Adapter.Connect is not thread-safe on Mac OS

[erik]

When running adapter.Connect calls concurrently, I've noticed that connection attempts to a non-existent address sometimes successfully return with the data of other devices.

Minimal repro:

adapter := bluetooth.DefaultAdapter
wg := sync.WaitGroup()

retryConnect := func(addr bluetooth.Addresser, device *bluetooth.Device) {
  wg.Add(1)
  // Keep retrying until we successfully connect
  for {
    if d, err := adapter.Connect(addr, bluetooth.ConnectionParams{}); err == nil {
        *device = d
        break
    }
    println("Retrying connection to", addr)  
  }
  wg.Done()
}

var dev1, dev2 bluetooth.Device

go retryConnection("addr1", &dev1)
go retryConnection("addr2", &dev2)

wg.Wait()

// At this point, dev1 and dev2 may be swapped or the same device.

In the Mac OS implementation of Adapter, newly connected devices are sent over a channel which is shared between all callers (Adapter.connectChan). In Adapter.Connect, we then unconditionally take the first device sent across this channel and return it.

This can lead to a data race in the case where we have multiple goroutines connecting to different devices at once, resulting in adapter.Connect returning successfully with the wrong device.