Multiple services/characteristics with the same UUID should not be an error

[AGlass0fMilk]

• bleak version: 0.9.1
• Python version: 3.9.0
• Operating System: Ubuntu 20.04
• BlueZ version (bluetoothctl -v) in case of Linux: 5.53

Description

There is no universal requirement in the BT specification that a GATT Server cannot have multiple instances of the same service in its profile, or multiple instances of the same characteristic in a service. This restriction may be specified by a GATT Service specification, but it is not the default nature of GATT services.

Currently, bleak stores services in the client object inside a dictionary using the UUID as the key. This innately disallows multiple services with the same UUID. In fact, it raises an exception here:

bleak/bleak/backends/service.py

Lines 107 to 117 in d7e9558

	def add_service(self, service: BleakGATTService):
	"""Add a :py:class:`~BleakGATTService` to the service collection.
	Should not be used by end user, but rather by `bleak` itself.
	"""
	if service.uuid not in self.__services:
	self.__services[service.uuid] = service
	else:
	raise BleakError(
	"This service is already present in this BleakGATTServiceCollection!"
	)

I am implementing a DFU Service for Mbed OS and I was planning to use bleak as the platform for a desktop-based update tool. Through our discussion of the DFU Service specification, we want to allow multiple DFU Services to exist on the GATT Server so that different embedded devices within the same product may be updated over BLE.

This limitation of bleak will make it impossible to support multiple DFU services on the same GATT server. Another example of a service (one that is BT SIG specified, in fact) that allows multiple instances in this way is the common Battery Service:

The suggested fix would be to:

1. Change from storing any attributes (services/characteristics/descriptors, etc) in a map using their UUID as a key. The key should be the handle of the attribute.
2. Implement new functions get_services_by_uuid(), get_characteristics_by_uuid(), and so on to return Lists of BleakGattService, BleakGattCharacteristic, ...
3. Deprecate the get_service(), get_descriptor(), etc functions and in the mean time have them return the first occurrence of a service/descriptor by UUID (ie: return `get_services_by_uuid(uuid)[0])

[hbldh]

Characteristics and descriptors already use handles, but for services I was not sure I could get handles in a good fashion in all OS backends, so I let that remain as uuids. I assumed that no one would ever want to split characteristics over two different services with the same uuid...

I haven’t got time to fix this in the foreseeable future, but feel free to have a go by yourselves! It does seem to be an error in implementation since it apparently says that it should be allowed in some spec.

[AGlass0fMilk]

Characteristics and descriptors already use handles, but for services I was not sure I could get handles in a good fashion in all OS backends, so I let that remain as uuids. I assumed that no one would ever want to split characteristics over two different services with the same uuid...

I haven’t got time to fix this in the foreseeable future, but feel free to have a go by yourselves! It does seem to be an error in implementation since it apparently says that it should be allowed in some spec.

I'll take a look into fixing this as I develop my DFU tooling. Hopefully the service handles are available on all OS's... I do not have a Mac to test with but I can test on Ubuntu 20.04 and Windows 10.