fix(jni): set app cache dir as temp dir and wrap callbacks in Handler

- Set app cache dir as temp directory for mkstemp() to avoid EACCES
  on Android sandbox (iperf_set_test_template).
- Wrap all MethodChannel callbacks in MainThreadCallback with
  Handler(Looper.getMainLooper()) to prevent SIGABRT from calling
  invokeMethod from native reader thread.
- Add setTempDir JNI function and logging for test return value.

android/app/src/main/cpp/iperf_jni.c

@@ -36,6 +36,7 @@
 /* Global state for controlling the iperf test */
 static struct iperf_test *g_test = NULL;
 static volatile int g_stop = 0;
+static char g_tmp_template[1024] = "/data/local/tmp/iperf3.XXXXXX";
 
 /* Per-call state to pass between JNI and the reader thread.
  * We store the callback object (global ref) and method IDs.
@@ -283,7 +284,7 @@ Java_com_novoyuuparosk_iperf3gui_IperfRunner_runIperfLive(
         char errbuf[256];
         snprintf(errbuf, sizeof(errbuf),
                  "Argument parse error: %s", iperf_strerror(i_errno));
-        LOGE("%s", errbuf);
+        LOGE("Parse error: %s", errbuf);
         free_argv(argv, total_argc);
         iperf_free_test(test);
         g_test = NULL;
@@ -339,6 +340,12 @@ Java_com_novoyuuparosk_iperf3gui_IperfRunner_runIperfLive(
     test->outfile = state->outfile;
     setvbuf(test->outfile, NULL, _IOLBF, 0); /* Line-buffered */
 
+    /* Set temp directory for stream buffers (mkstemp in iperf_new_stream).
+     * On Android, /data/local/tmp is NOT writable from the app sandbox —
+     * use the app's cache dir instead. iperf_set_test_template() handles
+     * the strdup internally. */
+    iperf_set_test_template(test, g_tmp_template);
+
     /*
      * Set up signal handling BEFORE calling iperf_run_client().
      * iperf3's run() function uses setjmp/longjmp for signal handling.
@@ -374,6 +381,7 @@ Java_com_novoyuuparosk_iperf3gui_IperfRunner_runIperfLive(
         /* Run the test */
         g_stop = 0;
         int ret = iperf_run_client(test);
+        LOGI("iperf_run_client returned: %d i_errno=%d strerror=%s ctrl_sck=%d", ret, i_errno, iperf_strerror(i_errno), test->ctrl_sck);
 
         /*
          * We get here after iperf_run_client returns normally.
@@ -447,3 +455,20 @@ Java_com_novoyuuparosk_iperf3gui_IperfRunner_isTestRunning(JNIEnv *env,
                                                            jobject thiz) {
     return (g_test != NULL && !g_test->done);
 }
+
+/* ------------------------------------------------------------------ */
+/* JNI: Set the temp directory for stream buffers.                    */
+/* Signature: void setTempDir(String tmpDir)                          */
+/* ------------------------------------------------------------------ */
+JNIEXPORT void JNICALL
+Java_com_novoyuuparosk_iperf3gui_IperfRunner_setTempDir(JNIEnv *env,
+                                                         jobject thiz,
+                                                         jstring tmpDir) {
+    if (!tmpDir) return;
+    const char *cstr = (*env)->GetStringUTFChars(env, tmpDir, NULL);
+    if (cstr) {
+        snprintf(g_tmp_template, sizeof(g_tmp_template), "%s/iperf3.XXXXXX", cstr);
+        (*env)->ReleaseStringUTFChars(env, tmpDir, cstr);
+        LOGI("Temp dir set to: %s", g_tmp_template);
+    }
+}

android/app/src/main/kotlin/com/novoyuuparosk/iperf3gui/IperfRunner.kt

@@ -1,24 +1,24 @@
 package com.novoyuuparosk.iperf3gui
 
+import android.os.Handler
+import android.os.Looper
 import io.flutter.embedding.engine.plugins.FlutterPlugin
 import io.flutter.plugin.common.MethodChannel
 
 /**
  * Kotlin binding layer for native iperf3 via JNI.
  *
- * This class loads the native library (libiperf3jni.so) and exposes
- * iperf3 functionality to Flutter via MethodChannel callbacks.
- *
  * iperf3 runs natively inside the app process — no Process.start() needed,
  * no SELinux restrictions. Output is streamed live to the Flutter UI via
- * the IperfCallback interface.
+ * the IperfCallback interface, with all callbacks posted to the main thread.
  */
 object IperfRunner : FlutterPlugin {
 
     private var channel: MethodChannel? = null
+    private var appContext: android.content.Context? = null
+    private val mainHandler = Handler(Looper.getMainLooper())
 
     init {
-        // Load native library at class initialization
         try {
             android.util.Log.i("IperfRunner", "Loading native library...")
             System.loadLibrary("iperf3jni")
@@ -30,48 +30,55 @@ object IperfRunner : FlutterPlugin {
     }
 
     override fun onAttachedToEngine(binding: FlutterPlugin.FlutterPluginBinding) {
+        appContext = binding.applicationContext
         channel = MethodChannel(binding.binaryMessenger, "com.novoyuuparosk.iperf3gui/iperf")
         channel?.setMethodCallHandler { call, result ->
             when (call.method) {
                 "runIperf" -> {
                     val args = call.argument<List<String>>("args")?.toTypedArray()
                     if (args != null) {
-                        runIperfLive(args, object : IperfCallback {
-                            override fun onOutput(line: String) {
+                        // Set temp dir to app's cache dir — /data/local/tmp is NOT writable
+                        appContext?.cacheDir?.absolutePath?.let { cacheDir ->
+                            setTempDir(cacheDir)
+                        }
+                        // Wrap callbacks so they always run on the main thread.
+                        // The native reader thread cannot call invokeMethod directly.
+                        val safeCallback = MainThreadCallback(
+                            onOutput = { line ->
                                 channel?.invokeMethod("onOutput", mapOf("output" to line))
-                            }
-                            override fun onError(error: String) {
+                            },
+                            onError = { error ->
                                 channel?.invokeMethod("onError", mapOf("error" to error))
-                            }
-                            override fun onComplete() {
+                            },
+                            onComplete = {
                                 channel?.invokeMethod("onComplete", emptyMap<String, Any>())
                             }
-                        })
+                        )
+                        runIperfLive(args, safeCallback)
                         result.success(null)
                     } else {
                         result.error("INVALID_ARGS", "args must be a String array", null)
                     }
                 }
                 "stopIperf" -> {
-                    forceStopIperfTest(object : IperfCallback {
-                        override fun onOutput(line: String) {
+                    val safeCallback = MainThreadCallback(
+                        onOutput = { line ->
                             channel?.invokeMethod("onOutput", mapOf("output" to line))
-                        }
-                        override fun onError(error: String) {
+                        },
+                        onError = { error ->
                             channel?.invokeMethod("onError", mapOf("error" to error))
-                        }
-                        override fun onComplete() {
+                        },
+                        onComplete = {
                             channel?.invokeMethod("onComplete", emptyMap<String, Any>())
                         }
-                    })
+                    )
+                    forceStopIperfTest(safeCallback)
                     result.success(null)
                 }
                 "isTestRunning" -> {
                     result.success(isTestRunning())
                 }
-                else -> {
-                    result.notImplemented()
-                }
+                else -> result.notImplemented()
             }
         }
     }
@@ -84,37 +91,39 @@ object IperfRunner : FlutterPlugin {
     // Native method bindings
     // ─────────────────────────────────────────────────────────────────
 
-    /**
-     * Start an iperf3 test with the given arguments.
-     * This calls the native C function via JNI.
-     */
     external fun runIperfLive(arguments: Array<String>, callback: IperfCallback)
-
-    /**
-     * Gracefully stop a running iperf3 test.
-     */
     external fun forceStopIperfTest(callback: IperfCallback)
-
-    /**
-     * Check if an iperf3 test is currently running.
-     * @return true if a test is in progress, false otherwise
-     */
+    external fun setTempDir(tmpDir: String)
     external fun isTestRunning(): Boolean
 
     /**
      * Callback interface for iperf3 test output.
-     *
-     * Used by the native layer to stream test results back to Flutter
-     * in real-time.
+     * Used by the native layer to stream results back to Flutter in real-time.
      */
     interface IperfCallback {
-        /** Called with each line of iperf3 output (stdout/stderr) */
         fun onOutput(line: String)
-
-        /** Called when an error occurs */
         fun onError(error: String)
-
-        /** Called when the test completes (success or failure) */
         fun onComplete()
     }
-}
+
+    /**
+     * Thread-safe wrapper: posts all callbacks to the main thread.
+     * This is required because the native reader thread cannot safely
+     * call MethodChannel.invokeMethod from a background thread.
+     */
+    private class MainThreadCallback(
+        private val onOutput: (String) -> Unit,
+        private val onError: (String) -> Unit,
+        private val onComplete: () -> Unit
+    ) : IperfCallback {
+        override fun onOutput(line: String) {
+            mainHandler.post { onOutput(line) }
+        }
+        override fun onError(error: String) {
+            mainHandler.post { onError(error) }
+        }
+        override fun onComplete() {
+            mainHandler.post { onComplete() }
+        }
+    }
+}