Github user uce commented on the pull request:
https://github.com/apache/flink/pull/773#issuecomment-108628742
No, there are no backwards events *after* the channel is closed. The sync
sends out the backwards events, then closes. But the close could overtake the
unflushed backwards events. This lead to a deadlock, because the head was
waiting on termination events, which never arrived.
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