Manage Python GIL state for thread safety

Introduces _mainThreadState to save and restore the Python GIL using PyEval_SaveThread and PyEval_RestoreThread, ensuring thread safety when running Python code in isolates. This prevents crashes on repeated interpreter initialization and allows other threads to acquire the GIL as needed.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -14,6 +14,7 @@ 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;
 
 CPython getCPython(String dynamicLibPath) {
   return _cpython ??= _cpython = CPython(DynamicLibrary.open(dynamicLibPath));
@@ -56,6 +57,8 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
   final cpython = getCPython(dynamicLibPath);
   if (!_pythonInitialized) {
     cpython.Py_Initialize();
+    // Release the GIL from the init thread so other threads can reacquire it.
+    _mainThreadState = cpython.PyEval_SaveThread();
     _pythonInitialized = true;
     debugPrint("after Py_Initialize()");
   } else {
@@ -64,23 +67,37 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
 
   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);
+  // 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");
+  }
+
+  cpython.PyEval_RestoreThread(mainState);
+
+  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 {
+    // Drop the GIL again so subsequent runs/threads can re-acquire it cleanly.
+    _mainThreadState = cpython.PyEval_SaveThread();
   }
 
   // cpython.Py_Finalize();