RE: Comments on the ConnectDevice API function

[Martin Woolley <mwoolley@bluetooth.com>]

Hi Luiz

thanks for your response. Much appreciated. I must confess this is the first time I've used the BlueZ APIs directly (via D-Bus). 

I'm all for hiding implementation details where possible in APIs and making the API itself take care of conditional aspects if possible, but these were just my $0.02, no more than that. I wasn't aware of the background or the philosophy, so thanks for that insight as well.

Having the API take care of the AlreadyExists error by providing a connection to the already discovered device, transparently does seem a nice touch for the application developer but certainly not essential.

All the best

Martin

-----Original Message-----
From: Luiz Augusto von Dentz <luiz.dentz@gmail.com> 
Sent: 22 May 2020 18:49
To: Martin Woolley <mwoolley@bluetooth.com>
Cc: Linux-bluetooth@vger.kernel.org
Subject: Re: Comments on the ConnectDevice API function

Hi Martin,

On Fri, May 22, 2020 at 1:25 AM Martin Woolley <mwoolley@bluetooth.com> wrote:
>
> Hello
>
> I've recently been working with BlueZ via D-Bus and have a situation which requires me to be able to connect to a device whose Bluetooth device address is known, but without first scanning. This is a link layer state transition with the specification allows.
>
> BlueZ currently supports this via an API adapter function called ConnectDevice, whose status is currently "experimental". From my experience of using this function, it seems to behave like this:
>
> If the BlueZ instance has not scanned yet, so that the target device is not known to it, the ConnectDevice call results in scanning taking place and then if the target device is found, it is connected to. Success!
>
> But if scanning has previously been performed, regardless of the state of the actual device (e.g. advertising and ready to accept connections), an exception is thrown with a message whose text value is "Already Exists".
>
> I was wondering if I could influence the design of the API before the ConnectDevice experimental status is removed?
>
> I would like to suggest that there should be no need for a special API to connect directly to a device without first scanning. Why burden the application developer needing to call it just in case this condition applies, catching the BlueZ exception ("Already Exists") and responding by then calling the normal Connect API?

I guess the intention was to have the application use the intended API for devices already present _before_ calling ConnectDevice, so before entering the address manually the application would enumerate the existing devices and figure out if that was already present.

> An alternative would be to accommodate this special case (not scanned before) in the implementation of the standard device Connect(bdaddr) function or if that makes no sense because Device objects must correspond to previously discovered, physical devices, then at least the adapter ConnectDevice function could take care of the two possible paths and simplify matters for the application developer.

I guess you probably know this but just in case someone look at the archives it is better that we make some things clearer, while the core spec allows connecting without scanning D-Bus are intend to be a higher level API thus why ConnectDevice was not really necessary for a long time and we just introduced it for qualification purpose or when there are multiple adapter where one acts as scanner. Also ever since the introduction of privacy (random addresses) APIs that takes addresses becomes rather complicated to be used directly, and there exists ways to scan for a specific address with pattern filtering:
https://nam12.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit.kernel.org%2Fpub%2Fscm%2Fbluetooth%2Fbluez.git%2Ftree%2Fdoc%2Fadapter-api.txt%23n122&amp;data=02%7C01%7C%7Cfafcb0f940054867612508d7fe7866fb%7Ce4e0fec5fc6c4dd6ae374bdb30e156b9%7C0%7C0%7C637257665400787125&amp;sdata=Iy%2FwWkxs%2FyW3gL239FLWdoDRGa0apb63WxMhYwRoneM%3D&amp;reserved=0

That said I don't oppose to remove Already Exists error, but we should be very clear that the use of such API should only be recommended with users input and does not substitute the likes of Device.Connect.

--
Luiz Augusto von Dentz