Improve CPython isolate handling and logcat setup

Ensures logcat forwarding is idempotent across Dart isolate restarts and avoids finalizing the CPython interpreter between runs to prevent native crashes. Also refines isolate management to prevent killing isolates that may leave the interpreter in a bad state, and improves error handling for logcat forwarding setup.

Author: FeodorFitsner

src/serious_python_android/lib/src/cpython.dart

@@ -11,8 +11,15 @@ import 'gen.dart';
 export 'gen.dart';
 
 CPython? _cpython;
+String? _logcatForwardingError;
 const _logcatInitScript = r'''
 import sys, logging
+
+# Make this init idempotent across Dart isolate restarts.
+if getattr(sys, "__serious_python_logcat_configured__", False):
+    raise SystemExit
+sys.__serious_python_logcat_configured__ = True
+
 from ctypes import cdll
 liblog = cdll.LoadLibrary("liblog.so")
 ANDROID_LOG_INFO = 4
@@ -73,19 +80,23 @@ Future<String> runPythonProgramFFI(bool sync, String dynamicLibPath,
   final receivePort = ReceivePort();
   if (sync) {
     // sync run
-    return await runPythonProgramInIsolate(
-        [receivePort.sendPort, dynamicLibPath, pythonProgramPath, script]);
+    try {
+      return await runPythonProgramInIsolate(
+          [receivePort.sendPort, dynamicLibPath, pythonProgramPath, script]);
+    } finally {
+      receivePort.close();
+    }
   } else {
-    var completer = Completer<String>();
     // async run
-    final isolate = await Isolate.spawn(runPythonProgramInIsolate,
+    //
+    // IMPORTANT: do not `isolate.kill()` here. Killing the isolate can abort the
+    // underlying OS thread while it still interacts with CPython, leaving the
+    // interpreter/GIL in a bad state for subsequent runs.
+    await Isolate.spawn(runPythonProgramInIsolate,
         [receivePort.sendPort, dynamicLibPath, pythonProgramPath, script]);
-    receivePort.listen((message) {
-      receivePort.close();
-      isolate.kill();
-      completer.complete(message);
-    });
-    return completer.future;
+    final message = await receivePort.first;
+    receivePort.close();
+    return message as String;
   }
 }
 
@@ -146,9 +157,9 @@ Future<String> runPythonProgramInIsolate(List<Object> arguments) async {
       return "";
     });
   } finally {
-    // Finalize interpreter so subsequent runs start clean and GIL is free.
-    _finalizeInterpreter(cpython);
-    _cpython = null;
+    // Keep interpreter alive between runs. Finalizing + re-initializing the
+    // interpreter is not reliably supported and has caused native crashes
+    // (e.g. during _ctypes re-import) on Android.
   }
 
   sendPort.send(result);
@@ -208,9 +219,11 @@ String? _setupLogcatForwarding(CPython cpython) {
   malloc.free(setupPtr);
 
   if (result != 0) {
-    return getPythonError(cpython);
+    _logcatForwardingError = getPythonError(cpython);
+    return _logcatForwardingError;
   }
 
   _debug("logcat forwarding configured");
+  _logcatForwardingError = null;
   return null;
 }