fix(android-ble): prevent ANR by moving blocking Bluetooth calls off main thread

rns-host/src/main/kotlin/network/columba/app/rns/host/ble/client/BleGattClient.kt

@@ -13,11 +13,10 @@ import android.bluetooth.BluetoothProfile
 import android.content.Context
 import android.content.pm.PackageManager
 import android.os.Build
-import android.os.Handler
-import android.os.Looper
 import android.util.Log
 import androidx.core.content.ContextCompat
 import network.columba.app.rns.host.ble.model.BleConstants
+import network.columba.app.rns.host.ble.util.BleDispatchers
 import network.columba.app.rns.host.ble.util.BleOperationQueue
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
@@ -206,7 +205,7 @@ class BleGattClient(
      * @return Result with Unit on success, exception on failure
      */
     suspend fun connect(address: String): Result<Unit> =
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 // Check if already connected
                 connectionsMutex.withLock {
@@ -297,14 +296,14 @@ class BleGattClient(
             }
 
             val connData =
-                withContext(Dispatchers.Main) {
+                withContext(BleDispatchers.ble) {
                     connectionsMutex.withLock {
                         connections.remove(address)
                     }
                 }
 
             if (connData != null) {
-                withContext(Dispatchers.Main) {
+                withContext(BleDispatchers.ble) {
                     connData.connectionJob?.cancel()
                     safeGattTeardown(connData.gatt, "manual disconnect $address")
                 }
@@ -446,7 +445,7 @@ class BleGattClient(
                 }
 
                 // Request high connection priority for better stability and throughput
-                withContext(Dispatchers.Main) {
+                withContext(BleDispatchers.ble) {
                     try {
                         val priorityResult =
                             gatt.requestConnectionPriority(
@@ -461,11 +460,11 @@ class BleGattClient(
                 // Small delay to let BLE stack apply parameters
                 delay(100)
 
-                // Discover services (post to main thread for older Android versions)
-                withContext(Dispatchers.Main) {
-                    Handler(Looper.getMainLooper()).post {
-                        gatt.discoverServices()
-                    }
+                // Discover services off the main thread: on Android 13+ discoverServices()
+                // is a synchronous binder call that can block for seconds when the
+                // Bluetooth stack is congested, which caused Background ANRs.
+                withContext(BleDispatchers.ble) {
+                    gatt.discoverServices()
                 }
             }
 
@@ -523,7 +522,7 @@ class BleGattClient(
             connectionsMutex.withLock {
                 connections.remove(address)
             }
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 safeGattTeardown(gatt, "service discovery failed status=$status")
             }
 
@@ -544,7 +543,7 @@ class BleGattClient(
             connectionsMutex.withLock {
                 connections.remove(address)
             }
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 safeGattTeardown(gatt, "Reticulum service not found")
             }
 
@@ -568,7 +567,7 @@ class BleGattClient(
             connectionsMutex.withLock {
                 connections.remove(address)
             }
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 safeGattTeardown(gatt, "required characteristics not found")
             }
 
@@ -853,7 +852,7 @@ class BleGattClient(
                 }
 
             if (connData != null) {
-                withContext(Dispatchers.Main) {
+                withContext(BleDispatchers.ble) {
                     safeGattTeardown(connData.gatt, "failed to enable notifications")
                 }
             }
@@ -1081,7 +1080,7 @@ class BleGattClient(
 
         val connData = connectionsMutex.withLock { connections.remove(address) }
         if (connData != null) {
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 safeGattTeardown(connData.gatt, "connection timeout")
             }
             onConnectionFailed?.invoke(address, "Connection timeout")

rns-host/src/main/kotlin/network/columba/app/rns/host/ble/client/BleScanner.kt

@@ -17,6 +17,7 @@ import androidx.core.content.ContextCompat
 import network.columba.app.rns.host.ble.model.BleConstants
 import network.columba.app.rns.host.ble.model.BleDevice
 import network.columba.app.rns.host.ble.model.BlePowerSettings
+import network.columba.app.rns.host.ble.util.BleDispatchers
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
@@ -144,7 +145,7 @@ class BleScanner(
      * @return Result indicating success or failure
      */
     suspend fun startScanning(minRssi: Int = BleConstants.MIN_RSSI_DBM): Result<Unit> {
-        return withContext(Dispatchers.Main) {
+        return withContext(BleDispatchers.ble) {
             try {
                 // Check if already scanning
                 if (_isScanning.value) {
@@ -206,7 +207,7 @@ class BleScanner(
             scanJob?.cancel()
             scanJob = null
 
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 if (_isScanning.value && bluetoothLeScanner != null) {
                     bluetoothLeScanner.stopScan(scanCallback)
                     _isScanning.value = false
@@ -248,7 +249,7 @@ class BleScanner(
      * Perform a single scan.
      */
     private suspend fun performScan(minRssi: Int) {
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 val scanner = bluetoothLeScanner ?: return@withContext
 

rns-host/src/main/kotlin/network/columba/app/rns/host/ble/server/BleAdvertiser.kt

@@ -15,6 +15,7 @@ import android.util.Log
 import androidx.core.content.ContextCompat
 import network.columba.app.rns.host.ble.model.BleConstants
 import network.columba.app.rns.host.ble.model.BlePowerSettings
+import network.columba.app.rns.host.ble.util.BleDispatchers
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.Job
@@ -142,7 +143,7 @@ class BleAdvertiser(
      * @return Result indicating success or failure
      */
     suspend fun startAdvertising(deviceName: String = BleConstants.DEFAULT_DEVICE_NAME_PREFIX): Result<Unit> =
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 // Check if already advertising
                 if (_isAdvertising.value) {
@@ -243,7 +244,7 @@ class BleAdvertiser(
             // Stop refresh job first
             stopRefreshJob()
 
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 if (_isAdvertising.value && bluetoothLeAdvertiser != null) {
                     bluetoothLeAdvertiser.stopAdvertising(advertiseCallback)
                     _isAdvertising.value = false
@@ -366,7 +367,7 @@ class BleAdvertiser(
         isRefreshing = true
         try {
             // Stop current advertising
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 try {
                     bluetoothLeAdvertiser?.stopAdvertising(advertiseCallback)
                 } catch (e: Exception) {
@@ -377,7 +378,7 @@ class BleAdvertiser(
             delay(100) // Brief delay for cleanup
 
             // Restart advertising
-            withContext(Dispatchers.Main) {
+            withContext(BleDispatchers.ble) {
                 startAdvertisingInternal()
             }
             Log.d(TAG, "Advertising refreshed successfully")

rns-host/src/main/kotlin/network/columba/app/rns/host/ble/server/BleGattServer.kt

@@ -18,6 +18,7 @@ import android.os.Build
 import android.util.Log
 import androidx.core.content.ContextCompat
 import network.columba.app.rns.host.ble.model.BleConstants
+import network.columba.app.rns.host.ble.util.BleDispatchers
 import kotlinx.coroutines.CompletableDeferred
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
@@ -262,7 +263,7 @@ class BleGattServer(
      * @return Result indicating success or failure
      */
     suspend fun open(): Result<Unit> =
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 // Check if already open
                 if (_isServerOpen.value) {
@@ -358,7 +359,7 @@ class BleGattServer(
      * Close the GATT server.
      */
     suspend fun close() =
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 // Clear connected centrals
                 centralsMutex.withLock {
@@ -431,7 +432,7 @@ class BleGattServer(
                 IllegalStateException("TX characteristic not available"),
             )
 
-        return withContext(Dispatchers.Main) {
+        return withContext(BleDispatchers.ble) {
             try {
                 if (!hasConnectPermission()) {
                     return@withContext Result.failure(
@@ -492,7 +493,7 @@ class BleGattServer(
      * We must manually clean up state and fire the disconnect callback.
      */
     suspend fun disconnectCentral(address: String) =
-        withContext(Dispatchers.Main) {
+        withContext(BleDispatchers.ble) {
             try {
                 if (!hasConnectPermission()) {
                     Log.w(TAG, "Cannot disconnect central, missing permission")
@@ -670,7 +671,7 @@ class BleGattServer(
         requestId: Int,
         offset: Int,
         characteristic: BluetoothGattCharacteristic,
-    ) = withContext(Dispatchers.Main) {
+    ) = withContext(BleDispatchers.ble) {
         try {
             if (!hasConnectPermission()) {
                 return@withContext
@@ -735,7 +736,7 @@ class BleGattServer(
         responseNeeded: Boolean,
         offset: Int,
         value: ByteArray,
-    ) = withContext(Dispatchers.Main) {
+    ) = withContext(BleDispatchers.ble) {
         try {
             if (!hasConnectPermission()) {
                 return@withContext
@@ -827,7 +828,7 @@ class BleGattServer(
         requestId: Int,
         offset: Int,
         descriptor: BluetoothGattDescriptor,
-    ) = withContext(Dispatchers.Main) {
+    ) = withContext(BleDispatchers.ble) {
         try {
             if (!hasConnectPermission()) {
                 return@withContext
@@ -869,7 +870,7 @@ class BleGattServer(
         responseNeeded: Boolean,
         offset: Int,
         value: ByteArray,
-    ) = withContext(Dispatchers.Main) {
+    ) = withContext(BleDispatchers.ble) {
         try {
             if (!hasConnectPermission()) {
                 return@withContext

rns-host/src/main/kotlin/network/columba/app/rns/host/ble/util/BleDispatchers.kt

@@ -0,0 +1,22 @@
+package network.columba.app.rns.host.ble.util
+
+import kotlinx.coroutines.CoroutineDispatcher
+import kotlinx.coroutines.Dispatchers
+
+/**
+ * Dispatchers for Android Bluetooth framework calls.
+ *
+ * Since Android 13, many framework Bluetooth APIs (`BluetoothLeScanner.startScan`/`stopScan`,
+ * `BluetoothGatt.discoverServices`/`requestConnectionPriority`/`disconnect`,
+ * `BluetoothLeAdvertiser.startAdvertising`, `BluetoothGattServer.sendResponse`, ...) are
+ * synchronous binder calls that block the caller via `SynchronousResultReceiver` until the
+ * Bluetooth process replies. When the Bluetooth stack is congested (e.g. heavy RNode GATT
+ * writes) these calls can block for seconds, so they must NEVER run on the main thread or the
+ * process gets a (Background) ANR.
+ *
+ * [ble] is a single-threaded view of [Dispatchers.IO]: it keeps the serialized, one-at-a-time
+ * semantics the code previously got from [Dispatchers.Main], without blocking the main looper.
+ */
+object BleDispatchers {
+    val ble: CoroutineDispatcher = Dispatchers.IO.limitedParallelism(1)
+}