[DISCUSS] Return the message ID of the first chunk when sending chunked messages

[Zike Yang]

Hi Pulsar Community,


Currently, when we send chunked messages, the producer returns the
message-id of the last chunk. This can cause some problems. For example,
when we use this message-id to seek, it will cause the consumer to consume
from the position of the last chunk, and the consumer will mistakenly think
that the previous chunks are lost and choose to skip the current message.
If we use the inclusive seek, the consumer may skip the first message,
which brings the wrong behavior.


Here is the simple code used to demonstrate the problem.

```

var msgId = producer.send(...); // eg. return 0:1:-1

var otherMsg = producer.send(...); // return 0:2:-1

consumer.seek(msgId); // inclusive seek

var receiveMsgId = consumer.receive().getMessageId(); // it may skip the
first message and return like 0:2:-1

Assert.assertEquals(msgId, receiveMsgId); // fail

```


To fix this, I think we could return the message ID of the first chunk when
sending chunked messages. I would like to know if this solution will bring
other problems. Any ideas on this?


Thanks
-- 
Zike Yang