Refactor GIL management for Python interpreter

Replaces manual thread state management with PyGILState_Ensure and PyGILState_Release for safer and more robust GIL handling across isolates. Removes global variables and simplifies initialization logic to avoid crashes on repeated interpreter use.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -11,10 +11,6 @@ import 'gen.dart';
 export 'gen.dart';
 
 CPython? _cpython;
-// Keep a single interpreter per process; repeated init/finalize of CPython 3.12
-// from an embedder is fragile and was crashing on second launch.
-bool _pythonInitialized = false;
-Pointer<PyThreadState>? _mainThreadState;
 
 void _pyLog(String msg) {
   debugPrint("[PY] ${DateTime.now().toIso8601String()} $msg");
@@ -60,28 +56,21 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
   _pyLog("script provided: ${script.isNotEmpty} length=${script.length}");
 
   final cpython = getCPython(dynamicLibPath);
-  if (!_pythonInitialized) {
+  final alreadyInit = cpython.Py_IsInitialized() != 0;
+  _pyLog("Py_IsInitialized: $alreadyInit");
+  if (!alreadyInit) {
     _pyLog("calling Py_Initialize()");
     cpython.Py_Initialize();
-    // Release the GIL from the init thread so other threads can reacquire it.
-    _mainThreadState = cpython.PyEval_SaveThread();
-    _pythonInitialized = true;
-    _pyLog(
-        "after Py_Initialize(), saved thread state: ${_mainThreadState?.address.toRadixString(16)}");
+    _pyLog("after Py_Initialize()");
   } else {
     _pyLog("Python already initialized; reusing interpreter");
   }
 
   var result = "";
 
-  // Reacquire the GIL using the saved main thread state for this process.
-  final mainState = _mainThreadState;
-  if (mainState == null) {
-    throw StateError("Python main thread state is null after initialization");
-  }
-
-  _pyLog("restoring thread state ${mainState.address.toRadixString(16)}");
-  cpython.PyEval_RestoreThread(mainState);
+  // Ensure the calling thread owns the GIL; this is safe across isolates.
+  final gilState = cpython.PyGILState_Ensure();
+  _pyLog("PyGILState_Ensure -> $gilState");
 
   try {
     if (script != "") {
@@ -98,20 +87,19 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
       // run program
       final moduleNamePtr = programModuleName.toNativeUtf8();
       _pyLog("calling PyImport_ImportModule for $programModuleName");
-      var modulePtr =
-          cpython.PyImport_ImportModule(moduleNamePtr.cast<Char>());
+      var modulePtr = cpython.PyImport_ImportModule(moduleNamePtr.cast<Char>());
       if (modulePtr == nullptr) {
         result = getPythonError(cpython);
       } else {
-        _pyLog("PyImport_ImportModule returned ${modulePtr.address.toRadixString(16)}");
+        _pyLog(
+            "PyImport_ImportModule returned ${modulePtr.address.toRadixString(16)}");
       }
       malloc.free(moduleNamePtr);
     }
   } finally {
     // Drop the GIL again so subsequent runs/threads can re-acquire it cleanly.
-    _mainThreadState = cpython.PyEval_SaveThread();
-    _pyLog(
-        "saved thread state after run: ${_mainThreadState?.address.toRadixString(16)}");
+    _pyLog("PyGILState_Release($gilState)");
+    cpython.PyGILState_Release(gilState);
   }
 
   // cpython.Py_Finalize();