Crash in generated marshal method returning View

[radekdoulik]

When using JNI marshal methods with Xamarin.Forms app, we get crash after following marshal method is used:

public static IntPtr n_onCreateView_Landroid_view_LayoutInflater_Landroid_view_ViewGroup_Landroid_os_Bundle_ (IntPtr __jnienv, IntPtr __this, IntPtr inflater, IntPtr container, IntPtr savedInstanceState)
{
	JniTransition jniTransition = new JniTransition (__jnienv);
	JniRuntime runtime = default(JniRuntime);
	JniObjectReference value5 = default(JniObjectReference);
	try {
		runtime = JniEnvironment.Runtime;
		JniRuntime.JniValueManager valueManager = runtime.ValueManager;
		valueManager.WaitForGCBridgeProcessing ();
		FragmentContainer value = valueManager.GetValue<FragmentContainer> (__this);
		LayoutInflater value2 = valueManager.GetValue<LayoutInflater> (inflater);
		ViewGroup value3 = valueManager.GetValue<ViewGroup> (container);
		Bundle value4 = valueManager.GetValue<Bundle> (savedInstanceState);
		global::Android.Views.View view = value.OnCreateView (value2, value3, value4);
		value5 = ((view != null) ? view.PeerReference : default(JniObjectReference));
		IntPtr handle = value5.Handle;
		return JniEnvironment.References.NewReturnToJniRef (value5);
		IL_0087:;
	} catch (Exception ex) when (runtime.ExceptionShouldTransitionToJni (ex)) {
		jniTransition.SetPendingException (ex);
		return default(IntPtr);
		IL_00c5:;
	} finally {
		JniObjectReference.Dispose (ref value5);
		jniTransition.Dispose ();
	}
	IntPtr intPtr = default(IntPtr);
	return intPtr;
}

The generated method has few issues:

• it should not call JniObjectReference.Dispose in the finally block as it will release the global reference
• there seems to be unnecessary leave in try and catch blocks
• unsed handle variable

The probable cause of the crash is that we are returning released gref. I need to look at JniEnvironment.References.NewReturnToJniRef in detail to confirm that.

[jonpryor]

The problem is this line:

https://github.com/xamarin/java.interop/blob/8d5bcec11b6005447b736537af17faa50daa7a87/src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs#L601

Instead of calling r.Dispose(), it should instead call r.DisposeUnlessReferenced().

[jonpryor]

there seems to be unnecessary leave in try and catch blocks

This may be due to the decompiler, not the IL. Reading the actual IL might be illuminating here.

[jonpryor]

unused handle variable

I think this is a "bug"/"thinko": https://github.com/xamarin/java.interop/blob/8d5bcec11b6005447b736537af17faa50daa7a87/src/Java.Interop/Java.Interop/JniRuntime.JniValueManager.cs#L609-L614

CreateParameterFromManagedExpression() returns an Expression, but CreateReturnValueFromManagedExpression() doesn't use that return value. Why doesn't it? In retrospect, I think it would make more sense if CreateReturnValueFromManagedExpression() did:

var r = CreateParameterFromManagedExpression (context, sourceValue, 0);
return ReturnObjectReferenceToJni (context, sourceValue.Name, r);

That way, the return value isn't entirely ignored, instead of using a "+_ref" suffix logic to lookup a variable created elsewhere. The current code seems ugly.