MQTT camera can't use encoding: b64 with availability topic

[mdegat01]

The problem

If you configure an MQTT camera with encoding: b64 then you cannot provide an availability_topic. If you do when the integration attempts to read messages from the availability topic then you receive errors like this:

Logger: homeassistant
Source: components/mqtt/client.py:586
First occurred: 6:21:14 PM (14 occurrences)
Last logged: 6:21:29 PM

    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5ef80>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5f060>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5f300>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff87f78040>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8d381690>)

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "/usr/src/homeassistant/homeassistant/components/mqtt/client.py", line 586, in _mqtt_handle_message
    payload = msg.payload.decode(subscription.encoding)
LookupError: unknown encoding: b64

As you can see it tries to provide that value to paho mqtt which does not understand it. Removing the availability_topic fixes it because this config option is handled correctly for normal messages to the cameras topic here:

core/homeassistant/components/mqtt/camera.py

Lines 127 to 128 in 04c6b9c

	if self._config[CONF_ENCODING] == "b64":
	self._last_image = b64decode(msg.payload)

What version of Home Assistant Core has the issue?

core-2022.7.6

What was the last working version of Home Assistant Core?

No response

What type of installation are you running?

Home Assistant OS

Integration causing the issue

camera.mqtt

Link to integration documentation on our website

https://www.home-assistant.io/integrations/camera.mqtt/

Diagnostics information

No response

Example YAML snippet

mqtt:
  camera:
  - name: Family Room alert images test
    availability:
      - topic: blueiris/family-room/state
    topic: "blueiris/family-room/alert-image-b64"
    icon: mdi:cctv
    encoding: b64

Anything in the logs that might be useful for us?

Logger: homeassistant
Source: components/mqtt/client.py:586
First occurred: 6:21:14 PM (14 occurrences)
Last logged: 6:21:29 PM

    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5ef80>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5f060>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8bc5f300>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff87f78040>)
    Error doing job: Exception in callback MQTT._mqtt_handle_message(<paho.mqtt.cl...xffff8d381690>)

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/asyncio/events.py", line 80, in _run
    self._context.run(self._callback, *self._args)
  File "/usr/src/homeassistant/homeassistant/components/mqtt/client.py", line 586, in _mqtt_handle_message
    payload = msg.payload.decode(subscription.encoding)
LookupError: unknown encoding: b64

Additional information

No response

[probot-home-assistant]

mqtt documentation
mqtt source
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[probot-home-assistant]

Hey there @emontnemery, mind taking a look at this issue as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!
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[jbouwh]

The problem is that the encoding parameter is used in two ways. For availability it should be utf-8 which is the default.
The linked PR will introduce a new image_encoding parameter to set the image encoding.
Setting encoding to b64 will work too with this fix, but is deprecated and will give a warning message in the log.