Correct MQTT device trigger reconfiguration with same topic

[emontnemery]

Proposed change

Correct MQTT device trigger reconfiguration with same topic:

• If the reconfiguration does not change the topic, do not resubscribe+subscribe because it will happen in the wrong order.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes Automation trigger based on MQTT dicovery device works only once #36101

Checklist

• The code change is tested and works locally.
• Local tests pass. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.
• I have followed the development checklist
• The code has been formatted using Black (black --fast homeassistant tests)
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io

If the code communicates with devices, web services, or third-party tools:

• The manifest file has all fields filled out correctly.
  Updated and included derived files by running: python3 -m script.hassfest.
• New or updated dependencies have been added to requirements_all.txt.
  Updated by running python3 -m script.gen_requirements_all.
• Untested files have been added to .coveragerc.

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[probot-home-assistant]

Hey there @home-assistant/core, mind taking a look at this pull request as its been labeled with a integration (mqtt) you are listed as a codeowner for? Thanks!
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[emontnemery]

The unsubscribe is scheduled as a task:

core/homeassistant/components/mqtt/__init__.py

Lines 880 to 881 in e54bae4

	if self.connected:
	self.hass.async_create_task(self._async_unsubscribe(topic))

But the subscribe is not:

core/homeassistant/components/mqtt/__init__.py

Lines 865 to 866 in e54bae4

	if self.connected:
	await self._async_perform_subscription(topic, qos)

This asymmetry is not very intuitive.

Making the async_remove() a coroutine would break custom components, I'm not sure if that's OK.

If async_remove() is changed to return the task, it's not a breaking change, maybe that's better?
Edit: Making async_remove() return the task doesn't solve the asymmetry but at least makes it possible to have a predictable execution of the unsubscribe.

@MartinHjelmare, @balloob what do you think?

[balloob]

I think that returning the task is good improvement to make it work better.

Somehow back in the days I wanted all functions that integrations exposed never wait for the work to be done, instead of allowing the caller to decide to wait or not.