Current implementation of DBCPConnectionPool was attempting to test if connection could be obtained via dataSource.getConnection().
Such call is naturally a blocking call and the duration of the block is dependent on driver implementation. Some drivers (e.g., Phoenix - https://phoenix.apache.org/installation.html)
attempts numerous retries before failing creating a deadlock when attempt was made to disable DBCPConnectionPool which was still being enabled.
This fix removes the connection test from DBCPConnectionPool.onConfigured() operation returning successfully upon creation of DataSource.
For more details see comments in https://issues.apache.org/jira/browse/NIFI-1061
Oleg Zhurakousky
joewitt