In test environment, our zookeeper version is old that our requirement version. Not use solr default 3.3.6 version.
The overseer collection processor stop work. Trace the dump, the overseer wait for LatchChildWatcher.await.
Check the zookeeper /overseer/collection-queue-work, block a lot of operation for collection.
Check the logic, suspect the zookeeper client doesn't call back the watchevent that register the path "/overseer/collection-queue-work", unlucky the log level is debug.
This case doesn't happen often, very little. But if it happen, it is fatal, we have to stop the leader server.
Suggest the compensate solution, that doesn't await until notify. Only wait some time that maybe it is ten minutes or a half of hour or other value to recheck the queue again. Of cause if get the notify, that can direct work normal.