Resolve race condition for UNSUBACK

[ch4nsuk3]

When attempting to unsubscribe from a topic it was possible to receive the following error:

Traceback (most recent call last): File "code.py", line 79, in <module> File "/lib/adafruit_minimqtt/adafruit_minimqtt.py", line 869, in unsubscribe MMQTTException: ('invalid message received as response to UNSUBSCRIBE: 0x30', None)

This is due to the unsubscribe function assuming the next packet received will either be the UNSUBACK or an error code, rather than accounting for possible PUBLISH packets coming from the MQTT broker that may have already been in flight. Looking at some packet captures taken from the broker side its possible to get this error before the broker even receives the UNSUB request.

This fix essentially sets the unsubscribe function to match the existing behavior in the subscribe function, which ignores publish messages while waiting for the SUBACK.

Additionally I added named constant values for the SUBACK and UNSUBACK opcodes, replacing the 'magic number' values for comparing returned opcodes to increase clarity.

[vladak]

you might want to add a comment similar to the one in subscribe(), explain the rationale.

[vladak]

Looks good and make sense as The MQTT 3.1.1 spec says in section 3.10.4 Response:

If a Server deletes a Subscription:

• It MUST stop adding any new messages for delivery to the Client [MQTT-3.10.4-2].
• It MUST complete the delivery of any QoS 1 or QoS 2 messages which it has started to send to the Client [MQTT-3.10.4-3].
• It MAY continue to deliver any existing messages buffered for delivery to the Client.

So the last bullet applies to this case.

[ch4nsuk3]

Good call on the comment. Its been added.

[vladak]

It would be nice if the testdata packet sequences in https://github.com/adafruit/Adafruit_CircuitPython_MiniMQTT/blob/main/tests/test_unsubscribe.py were augmented by the PUBLISH+UNSUBACK case like it is done for the SUBSCRIBE case:

Adafruit_CircuitPython_MiniMQTT/tests/test_subscribe.py

Lines 93 to 137 in 62abc1a

	# SUBSCRIBE responded to by PUBLISH followed by SUBACK
	(
	"foo/bar",
	bytearray(
	[
	0x30, # PUBLISH
	0x0C,
	0x00,
	0x07,
	0x66,
	0x6F,
	0x6F,
	0x2F,
	0x62,
	0x61,
	0x72,
	0x66,
	0x6F,
	0x6F,
	0x90, # SUBACK
	0x03,
	0x00,
	0x01,
	0x00,
	]
	),
	bytearray(
	[
	0x82, # fixed header
	0x0C, # remaining length
	0x00,
	0x01, # message ID
	0x00,
	0x07, # topic length
	0x66, # topic
	0x6F,
	0x6F,
	0x2F,
	0x62,
	0x61,
	0x72,
	0x00, # QoS
	]
	),
	),

[ch4nsuk3]

I should be able to make these changes soon.