Ensure single CPython interpreter per process

Introduces a guard to prevent repeated initialization and finalization of the CPython interpreter, addressing crashes on second launch with CPython 3.12. Also ensures the current native thread is registered with the GIL when running Python code, improving stability when re-entering Python from new Dart isolates or threads.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -11,6 +11,9 @@ 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;
 
 CPython getCPython(String dynamicLibPath) {
   return _cpython ??= _cpython = CPython(DynamicLibrary.open(dynamicLibPath));
@@ -51,33 +54,43 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
   debugPrint("programModuleName: $programModuleName");
 
   final cpython = getCPython(dynamicLibPath);
-  cpython.Py_Initialize();
-  debugPrint("after Py_Initialize()");
+  if (!_pythonInitialized) {
+    cpython.Py_Initialize();
+    _pythonInitialized = true;
+    debugPrint("after Py_Initialize()");
+  } else {
+    debugPrint("Python already initialized; reusing interpreter");
+  }
 
   var result = "";
-
-  if (script != "") {
-    // run script
-    final scriptPtr = script.toNativeUtf8();
-    int sr = cpython.PyRun_SimpleString(scriptPtr.cast<Char>());
-    debugPrint("PyRun_SimpleString for script result: $sr");
-    malloc.free(scriptPtr);
-    if (sr != 0) {
-      result = getPythonError(cpython);
-    }
-  } else {
-    // run program
-    final moduleNamePtr = programModuleName.toNativeUtf8();
-    var modulePtr = cpython.PyImport_ImportModule(moduleNamePtr.cast<Char>());
-    if (modulePtr == nullptr) {
-      result = getPythonError(cpython);
+  // Ensure the current native thread is registered with the GIL; this is
+  // required if we re-enter Python from a new Dart isolate/thread.
+  final gilState = cpython.PyGILState_Ensure();
+
+  try {
+    if (script != "") {
+      // run script
+      final scriptPtr = script.toNativeUtf8();
+      int sr = cpython.PyRun_SimpleString(scriptPtr.cast<Char>());
+      debugPrint("PyRun_SimpleString for script result: $sr");
+      malloc.free(scriptPtr);
+      if (sr != 0) {
+        result = getPythonError(cpython);
+      }
+    } else {
+      // run program
+      final moduleNamePtr = programModuleName.toNativeUtf8();
+      var modulePtr =
+          cpython.PyImport_ImportModule(moduleNamePtr.cast<Char>());
+      if (modulePtr == nullptr) {
+        result = getPythonError(cpython);
+      }
+      malloc.free(moduleNamePtr);
     }
-    malloc.free(moduleNamePtr);
+  } finally {
+    cpython.PyGILState_Release(gilState);
   }
 
-  cpython.Py_Finalize();
-  debugPrint("after Py_Finalize()");
-
   sendPort.send(result);
 
   return result;