HIVE-15803 fixes the msck hang issue in HiveMetaStoreChecker.checkPartitionDirs method by adding a check to see if the Threadpool has any spare threads. If not it uses single threaded listing of the files.
Based on the java doc of getActiveCount() below
Returns the approximate number of threads that are actively executing tasks.
it returns only approximate number of threads and it cannot be guaranteed that it always returns the exact number of active threads. This still exposes the method implementation to the msck hang bug in rare corner cases.
We could either:
1. Use a atomic counter to track exactly how many threads are actively running
2. Relook at the method itself to make it much simpler. Like eg, look into the possibility of changing the recursive implementation to an iterative implementation where worker threads pick tasks from a queue until the queue is empty.