Exposing the bluetoothd version over D-Bus

[Kevin Dewald <kevin@simpleble.org>]

Hi Luiz,

Quick question before I write any code: would you take a patch that
exposes the version of the running bluetoothd over D-Bus?

In SimpleBLE we use introspection to check which interfaces and
properties are available, and that works well for new APIs. Where it
falls short is behaviour changes behind the same API. For example,
since 5.88 Discovering no longer gets stuck after a failed
StopDiscovery, but a client can't tell whether the daemon it's talking
to has that fix. Knowing the version would let us apply workarounds
only where they're needed, and it would help with bug reports too.
BlueR asked for the same thing in
https://github.com/bluez/bluez/issues/619.

Running bluetoothd --version isn't reliable from a client, since the
binary on disk might not be the one running, or might not be reachable
from a container. The version in Adapter1.Modalias also changes or goes
away when DeviceID is set in main.conf.

I know it would only help from the release it lands in, but I think
it's still worth having going forward.

If you're open to it, where would you like it? Adapter1.Version is
already the controller version, and an adapter property wouldn't be
there when no adapter is present, so I was thinking of a read-only
property on a small new interface at /org/bluez. Happy to put it
wherever you prefer, with docs and a functional test.

Thanks,
Kevin