Use sub-interpreter for each Python run in isolate

Refactors the Python execution logic to create a fresh sub-interpreter for each run, avoiding reuse of potentially wedged state from previous runs. Removes global signal patching in favor of patching within each sub-interpreter, and ensures the asyncio event loop is reset per run. This improves reliability when running Python code in isolates.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -15,7 +15,6 @@ CPython? _cpython;
 // "main thread" identity across invocations.
 Isolate? _pythonIsolate;
 SendPort? _pythonSendPort;
-bool _signalPatched = false;
 const _pythonRunTimeout = Duration(seconds: 30);
 
 void _pyLog(String msg) {
@@ -122,37 +121,33 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
     cpython.Py_Initialize();
     _pyLog("after Py_Initialize()");
   } else {
-    // Interpreter was left running (likely from a previous app session). Tear
-    // it down and start fresh to avoid reusing a wedged event loop.
-    _pyLog("Python already initialized; finalizing stale interpreter");
-    cpython.PyGILState_Ensure(); // ensure we own the GIL before finalizing
-    cpython.Py_FinalizeEx();
-    _pyLog("after Py_FinalizeEx(), reinitializing");
-    cpython.Py_Initialize();
-    _pyLog("after Py_Initialize() (fresh)");
+    _pyLog("Python already initialized; reusing runtime");
   }
 
   var result = "";
 
-  // Ensure the calling thread owns the GIL; this is safe across isolates.
+  // Use a fresh sub-interpreter per run to avoid wedged state.
   final gilState = cpython.PyGILState_Ensure();
   _pyLog("PyGILState_Ensure -> $gilState");
+  final mainState = cpython.PyThreadState_Get();
+  _pyLog("main thread state: 0x${mainState.address.toRadixString(16)}");
+  final subState = cpython.Py_NewInterpreter();
+  if (subState == nullptr) {
+    _pyLog("Py_NewInterpreter returned null");
+    cpython.PyGILState_Release(gilState);
+    sendPort.send("Py_NewInterpreter failed");
+    return "Py_NewInterpreter failed";
+  }
+  _pyLog("created sub-interpreter: 0x${subState.address.toRadixString(16)}");
 
-  // Patch signal handling if we're not on the interpreter's main thread; some
-  // libraries (e.g., asyncio) call signal.signal which would otherwise raise.
-  if (!_signalPatched) {
-    const patch = """
+  // Within the sub-interpreter: patch signals and reset asyncio loop.
+  const patchSignals = """
 import threading, signal
 if threading.current_thread() is not threading.main_thread():
     signal.signal = lambda *args, **kwargs: None
 """;
-    _pyLog("patching signal.signal for non-main thread use");
-    cpython.PyRun_SimpleString(patch.toNativeUtf8().cast());
-    _signalPatched = true;
-  }
+  cpython.PyRun_SimpleString(patchSignals.toNativeUtf8().cast());
 
-  // Reset/replace asyncio loop between runs to avoid reusing a running loop from
-  // a previous app session that could block new runs.
   const loopReset = """
 import asyncio
 try:
@@ -161,10 +156,9 @@ try:
         loop.stop()
         loop.close()
     asyncio.set_event_loop(asyncio.new_event_loop())
-except Exception as e:
+except Exception:
     pass
 """;
-  _pyLog("resetting asyncio loop");
   cpython.PyRun_SimpleString(loopReset.toNativeUtf8().cast());
 
   try {
@@ -192,7 +186,9 @@ except Exception as e:
       malloc.free(moduleNamePtr);
     }
   } finally {
-    // Drop the GIL again so subsequent runs/threads can re-acquire it cleanly.
+    _pyLog("ending sub-interpreter 0x${subState.address.toRadixString(16)}");
+    cpython.Py_EndInterpreter(subState);
+    cpython.PyThreadState_Swap(mainState);
     _pyLog("PyGILState_Release($gilState)");
     cpython.PyGILState_Release(gilState);
   }