Recently, during 0.92 rc testing, we found distributed log splitting hangs there forever. Please see attached screen shot.
I looked into it and here is what happened I think:
1. One rs died, the servershutdownhandler found it out and started the distributed log splitting;
2. All three tasks failed, so the three tasks were deleted, asynchronously;
3. Servershutdownhandler retried the log splitting;
4. During the retrial, it created these three tasks again, and put them in a hashmap (tasks);
5. The asynchronously deletion in step 2 finally happened for one task, in the callback, it removed one
task in the hashmap;
6. One of the newly submitted tasks' zookeeper watcher found out that task is unassigned, and it is not
in the hashmap, so it created a new orphan task.
7. All three tasks failed, but that task created in step 6 is an orphan so the batch.err counter was one short,
so the log splitting hangs there and keeps waiting for the last task to finish which is never going to happen.
So I think the problem is step 2. The fix is to make deletion sync, instead of async, so that the retry will have
a clean start.
Async deleteNode will mess up with split log retrial. In extreme situation, if async deleteNode doesn't happen
soon enough, some node created during the retrial could be deleted.
deleteNode should be sync.