I have encountered a bug whereby the shade plugin gets stuck in an infinite loop in the step of creating a dependency reduced POM.
I have attached a minimal POM that reproduces the issue and done some debugging on this. The problem seems to be caused by using both the jar and test-jar version of a dependency (jena-fuseki in my example) while also including a jar dependency that is a transitive dependency of the other dependencies (jena-arq in my example). Removing either the jar/test-jar resolves the problem and so does removing the other jar dependency.
The problem appears to be that the plugin is getting stuck endlessly trying to produce the dependency-reduced-pom.xml
If I watch the file over time it is repeatedly deleted and rewritten, each time it is written it gets larger. When I manage to kill the process at a point where the file is non-empty I can see that the plugin is endlessly adding the same set of <exclusions> to one of the jena-fuseki dependencies.