Added topic alias support.

[redboltz]

TopicAlias lifetime is the same as Session lifetime
It is different from MQTT v5 spec but practical choice.
See
https://lists.oasis-open.org/archives/mqtt-comment/202009/msg00000.html

Related fix:

When topic is empty and no topic alias entry is found, then
protocol_error.
The broker send DISCONNECT packet with protocol_error reason code.

process_disconnect implementation was simply doesn't call
on_mqtt_message_processed() but it was bad.
It caused exit from ioc.run() loop because the next async_read was not
called.
The correct behavior is calling on_mqtt_message_processed() and close
socket. The endpoint can detect the socket close by on_error().

[codecov]

Codecov Report

Merging #660 into master will increase coverage by 0.55%.
The diff coverage is 90.78%.

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
+ Coverage   82.13%   82.68%   +0.55%     
==========================================
  Files          45       46       +1     
  Lines        7012     7076      +64     
==========================================
+ Hits         5759     5851      +92     
+ Misses       1253     1225      -28     

[jonesmz]

I'm not sure that this is actually a safe change to make.

What happens if an MQTT client attempts to bind the topic alias a second time?

Could there be MQTT clients that attempt to use a topic alias upon first connecting to test that the broker does not have the topic alias?

[redboltz]

I'm not sure that this is actually a safe change to make.

What happens if an MQTT client attempts to bind the topic alias a second time?

The exising topic - topic_alias mapping is overritten.

The spec said that as follows:

A sender can modify the Topic Alias mapping by sending another PUBLISH in the same Network Connection with the same Topic Alias value and a different non-zero length Topic Name.

See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901113

Could there be MQTT clients that attempt to use a topic alias upon first connecting to test that the broker does not have the topic alias?

No, it is a protocol violation.
The spec said that as follows:

3. If the receiver does not already have a mapping for this Topic Alias

a) If the packet has a zero length Topic Name field it is a Protocol Error and the receiver uses DISCONNECT with Reason Code of 0x82 (Protocol Error) as described in section 4.13.

See https://docs.oasis-open.org/mqtt/mqtt/v5.0/os/mqtt-v5.0-os.html#_Toc3901120

[redboltz]

sequence	publish(topic, alias)	result
1	"",1	protocol error (entry not found)

sequence	publish(topic, alias)	result
1	"topic1",1	register "topic1" - 1
2	"",1	find "topic1" by topic alias 1
3	"topic2",1	register(overwrite) "topic2" - 1
4	"",1	find "topic2" by topic alias 1

[redboltz]

I will merge the PR tomorrow morning (JST).