Fix incorrect last sequence id when sending messages in batch

[zhanglistar]

Fixes #531

Problem

When sending messages with sendAsync and then flush(), getLastSequenceId() returns a wrong value when batching is enabled.

• After sending 3 messages (sequence ids [0, 1, 2]), the result is 4 instead of 2.
• After sending 5 messages, the result is 8 instead of 4.

The sequence id is effectively doubled per message in the batch.

Root cause

1. Batch metadata
   Commands::serializeSingleMessagesToBatchPayload used the last message's sequence_id (messages.back()) for the batch metadata. The client then assumed the broker ack referred to the first id and computed lastSequenceIdPublished_ = sequenceId + messagesCount - 1, which is wrong when the broker echoes or uses the last id.

2. Ack handling
   In ProducerImpl::ackReceived, the code assumed the broker always sends the first sequence id of the batch and set lastSequenceIdPublished_ = sequenceId + messagesCount - 1. When the broker sends the last sequence id (common for batch acks), this formula double-counts and yields the wrong last sequence id.

Solution

1. lib/Commands.cc
   In serializeSingleMessagesToBatchPayload, return the first message's sequence_id (messages.front()) for the batch metadata so the batch is identified by the first message's id.

2. lib/ProducerImpl.cc
   In ackReceived:

   • Treat the ack as matching the pending op when sequenceId is in [expectedFirstSequenceId, expectedLastSequenceId].
   • If the broker sent the first id: lastSequenceIdPublished_ = expectedLastSequenceId.
   • If the broker sent the last id: lastSequenceIdPublished_ = sequenceId.

3. tests/ProducerTest.cc
   Add testGetLastSequenceIdAfterBatchFlush to verify:

   • After 3 messages in a batch and flush(), getLastSequenceId() is 2.
   • After 5 messages total, getLastSequenceId() is 4.

How to verify

./tests/pulsar_tests --gtest_filter="ProducerTest.testGetLastSequenceIdAfterBatchFlush"