The car can learn which paired phone is on the cable

A VW MIB2 drops an already paired phone's Bluetooth when Gearslip
connects over USB, because nothing tells it the two are the same phone
(#28). Android Auto says so on the car's Bluetooth channel with the
phone's address, which Android hides from apps. Settings now takes the
address typed in by hand; with it, Gearslip opens that channel on every
connection to a paired car and sends the address. Without it, nothing
changes.

gearslip/src/main/java/app/seb3thehacker/gearslip/car/CarBluetooth.kt

@@ -40,6 +40,11 @@ import java.security.MessageDigest
  * phone's own Bluetooth address (apps get a placeholder), and confirming the pairing code without
  * asking. So a car may turn the request down, and the phone always shows its pairing dialog. The
  * channel stays closed unless the driver taps Pair: every new channel is a risk to cars that work.
+ *
+ * The one exception is a driver who typed the phone's real address into Settings. A VW MIB2 drops
+ * an already paired phone's Bluetooth when it connects over USB, because nothing tells it the two
+ * are the same phone (issue #28). With the address, Gearslip opens the channel on every connection
+ * and sends the same request, which is how Android Auto says so.
  */
 object CarBluetooth {
 
@@ -63,13 +68,15 @@ object CarBluetooth {
     @Volatile private var bondStarted = false
     /** From the Pair tap until it succeeds or fails, whether or not the card still shows. */
     @Volatile private var pairing = false
+    /** Already paired: the channel is open only to tell the car which phone this is. */
+    @Volatile private var announcing = false
     private var receiver: BroadcastReceiver? = null
 
     /** The car's Bluetooth address, when it named one in its service list. */
     val carAddress: String? get() = car?.service?.carAddress
 
     /** The car's Bluetooth channel once the driver has asked to pair, -1 otherwise. */
-    val channelId: Int get() = car?.takeIf { pairing }?.service?.serviceId ?: -1
+    val channelId: Int get() = car?.takeIf { pairing || announcing }?.service?.serviceId ?: -1
 
     fun init(context: Context) {
         appContext = context.applicationContext
@@ -83,9 +90,15 @@ object CarBluetooth {
     ) {
         car = Car(service, open, send)
         bondStarted = false
+        announcing = false
         log.i("the car offers Bluetooth pairing, methods=${service.pairingMethods}")
         val device = device() ?: return
         when {
+            device.bondState == BluetoothDevice.BOND_BONDED && typedAddress() != null -> {
+                log.i("already paired with this car - telling it which phone this is")
+                announcing = true
+                open()
+            }
             device.bondState == BluetoothDevice.BOND_BONDED -> log.i("already paired with this car")
             declined(service.carAddress) -> log.i("not paired, and the driver said not now for this car")
             else -> _prompt.value = Prompt.ASK
@@ -95,6 +108,7 @@ object CarBluetooth {
     fun onCarDisconnected() {
         car = null
         pairing = false
+        announcing = false
         main.post {
             stopWatching()
             _prompt.value = Prompt.NONE
@@ -139,7 +153,8 @@ object CarBluetooth {
                 val status = Protobuf.readInt32Field(body, 1) ?: 0
                 log.i("<- ChannelOpenResponse: status=$status")
                 if (status != 0) {
-                    main.post { startBond("the car wouldn't open its Bluetooth channel") }
+                    if (announcing) log.i("the car wouldn't open its Bluetooth channel")
+                    else main.post { startBond("the car wouldn't open its Bluetooth channel") }
                     return
                 }
                 val method = c.service.pairingMethods.firstOrNull { it == METHOD_NUMERIC_COMPARISON }
@@ -153,6 +168,7 @@ object CarBluetooth {
             MSG_PAIRING_RESPONSE -> {
                 val fields = Wire.fields(body)
                 log.i("<- PairingResponse status=${Wire.varint(fields, 1)} already_paired=${Wire.varint(fields, 2) == 1L}")
+                if (!pairing) return
                 // Some cars start pairing themselves once they answer; give them a moment first.
                 main.postDelayed({ startBond("the car answered") }, CAR_BOND_WAIT_MS)
             }
@@ -229,12 +245,30 @@ object CarBluetooth {
         return adapter.getRemoteDevice(address)
     }
 
-    /** Apps get 02:00:00:00:00:00 here, not the real address. Sent anyway, as the field is required. */
+    /**
+     * Apps get 02:00:00:00:00:00 here, not the real address, so the one the driver typed in wins.
+     * Sent either way, as the field is required. Never logged: it identifies the phone.
+     */
     @SuppressLint("MissingPermission", "HardwareIds")
     private fun phoneAddress(): String =
-        runCatching { appContext?.getSystemService(BluetoothManager::class.java)?.adapter?.address }.getOrNull()
+        typedAddress()
+            ?: runCatching { appContext?.getSystemService(BluetoothManager::class.java)?.adapter?.address }.getOrNull()
             ?: "02:00:00:00:00:00"
 
+    private fun typedAddress(): String? = appContext?.let { typedAddress(it) }
+
+    /** The phone's own address as the driver typed it in Settings, or null. */
+    fun typedAddress(context: Context): String? =
+        AppSettings.getString(context, KEY_PHONE_ADDRESS, "").takeIf { BluetoothAdapter.checkBluetoothAddress(it) }
+
+    /** Saves [address] after tidying it up; false when it isn't one. Empty clears it. */
+    fun setTypedAddress(context: Context, address: String): Boolean {
+        val tidy = address.trim().uppercase().replace('-', ':')
+        if (tidy.isNotEmpty() && !BluetoothAdapter.checkBluetoothAddress(tidy)) return false
+        AppSettings.putString(context, KEY_PHONE_ADDRESS, tidy)
+        return true
+    }
+
     private fun declined(address: String): Boolean {
         val context = appContext ?: return false
         return hash(address) in AppSettings.getString(context, KEY_DECLINED, "").split(",")
@@ -246,6 +280,7 @@ object CarBluetooth {
             .take(8).joinToString("") { "%02x".format(it) }
 
     private const val KEY_DECLINED = "bluetooth_pair_declined"
+    private const val KEY_PHONE_ADDRESS = "phone_bluetooth_address"
     private const val CAR_REPLY_WAIT_MS = 5_000L
     private const val CAR_BOND_WAIT_MS = 3_000L
     private const val GIVE_UP_MS = 90_000L

gearslip/src/main/java/app/seb3thehacker/gearslip/ui/SettingsScreen.kt

@@ -20,6 +20,8 @@ import androidx.compose.foundation.rememberScrollState
 import androidx.compose.foundation.selection.selectable
 import androidx.compose.foundation.selection.toggleable
 import androidx.compose.foundation.text.KeyboardOptions
+import androidx.compose.ui.text.input.KeyboardCapitalization
+import app.seb3thehacker.gearslip.car.CarBluetooth
 import androidx.compose.foundation.verticalScroll
 import androidx.compose.material.icons.Icons
 import androidx.compose.material.icons.automirrored.filled.ArrowBack
@@ -246,9 +248,55 @@ private fun ConnectionSection() {
             icon = Icons.Filled.Home,
             trailing = { Switch(checked = launcher, onCheckedChange = null) },
         )
+        var address by remember { mutableStateOf(CarBluetooth.typedAddress(context)) }
+        var editing by remember { mutableStateOf(false) }
+        SettingsRow(
+            stringResource(R.string.settings_bt_address),
+            Modifier.clickable { editing = true },
+            subtitle = address ?: stringResource(R.string.settings_bt_address_off),
+            icon = Icons.Filled.Info,
+        )
+        if (editing) {
+            AddressDialog(
+                initial = address.orEmpty(),
+                onDismiss = { editing = false },
+                onSave = { typed ->
+                    CarBluetooth.setTypedAddress(context, typed).also { saved ->
+                        if (saved) {
+                            address = CarBluetooth.typedAddress(context)
+                            editing = false
+                        }
+                    }
+                },
+            )
+        }
     }
 }
 
+/** The phone's Bluetooth address, typed in by hand: Android doesn't let apps read it. */
+@Composable
+private fun AddressDialog(initial: String, onDismiss: () -> Unit, onSave: (String) -> Boolean) {
+    var text by remember { mutableStateOf(initial) }
+    var invalid by remember { mutableStateOf(false) }
+    AlertDialog(
+        onDismissRequest = onDismiss,
+        title = { Text(stringResource(R.string.settings_bt_address)) },
+        text = {
+            OutlinedTextField(
+                value = text,
+                onValueChange = { text = it; invalid = false },
+                singleLine = true,
+                placeholder = { Text("12:34:56:78:9A:BC") },
+                keyboardOptions = KeyboardOptions(capitalization = KeyboardCapitalization.Characters, autoCorrectEnabled = false),
+                isError = invalid,
+                supportingText = { Text(stringResource(if (invalid) R.string.settings_bt_address_invalid else R.string.settings_bt_address_hint)) },
+            )
+        },
+        confirmButton = { TextButton(onClick = { invalid = !onSave(text) }) { Text(stringResource(R.string.save)) } },
+        dismissButton = { TextButton(onClick = onDismiss) { Text(stringResource(R.string.cancel)) } },
+    )
+}
+
 /** Tools for testing a debug build; release builds never show this section. */
 @Composable
 private fun DebugSection() {

gearslip/src/main/res/values/strings.xml

@@ -866,6 +866,10 @@
     <string name="status_no_car_graphene">The phone is charging, but no car has answered. Check that USB-C port control, under Security &amp; privacy, allows data and isn\'t set to Charging-only.</string>
     <string name="status_stuck_cable">The car hasn\'t shown Gearslip yet. Unplug the cable, wait a few seconds, and plug it back in. If it keeps happening, try another cable or USB port, and share the log in an issue.</string>
     <string name="status_stuck_wireless">The car hasn\'t shown Gearslip yet. Turn the car off and on again, or connect with a cable. If it keeps happening, share the log in an issue.</string>
+    <string name="settings_bt_address">Phone\'s Bluetooth address</string>
+    <string name="settings_bt_address_off">Not set. Only for a car that drops Bluetooth when Gearslip connects.</string>
+    <string name="settings_bt_address_hint">Copy it from Android\'s Settings, under About phone. Gearslip passes it to the car, so the car knows the phone on the cable is the one it\'s paired with.</string>
+    <string name="settings_bt_address_invalid">That isn\'t a Bluetooth address. It looks like 12:34:56:78:9A:BC.</string>
     <string name="settings_use_as_launcher">Use as launcher</string>
     <string name="settings_use_as_launcher_off">For a head unit or a tablet on the dash. Gearslip\'s car screen becomes this device\'s home screen.</string>
     <string name="settings_use_as_launcher_on">Gearslip can be this device\'s home screen. Pick it under Default home app.</string>