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When close(positive, TimeUnit.MILLISECONDS) is called, it will try to do a normal close first. If the normal close did not finish before timeout, it then close the producer forcefully. It will also wait the sender thread to finish.
If sender thread user calls close() in callback, it will block forever because the sender thread will try to join itself. So if a close() call is called from sender thread, an error message will be put in the log and close(0) will be called instead.
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