HIVE-15093 we have encountered deadlocks in the s3a connector. The TransferManager javadocs (http://docs.aws.amazon.com/AWSJavaSDK/latest/javadoc/com/amazonaws/services/s3/transfer/TransferManager.html) explain how this is possible:
It is not recommended to use a single threaded executor or a thread pool with a bounded work queue as control tasks may submit subtasks that can't complete until all sub tasks complete. Using an incorrectly configured thread pool may cause a deadlock (I.E. the work queue is filled with control tasks that can't finish until subtasks complete but subtasks can't execute because the queue is filled).