Re: Request for help with btmgmt

[Barry Byford <31baz66@gmail.com>]

Hello Keith,

On Wed, 23 Dec 2020 at 00:34, KeithG <ys3al35l@gmail.com> wrote:
>
> I am developing a RPi based audio player. We are trying to add
> bluetooth capability to this player. We use alsa and not pulse-audio,
> so we need a bit more granular info to get it to run. I have written a
> script which is designed to be triggered by udev when a BT device is
> added or removed. This script will then use bluetoothctl to get the
> capabilities of the device and also add or remove it from a dynamic
> database list of MACs in redis. I have a simple script working as I
> want from the cli. The key command I am using is 'btmgmt con' which is
> the only command I have found that tells me what MACs are currently
> connected. I can run this command from the command line as root. I can
> set the setuid bit of btmgmt and also get it to run as a user (http in
> this case). I cannot, ever, get this command to run when triggered
> from udev. I get no response and no error.
>
> 1) Why is this particular command (query attached BT MACs) not part of
> bluetoothctl?  Is there a security issue?
> 2) Why can I not run 'btmgmt con' in a script (bash or php) triggered
> by udev when the device is added or removed?
> 3) is there a 'better way' for me to know what device MAC addresses
> are attached? I have not found another simple way to get this info.
>
> What I am doing in the script is: if an audio source is attached, I
> want to run a bluealsa-aplay service. When an audio sink is attached,
> I want to re-set my audio output to the bluealsa device. If it is not
> an audio device, I do not care, but do keep track of what is attached
> in my database so that the next time the script runs that it is not
> queried and is 'known'.
>
> Thanks for the help.
>
> Keith

I would suggest that the D-Bus API would be a more appropriate way to
get the information you want.

The "get managed objects" functionality is a good way to iterate
through all the devices that BlueZ knows about.
There are examples at:
https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/test/bluezutils.py
https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/test/list-devices

The D-Bus API also allows your code to execute callbacks when a device
is added or its connection state changes.
This is done with the InterfacesAdded and PropertiesChanged signals.

If you are going to use Python I would suggest the
https://pypi.org/project/pydbus/ library is a more "Pythonic" set of
D-Bus bindings.

The API is documented at:
https://git.kernel.org/pub/scm/bluetooth/bluez.git/tree/doc

Regards,
Barry