Make EspFlasher cancellable to prevent resource leak

Add @Volatile cancelled flag with cancel() method. Check flag at 3
checkpoints (after initial sleep, after transport setup, before each
flash step) so the thread exits early and closes the USB connection
in the finally block. SettingsActivity.onDestroy now calls cancel().

Reviewed in codereview.md (recommendation 3.2).

android/app/src/main/java/com/eddy/atomrelay/EspFlasher.kt

@@ -25,7 +25,15 @@ class EspFlasher(private val context: Context) {
 
     private val handler = Handler(Looper.getMainLooper())
 
+    @Volatile
+    private var cancelled = false
+
+    fun cancel() {
+        cancelled = true
+    }
+
     fun flashDevice(usbManager: UsbManager, device: UsbDevice, callback: FlashCallback) {
+        cancelled = false
         Thread {
             try {
                 doFlash(usbManager, device, callback)
@@ -47,6 +55,7 @@ class EspFlasher(private val context: Context) {
 
     private fun doFlash(usbManager: UsbManager, device: UsbDevice, callback: FlashCallback) {
         Thread.sleep(500)
+        if (cancelled) return
 
         val connection = usbManager.openDevice(device)
             ?: throw IOException("Kan USB-apparaat niet openen")
@@ -67,6 +76,7 @@ class EspFlasher(private val context: Context) {
             postProgress(callback, 0, "Verbinden met ESP bootloader...")
 
             val transport = UsbSerialTransport(connection, epOut, epIn)
+            if (cancelled) return
 
             val assets = context.assets
             val bootloader = assets.open("firmware/bootloader.bin").readBytes()
@@ -107,6 +117,7 @@ class EspFlasher(private val context: Context) {
             stub.eraseFlash()
 
             for (step in steps) {
+                if (cancelled) return
                 val sizeStr = if (step.data.size > 1024) "${step.data.size / 1024}KB" else "${step.data.size} bytes"
                 postProgress(callback, step.baseProgress, "${step.name} schrijven ($sizeStr)...")
 

android/app/src/main/java/com/eddy/atomrelay/SettingsActivity.kt

@@ -43,6 +43,7 @@ class SettingsActivity : AppCompatActivity() {
     private var currentAction = UsbAction.NONE
     private var progressDialog: AlertDialog? = null
     private var dialog: AlertDialog? = null
+    private var flasher: EspFlasher? = null
     private val handler = Handler(Looper.getMainLooper())
 
     private val usbAttachedReceiver = object : BroadcastReceiver() {
@@ -120,6 +121,8 @@ class SettingsActivity : AppCompatActivity() {
     override fun onDestroy() {
         super.onDestroy()
         currentAction = UsbAction.NONE
+        flasher?.cancel()
+        flasher = null
         try { unregisterReceiver(usbAttachedReceiver) } catch (e: Exception) {}
         try { unregisterReceiver(usbPermissionReceiver) } catch (e: Exception) {}
         try { unregisterReceiver(usbDetachedReceiver) } catch (e: Exception) {}
@@ -203,8 +206,8 @@ class SettingsActivity : AppCompatActivity() {
     private fun doFlashing(usbManager: UsbManager, device: UsbDevice) {
         showProgressDialog("Firmware Flashen", "Initialiseren...", STYLE_HORIZONTAL)
 
-        val flasher = EspFlasher(this)
+        flasher = EspFlasher(this)
-        flasher.flashDevice(usbManager, device, object : EspFlasher.FlashCallback {
+        flasher?.flashDevice(usbManager, device, object : EspFlasher.FlashCallback {
             override fun onProgress(percent: Int, message: String) {
                 runOnUiThread {
                     val d = progressDialog ?: return@runOnUiThread