display-plugin-updates goal hang #354 investigation

[andrzejj0]

1. Stack traces are from maven-compat, not from maven-core
2. An old version of DisplayPluginUpdatesMojo specifically used the maven-compat interface:

PR mojohaus#538 switch to non-deprecated ModelInterpolator changed that.

The old model interpolator implements recursion using its own stack (interpolationTarget):

while ( !interpolationTargets.isEmpty() )
{
    Object obj = interpolationTargets.removeFirst();

    traverseObjectWithParents( obj.getClass(), obj );
}

The above code removes the currently processed element from interpolationTarget, just like a function call would do, but then places newly found elements to be processed into that collection.

This risks an infinite recursion loop if the analysed model contains cycles. So, it's possible that the function is fed a model with cycles in it.