Unable to start NiFi when controller service references other controller services

[Mark Bean <ma...@gmail.com>]

We are having issues when starting NiFi with a custom controller service
which allows dynamic properties. The dynamic properties are themselves
additional controller services. Enabling the custom controller service
after NiFi is running is fine. But, if the service is enabled on startup,
NiFi fails to start correctly and the UI is never presented.

When starting, as controller services are attempted to be enabled, they
first determine if there are referencing services required to be enabled
first. In the process of enabling each of these, the referenced controller
service is validated (so that its state can change from invalid to disabled
once all referencing services are enabled.)

However, the process never completes. It consistently stops at the exact
same point on each NiFi start attempt. The custom controller services
enables 6 of 19 referenced services, but fails on the 7th.

The thread enabling the controller service (created by
StandardControllerServiceNode) appears to simply stop.This occurs when
attempting to access the proxy controller service in
ControllerServiceDetails. I cannot explain why though.

Specific question: why does the thread hang when accessing the proxy
controller service?

General question: what purpose does the proxy controller service serve
versus the actual implementation?

Apache 1.10.0 (with patch applied for NIFI-6852)

Thanks,
Mark