Add timeout and restart logic for Python isolate

Introduces a 30-second timeout for running Python programs in the isolate. If a timeout or error occurs, the Python interpreter isolate is restarted to ensure reliability. Refactors isolate management into helper functions for clarity and robustness.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -16,6 +16,7 @@ CPython? _cpython;
 Isolate? _pythonIsolate;
 SendPort? _pythonSendPort;
 bool _signalPatched = false;
+const _pythonRunTimeout = Duration(seconds: 30);
 
 void _pyLog(String msg) {
   debugPrint("[PY] ${DateTime.now().toIso8601String()} $msg");
@@ -35,20 +36,27 @@ Future<String> runPythonProgramFFI(bool sync, String dynamicLibPath,
   }
 
   // Ensure a single long-lived isolate/thread to keep Python "main thread" consistent.
-  if (_pythonIsolate == null || _pythonSendPort == null) {
-    final readyPort = ReceivePort();
-    _pythonIsolate =
-        await Isolate.spawn(_pythonIsolateMain, readyPort.sendPort);
-    _pythonSendPort = await readyPort.first as SendPort;
-    readyPort.close();
-  }
+  await _ensurePythonIsolate();
 
   final response = ReceivePort();
-  _pythonSendPort!
-      .send([response.sendPort, dynamicLibPath, pythonProgramPath, script]);
-  final result = await response.first as String;
-  response.close();
-  return result;
+  _pythonSendPort!.send(
+      [response.sendPort, dynamicLibPath, pythonProgramPath, script]);
+  try {
+    final result = await response.first
+        .timeout(_pythonRunTimeout, onTimeout: () => "<timeout>");
+    response.close();
+    if (result == "<timeout>") {
+      _pyLog("Python run timed out; restarting interpreter isolate");
+      _restartPythonIsolate();
+      return "Python run timed out";
+    }
+    return result as String;
+  } catch (e) {
+    response.close();
+    _pyLog("Python run failed: $e");
+    _restartPythonIsolate();
+    rethrow;
+  }
 }
 
 // Long-lived isolate entry point to serialize all Python runs on one thread.
@@ -61,13 +69,37 @@ void _pythonIsolateMain(SendPort readyPort) {
     final dynamicLibPath = args[1] as String;
     final pythonProgramPath = args[2] as String;
     final script = args[3] as String;
-    final result = await runPythonProgramInIsolate(
-        [sendPort, dynamicLibPath, pythonProgramPath, script]);
-    // runPythonProgramInIsolate already sends the result, but return anyway.
-    sendPort.send(result);
+    try {
+      final result = await runPythonProgramInIsolate(
+          [sendPort, dynamicLibPath, pythonProgramPath, script]);
+      // runPythonProgramInIsolate already sends the result, but return anyway.
+      sendPort.send(result);
+    } catch (e) {
+      sendPort.send("Python isolate error: $e");
+    }
   });
 }
 
+Future<void> _ensurePythonIsolate() async {
+  if (_pythonIsolate != null && _pythonSendPort != null) {
+    return;
+  }
+  _pyLog("spawning Python isolate");
+  final readyPort = ReceivePort();
+  _pythonIsolate = await Isolate.spawn(_pythonIsolateMain, readyPort.sendPort);
+  _pythonSendPort = await readyPort.first as SendPort;
+  readyPort.close();
+  _signalPatched = false;
+}
+
+void _restartPythonIsolate() {
+  _pyLog("restarting Python isolate");
+  _pythonIsolate?.kill(priority: Isolate.immediate);
+  _pythonIsolate = null;
+  _pythonSendPort = null;
+  _signalPatched = false;
+}
+
 Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
   final sendPort = arguments[0] as SendPort;
   final dynamicLibPath = arguments[1] as String;