Another classloader deadlock?

The java processes spawned by our Lucene nightly benchmarks sometimes randomly hang, apparently while loading classes across threads, under contention.

I've opened this luceneutil issue with some details, but uschindler suggested I open an issue here too since he has been seeing this in CI builds too.

It is rare, maybe once a week in the nightly benchmarks (which spawn many java processes with many threads across 128 CPU cores).  It is clearly a deadlock – when it strikes, the process hangs forever until I notice and kill -9 it.  I posted a coupled jstacks in the issue above.

rcmuir suggested using classcycle to maybe statically dig into possible deadlocks ... I have not tried that yet.