[BRLTTY] Repeated Bluetooth pairing request on Android

[Robert Pösel]

Hi,

I am having annoing problem when using BRLTTY on Android with Braille 
device connected via Bluetooth. I'm using Humanware Brailliant BI 40 
which I paired with Android phone and then in BRLTTY settings I added 
and selected this device and everything works as expected. Then when I 
turn off the Braille device, BRLTTY after few seconds correctly realizes 
it's disconnected and plays a sound. But when I turn the Braille device 
on again, most of the time it shows the pairing dialog to pair the 
device again. I have tested different Android devices with different 
Android versions (Android 7.0 and Android 7.1.1) and the issue is 
happening on all of them.

There are 2 interesting things that I realized:

1) It doesn't unpair all the time, but it happens only some times. It 
feels like it is dependend on the specific moment when I turn on the 
device, but I'm unable to really determine what is the trigger. On 
average it happens in 50 % of attempts.

2) It seems to work correctly (without repeated pairing request) in 
BrailleBack app. I see as a difference that in BrailleBack I need to 
initiate connection to the Braille device manually via click in the 
options, while in BRLTTY app it tries to connect automatically every few 
seconds. So I thought that BRLTTY may somehow interrupt or cancel the 
paired state of the Braille device resulting in Android showing pairing 
dialog again.

And during writing of this e-mail I've found the issue and solution. 
Problem was indeed in BRLTTY code, namely in the 
BluetoothConnection.open() method. There is check "if (channel == 0)" 
based on which it uses either direct call on device object, or it uses 
reflection to call some hidden method on the device object. I don't 
understand the purpose of this channel argument and whole condition, 
could you clarify why it was written this way? In my case channel value 
was 1, so it was calling the hidden method via reflection and was 
causing the problems with pairing dialog I described. When I removed the 
if check and always used the correct call (as if channel value was 0), 
everything works correctly and I've experienced no pairing dialog ever 
again.

I think that if-block and reflection call to that hidden method should 
be removed completely.

Best regards
Robert Pösel