boolean isEndOfStream() {
      return isEndOfStream;
    }
This class manages the end-of-stream state of the streams in a task. Internally it keeps track of end-of-stream
messages received from upstream tasks for each system stream partition (ssp). If messages have been received from
all tasks, it will mark the ssp as end-of-stream. For a stream to be end-of-stream, all its partitions assigned to
the task need to be end-of-stream.

This class is thread-safe.
samza-core/src/main/java/org/apache/samza/operators/impl/EndOfStreamStates.java