Github user serranom commented on the issue: https://github.com/apache/twill/pull/23 I found that there is still a race condition in ApplicationMasterService.launchRunnable. If the number of instances is increased right after the original request is fullfilled, the current logic can result in the original request not being polled resulting in future requests hanging. This seems to be a fairly unlikely case in the real world, but I'll file a JIRA for it. For now, I've reworked the test to wait until launchRunnable has done the polling for the original request. This PR is now complete.
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