Keep Track of Active Notifiers. Make Notifier usable as ContextManager.

[zariiii9003]

• Make Notifier usable as a context manager to automatically call Notifier.stop(). Adapt examples to use with can.Notifier(...)
• Add a NotifierRegistry class to ensure that a bus is not added to multiple active Notifiers
• Fixes Unexpected behavior with multiple notifiers #1888

[pierreluctg]

@zariiii9003 can we make this NotifierRegistry a little more "public"?

For example we could have a Notifier factory (for a given Bus instance give me the existing Notifier or create a new one), etc.

[zariiii9003]

@zariiii9003 can we make this NotifierRegistry a little more "public"?

For example we could have a Notifier factory (for a given Bus instance give me the existing Notifier or create a new one), etc.

This would make things a bit more complicated.

• We could add a __new__ method to the Notifier where we check if a Notifier already exists and return that. But the Notifier can be instantiated with a list of BusABC objects, these might be already assigned to multiple other notifiers. And the listeners could be different in the existing instance, too. This might get difficult to handle, it's easier to just always raise an error in that case.
• The user should not directly call the register and unregister methods, so i'd rather keep the registry itself private.

However, a Notifier.find(bus) -> Notifier classmethod or something like that would be easy to add.

What kind of solution did you have in mind?

[bessman]

a Notifier.find(bus) -> Notifier classmethod

This would be great! If I understand correctly, without such a classmethod this PR in its current state does not solve the example I put forth in #1888 (comment). A Notifier with no remaining references won't be unregisterable unless there is some public way to access or search the registry.

[hardbyte]

Suggested change

	A :ref:`bus` or a list of buses to listen to.
	A :ref:`bus` or a list of buses to consume messages from.

[hardbyte]

self._bus_list won't be correct after a user adds an additional Bus via add_bus, instead prefer to create an empty bus list here in the initializer, but add them inside the add_bus method.

[zariiii9003]

Good catch. The same was true for self.bus, i changed that to a property.