source repository is a known project to Zuul and benefit from depends-on. But also if the external source is not attached to Zuul, the job will clone the master branch.

Use cases:
periodic job on a fedora dist-git to validate package still build with upstream source code
check pipeline job on a dist-git with depends-on on a source known to zuul