[registrar] Improve error message when failing to create a managed wr…

…apper for a native handle. (#4360)

* Convert it into a MM8027/MT8027 error (with documentation).
* Add information about the selector and managed method that triggered the error.

Now the problem reported in issue #4254 shows up as:

> ObjCRuntime.RuntimeException: Failed to marshal the Objective-C object 0x7f8080412810 (type: UIView). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'UIKit.UIView&' does not have a constructor that takes one IntPtr argument).
> Additional information:
> 	Selector: popoverController:willRepositionPopoverToRect:inView:
> 	Method: UIKit.UIPopoverController+_UIPopoverControllerDelegate.WillReposition(UIKit.UIPopoverController, CoreGraphics.CGRect ByRef, UIKit.UIView ByRef)

instead of just:

> ObjCRuntime.RuntimeException: Failed to marshal the Objective-C object 0x7f8080412810 (type: UIView). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'UIKit.UIView&' does not have a constructor that takes one IntPtr argument).

which makes it much easier to understand, track down, and fix/work around,
both for customers and ourselves.

docs/website/mmp-errors.md

@@ -477,3 +477,32 @@ This usually indicates a bug in Xamarin.Mac, because the dynamic registrar shoul
 It's possible to force the linker to keep the dynamic registrar by adding
 `--optimize=-remove-dynamic-registrar` to the additional mmp arguments in
 the project's Mac Build options.
+
+### <a name="MT8027"/>MT8027: Failed to marshal the Objective-C object {handle} (type: {managed_type}). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance.
+
+This occurs when the Xamarin.Mac runtime finds an Objective-C object without a
+corresponding managed wrapper object, and when trying to create that managed
+wrapper, it turns out it's not possible.
+
+There are a few reasons this may happen:
+
+* If it occurs when deserializing a storyboard/nib/xib and mentioning a
+  managed type that's used in that storyboard/nib/xib, then the fix is to add
+  a constructor that takes a single `IntPtr` argument to that managed type.
+  This constructor should not have any logic (because it's invoked before the
+  native and managed instances are fully created).
+
+* A managed wrapper existed at some point, but was collected by the GC. If the
+  native object is still alive, and later resurfaces to managed code, the
+  Xamarin.Mac runtime will try to re-create a managed wrapper instance. In
+  most cases the problem here is that the managed wrapper shouldn't have been
+  collected by the GC in the first place.
+
+  Possible causes include:
+
+  * Manually calling Dispose too early on the managed wrapper.
+  * Incorrect bindings for third-party libraries.
+  * Reference-counting bugs in third-party libraries.
+
+* It could be a bug in Xamarin.Mac. If this is the case, please file a bug at
+  [https://bugzilla.xamarin.com](https://bugzilla.xamarin.com/enter_bug.cgi?product=Xamarin.Mac).

docs/website/mtouch-errors.md

@@ -2482,3 +2482,32 @@ This usually indicates a bug in Xamarin.iOS, because the dynamic registrar shoul
 It's possible to force the linker to keep the dynamic registrar by adding
 `--optimize=-remove-dynamic-registrar` to the additional mtouch arguments in
 the project's iOS Build options.
+
+### <a name="MM8027"/>MM8027: Failed to marshal the Objective-C object {handle} (type: {managed_type}). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance.
+
+This occurs when the Xamarin.iOS runtime finds an Objective-C object without a
+corresponding managed wrapper object, and when trying to create that managed
+wrapper, it turns out it's not possible.
+
+There are a few reasons this may happen:
+
+* If it occurs when deserializing a storyboard/nib/xib and mentioning a
+  managed type that's used in that storyboard/nib/xib, then the fix is to add
+  a constructor that takes a single `IntPtr` argument to that managed type.
+  This constructor should not have any logic (because it's invoked before the
+  native and managed instances are fully created).
+
+* A managed wrapper existed at some point, but was collected by the GC. If the
+  native object is still alive, and later resurfaces to managed code, the
+  Xamarin.iOS runtime will try to re-create a managed wrapper instance. In
+  most cases the problem here is that the managed wrapper shouldn't have been
+  collected by the GC in the first place.
+
+  Possible causes include:
+
+  * Manually calling Dispose too early on the managed wrapper.
+  * Incorrect bindings for third-party libraries.
+  * Reference-counting bugs in third-party libraries.
+
+* It could be a bug in Xamarin.iOS. If this is the case, please file a bug at
+  [https://bugzilla.xamarin.com](https://bugzilla.xamarin.com/enter_bug.cgi?product=iOS).

runtime/delegates.t4

@@ -179,7 +179,9 @@
 		new XDelegate ("MonoObject *", "IntPtr", "xamarin_get_nsobject_with_type",
 			"id", "IntPtr", "obj",
 			"void *", "IntPtr", "type",
-			"int32_t *", "out bool", "created"
+			"int32_t *", "out bool", "created",
+			"SEL", "IntPtr", "selector",
+			"MonoReflectionMethod *", "IntPtr", "method"
 		) {
 			WrappedManagedFunction = "GetNSObjectWithType",
 			OnlyDynamicUsage = false,
@@ -271,6 +273,14 @@
 			WrappedManagedFunction = "ConvertNSStringToSmartEnum",
 			OnlyDynamicUsage = true,
 		},
+
+		new XDelegate ("int32_t", "int", "xamarin_create_runtime_exception",
+			"int32_t", "int", "code",
+			"const char *", "IntPtr", "message"
+		) {
+			WrappedManagedFunction = "CreateRuntimeException",
+			OnlyDynamicUsage = false,
+		},
 	};
 	delegates.CalculateLengths ();
 #><#+

runtime/runtime.m

@@ -257,17 +257,17 @@
 }
 
 MonoObject *
-xamarin_get_nsobject_with_type_for_ptr (id self, bool owns, MonoType* type, guint32 *exception_gchandle)
+xamarin_get_nsobject_with_type_for_ptr (id self, bool owns, MonoType* type, SEL selector, MonoMethod *managed_method, guint32 *exception_gchandle)
 {
 	// COOP: Reading managed data, must be in UNSAFE mode
 	MONO_ASSERT_GC_UNSAFE;
 
 	int32_t created;
-	return xamarin_get_nsobject_with_type_for_ptr_created (self, owns, type, &created, exception_gchandle);
+	return xamarin_get_nsobject_with_type_for_ptr_created (self, owns, type, &created, selector, managed_method, exception_gchandle);
 }
 
 MonoObject *
-xamarin_get_nsobject_with_type_for_ptr_created (id self, bool owns, MonoType *type, int32_t *created, guint32 *exception_gchandle)
+xamarin_get_nsobject_with_type_for_ptr_created (id self, bool owns, MonoType *type, int32_t *created, SEL selector, MonoMethod *managed_method, guint32 *exception_gchandle)
 {
 	// COOP: Reading managed data, must be in UNSAFE mode
 	MONO_ASSERT_GC_UNSAFE;
@@ -288,7 +288,7 @@
 			return mobj;
 	}
 
-	return xamarin_get_nsobject_with_type (self, mono_type_get_object (mono_domain_get (), type), created, exception_gchandle);
+	return xamarin_get_nsobject_with_type (self, mono_type_get_object (mono_domain_get (), type), created, selector, managed_method == NULL ? NULL : mono_method_get_object (mono_domain_get (), managed_method, NULL), exception_gchandle);
 }
 
 MonoObject *
@@ -672,9 +672,10 @@ -(void) xamarinSetGCHandle: (int) gc_handle;
 	char *type_name = xamarin_lookup_managed_type_name ([self class], exception_gchandle);
 	if (*exception_gchandle == 0) {
 		char *msg = xamarin_strdup_printf (m, self, object_getClassName (self), type_name, sel_getName (sel), method_full_name);
-		MonoException *mex = xamarin_create_exception (msg);
+		guint32 ex_handle = xamarin_create_runtime_exception (8027, msg, exception_gchandle);
 		xamarin_free (msg);
-		*exception_gchandle = mono_gchandle_new ((MonoObject *) mex, FALSE);
+		if (*exception_gchandle == 0)
+			*exception_gchandle = ex_handle;
 	}
 	mono_free (type_name);
 	mono_free (method_full_name);

runtime/trampolines-invoke.m

@@ -118,7 +118,7 @@
 	if (isCategoryInstance) {
 		// we know this must be an id
 		p = mono_signature_get_params (msig, &iter);
-		arg_ptrs [0] = xamarin_get_nsobject_with_type_for_ptr (self, false, p, &exception_gchandle);
+		arg_ptrs [0] = xamarin_get_nsobject_with_type_for_ptr (self, false, p, sel, method, &exception_gchandle);
 		if (exception_gchandle != 0)
 			goto exception_handling;
 		mofs = 1;
@@ -195,7 +195,7 @@
 									MonoObject *obj;
 									NSObject *targ = *(NSObject **) arg;
 
-									obj = xamarin_get_nsobject_with_type_for_ptr (targ, false, p, &exception_gchandle);
+									obj = xamarin_get_nsobject_with_type_for_ptr (targ, false, p, sel, method, &exception_gchandle);
 									if (exception_gchandle != 0)
 										goto exception_handling;
 #if DEBUG
@@ -324,7 +324,7 @@
 								} else {
 									MonoObject *obj;
 									id targ = [arr objectAtIndex: j];
-									obj = xamarin_get_nsobject_with_type_for_ptr (targ, false, e, &exception_gchandle);
+									obj = xamarin_get_nsobject_with_type_for_ptr (targ, false, e, sel, method, &exception_gchandle);
 									if (exception_gchandle != 0)
 										goto exception_handling;
 #if DEBUG
@@ -343,7 +343,7 @@
 							}
 							MonoObject *obj;
 							int32_t created = false;
-							obj = xamarin_get_nsobject_with_type_for_ptr_created (id_arg, false, p, &created, &exception_gchandle);
+							obj = xamarin_get_nsobject_with_type_for_ptr_created (id_arg, false, p, &created, sel, method, &exception_gchandle);
 							if (exception_gchandle != 0)
 								goto exception_handling;
 
@@ -380,7 +380,7 @@
 								[id_arg autorelease];
 							}
 							MonoObject *obj;
-							obj = xamarin_get_nsobject_with_type_for_ptr (id_arg, false, p, &exception_gchandle);
+							obj = xamarin_get_nsobject_with_type_for_ptr (id_arg, false, p, sel, method, &exception_gchandle);
 							if (exception_gchandle != 0)
 								goto exception_handling;
 #if DEBUG

runtime/trampolines.m

@@ -1122,7 +1122,7 @@
 		managed_method = xamarin_get_managed_method_for_token (context /* token ref */, exception_gchandle);
 		if (*exception_gchandle != 0) return NULL;
 
-		arg_ptrs [0] = xamarin_get_nsobject_with_type_for_ptr (value, false, xamarin_get_parameter_type (managed_method, 0), exception_gchandle);
+		arg_ptrs [0] = xamarin_get_nsobject_with_type_for_ptr (value, false, xamarin_get_parameter_type (managed_method, 0), NULL, NULL, exception_gchandle);
 		if (*exception_gchandle != 0) return NULL;
 
 		obj = mono_runtime_invoke (managed_method, NULL, arg_ptrs, &exception);

runtime/xamarin/runtime.h

@@ -173,8 +173,8 @@ bool			xamarin_is_class_nsstring (MonoClass *cls);
 bool			xamarin_is_class_nullable (MonoClass *cls, MonoClass **element_type, guint32 *exception_gchandle);
 MonoClass *		xamarin_get_nullable_type (MonoClass *cls, guint32 *exception_gchandle);
 MonoType *		xamarin_get_parameter_type (MonoMethod *managed_method, int index);
-MonoObject *	xamarin_get_nsobject_with_type_for_ptr (id self, bool owns, MonoType* type, guint32 *exception_gchandle);
-MonoObject *	xamarin_get_nsobject_with_type_for_ptr_created (id self, bool owns, MonoType *type, int32_t *created, guint32 *exception_gchandle);
+MonoObject *	xamarin_get_nsobject_with_type_for_ptr (id self, bool owns, MonoType* type, SEL selector, MonoMethod *managed_method, guint32 *exception_gchandle);
+MonoObject *	xamarin_get_nsobject_with_type_for_ptr_created (id self, bool owns, MonoType *type, int32_t *created, SEL selector, MonoMethod *managed_method, guint32 *exception_gchandle);
 int *			xamarin_get_delegate_for_block_parameter (MonoMethod *method, guint32 token_ref, int par, void *nativeBlock, guint32 *exception_gchandle);
 id              xamarin_get_block_for_delegate (MonoMethod *method, MonoObject *delegate, const char *signature /* NULL allowed, but requires the dynamic registrar at runtime to compute */, guint32 *exception_gchandle);
 id				xamarin_get_nsobject_handle (MonoObject *obj);

src/ObjCRuntime/DynamicRegistrar.cs

@@ -906,7 +906,7 @@ public void GetMethodDescriptionAndObject (Type type, IntPtr selector, bool is_s
 			if (res.IsInstanceCategory) {
 				mthis = IntPtr.Zero;
 			} else {
-				var nsobj = Runtime.GetNSObject (obj, Runtime.MissingCtorResolution.ThrowConstructor1NotFound, true);
+				var nsobj = Runtime.GetNSObject (obj, Runtime.MissingCtorResolution.ThrowConstructor1NotFound, true, selector, ObjectWrapper.Convert (res.Method));
 				mthis = ObjectWrapper.Convert (nsobj);
 				if (res.Method.ContainsGenericParameters) {
 					res.WriteUnmanagedDescription (desc, Runtime.FindClosedMethod (nsobj.GetType (), res.Method));

src/ObjCRuntime/Runtime.cs

@@ -416,6 +416,12 @@ static int CreateNSException (IntPtr ns_exception)
 			return GCHandle.ToIntPtr (GCHandle.Alloc (ex)).ToInt32 ();
 		}
 
+		static int CreateRuntimeException (int code, IntPtr message)
+		{
+			var ex = ErrorHelper.CreateError (code, Marshal.PtrToStringAuto (message));
+			return GCHandle.ToIntPtr (GCHandle.Alloc (ex)).ToInt32 ();
+		}
+
 		static IntPtr UnwrapNSException (int exc_handle)
 		{
 			var obj = GCHandle.FromIntPtr (new IntPtr (exc_handle)).Target;
@@ -698,11 +704,11 @@ static IntPtr GetINativeObject_Static (IntPtr ptr, bool owns, string typename, s
 			return ObjectWrapper.Convert (GetINativeObject (ptr, owns, iface, type));
 		}
 
-		static IntPtr GetNSObjectWithType (IntPtr ptr, IntPtr type_ptr, out bool created)
+		static IntPtr GetNSObjectWithType (IntPtr ptr, IntPtr type_ptr, out bool created, IntPtr selector, IntPtr method)
 		{
 			// It doesn't work to use System.Type in the signature, we get garbage.
 			var type = (System.Type) ObjectWrapper.Convert (type_ptr);
-			return ObjectWrapper.Convert (GetNSObject (ptr, type, MissingCtorResolution.ThrowConstructor1NotFound, true, out created));
+			return ObjectWrapper.Convert (GetNSObject (ptr, type, MissingCtorResolution.ThrowConstructor1NotFound, true, out created, selector, method));
 		}
 
 		static void Dispose (IntPtr mobj)
@@ -1016,7 +1022,7 @@ internal enum MissingCtorResolution {
 			Ignore,
 		}
 
-		static void MissingCtor (IntPtr ptr, IntPtr klass, Type type, MissingCtorResolution resolution)
+		static void MissingCtor (IntPtr ptr, IntPtr klass, Type type, MissingCtorResolution resolution, IntPtr selector = default (IntPtr), IntPtr method = default (IntPtr))
 		{
 			string msg;
 
@@ -1041,15 +1047,26 @@ static void MissingCtor (IntPtr ptr, IntPtr klass, Type type, MissingCtorResolut
 				return;
 			}
 
-			throw new Exception (string.Format (msg, ptr.ToString ("x"), new Class (klass).Name, type.FullName));
+			if (selector != IntPtr.Zero || method != IntPtr.Zero)
+				msg += "\nAdditional information:\n";
+
+			if (selector != IntPtr.Zero)
+				msg += $"\tSelector: {Selector.GetName (selector)}\n";
+			if (method != IntPtr.Zero) {
+				var mi = ObjectWrapper.Convert (method) as MethodBase;
+				if (mi != null)
+					msg += $"\tMethod: {mi.FullName}\n";
+			}
+
+			throw ErrorHelper.CreateError (8027, string.Format (msg, ptr.ToString ("x"), new Class (klass).Name, type.FullName));
 		}
 
-		static NSObject ConstructNSObject (IntPtr ptr, IntPtr klass, MissingCtorResolution missingCtorResolution)
+		static NSObject ConstructNSObject (IntPtr ptr, IntPtr klass, MissingCtorResolution missingCtorResolution, IntPtr selector = default (IntPtr), IntPtr method = default (IntPtr))
 		{
 			Type type = Class.Lookup (klass);
 
 			if (type != null) {
-				return ConstructNSObject<NSObject> (ptr, type, missingCtorResolution);
+				return ConstructNSObject<NSObject> (ptr, type, missingCtorResolution, selector, method);
 			} else {
 				return new NSObject (ptr);
 			}
@@ -1061,15 +1078,16 @@ static NSObject ConstructNSObject (IntPtr ptr, IntPtr klass, MissingCtorResoluti
 		}
 
 		// The generic argument T is only used to cast the return value.
-		static T ConstructNSObject<T> (IntPtr ptr, Type type, MissingCtorResolution missingCtorResolution) where T: class, INativeObject
+		// The 'selector' and 'method' arguments are only used in error messages.
+		static T ConstructNSObject<T> (IntPtr ptr, Type type, MissingCtorResolution missingCtorResolution, IntPtr selector = default (IntPtr), IntPtr method = default (IntPtr)) where T: class, INativeObject
 		{
 			if (type == null)
 				throw new ArgumentNullException ("type");
 
 			var ctor = GetIntPtrConstructor (type);
 
 			if (ctor == null) {
-				MissingCtor (ptr, IntPtr.Zero, type, missingCtorResolution);
+				MissingCtor (ptr, IntPtr.Zero, type, missingCtorResolution, selector, method);
 				return null;
 			}
 
@@ -1158,7 +1176,7 @@ internal static NSObject TryGetNSObject (IntPtr ptr, bool evenInFinalizerQueue =
 			return GetNSObject (ptr, MissingCtorResolution.ThrowConstructor1NotFound);
 		}
 
-		internal static NSObject GetNSObject (IntPtr ptr, MissingCtorResolution missingCtorResolution, bool evenInFinalizerQueue = false) {
+		internal static NSObject GetNSObject (IntPtr ptr, MissingCtorResolution missingCtorResolution, bool evenInFinalizerQueue = false, IntPtr selector = default (IntPtr), IntPtr method = default (IntPtr)) {
 			if (ptr == IntPtr.Zero)
 				return null;
 
@@ -1167,7 +1185,7 @@ internal static NSObject TryGetNSObject (IntPtr ptr, bool evenInFinalizerQueue =
 			if (o != null)
 				return o;
 
-			return ConstructNSObject (ptr, Class.GetClassForObject (ptr), missingCtorResolution);
+			return ConstructNSObject (ptr, Class.GetClassForObject (ptr), missingCtorResolution, selector, method);
 		}
 
 		static public T GetNSObject<T> (IntPtr ptr) where T : NSObject
@@ -1237,7 +1255,8 @@ internal static NSObject TryGetNSObject (IntPtr ptr, bool evenInFinalizerQueue =
 		// NSObject subclasses (the test case in #13518 should work even with type checks).
 		//
 
-		static NSObject GetNSObject (IntPtr ptr, Type target_type, MissingCtorResolution missingCtorResolution, bool evenInFinalizerQueue, out bool created) {
+		// The 'selector' and 'method' arguments are only used in error messages.
+		static NSObject GetNSObject (IntPtr ptr, Type target_type, MissingCtorResolution missingCtorResolution, bool evenInFinalizerQueue, out bool created, IntPtr selector = default (IntPtr), IntPtr method = default (IntPtr)) {
 			created = false;
 
 			if (ptr == IntPtr.Zero)
@@ -1269,7 +1288,7 @@ internal static NSObject TryGetNSObject (IntPtr ptr, bool evenInFinalizerQueue =
 			}
 
 			created = true;
-			return ConstructNSObject<NSObject> (ptr, target_type, MissingCtorResolution.ThrowConstructor1NotFound);
+			return ConstructNSObject<NSObject> (ptr, target_type, MissingCtorResolution.ThrowConstructor1NotFound, selector, method);
 		}
 
 		static Type LookupINativeObjectImplementation (IntPtr ptr, Type target_type, Type implementation = null)

tests/api-shared/ObjCRuntime/RegistrarTest.cs

@@ -31,31 +31,30 @@ public class RegistrarSharedTest {
 			}
 		}
 
+#if __UNIFIED__
 		[Test]
 		public void IntPtrCtor ()
 		{
 			IntPtr ptr = IntPtr.Zero;
 			try {
 				ptr = Messaging.IntPtr_objc_msgSend (Class.GetHandle (typeof (IntPtrCtorTestClass)), Selector.GetHandle ("alloc"));
 				ptr = Messaging.IntPtr_objc_msgSend (ptr, Selector.GetHandle ("init"));
-				var ex = Assert.Throws<Exception> (() => Messaging.bool_objc_msgSend_IntPtr (ptr, Selector.GetHandle ("conformsToProtocol:"), IntPtr.Zero));
+				var ex = Assert.Throws<RuntimeException> (() => Messaging.bool_objc_msgSend_IntPtr (ptr, Selector.GetHandle ("conformsToProtocol:"), IntPtr.Zero));
 				var msg = string.Format ("Failed to marshal the Objective-C object 0x{0} (type: IntPtrCtorTestClass). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'XamarinTests.ObjCRuntime.RegistrarSharedTest+IntPtrCtorTestClass' does not have a constructor that takes one IntPtr argument).", ptr.ToString ("x"));
+				msg += "\nAdditional information:\n\tSelector: conformsToProtocol:\n\tMethod: ";
+				// The difference between the registrars is basically whether this string
+				// was constructed by native mono API or managed API.
 				if (CurrentRegistrar == Registrars.Static) {
-					msg += "\nAdditional information:\n\tSelector: conformsToProtocol:\n\tMethod: ";
-#if !XAMCORE_2_0
-#if __IOS__
-					msg += "MonoTouch.";
-#else
-					msg += "MonoMac.";
-#endif
-#endif
 					msg += "Foundation.NSObject:InvokeConformsToProtocol (intptr)\n";
+				} else {
+					msg += "Foundation.NSObject.InvokeConformsToProtocol(IntPtr)\n";
 				}
 				Assert.AreEqual (msg, ex.Message, "#message");
 			} finally {
 				Messaging.void_objc_msgSend (ptr, Selector.GetHandle ("release"));
 			}
 		}
+#endif
 
 		[Register ("IntPtrCtorTestClass")]
 		class IntPtrCtorTestClass : NSObject {

tests/monotouch-test/ObjCRuntime/RegistrarTest.cs

@@ -2087,7 +2087,7 @@ public void TestCtors ()
 				ptr = Messaging.IntPtr_objc_msgSend (Class.GetHandle (typeof (D2)), Selector.GetHandle ("alloc"));
 				ptr = Messaging.IntPtr_objc_msgSend (ptr, Selector.GetHandle ("init"));
 				// Failed to marshal the Objective-C object 0x7adf5920 (type: AppDelegate_D2). Could not find an existing managed instance for this object, nor was it possible to create a new managed instance (because the type 'AppDelegate+D2' does not have a constructor that takes one IntPtr argument).
-				Assert.Throws<Exception> (() => Runtime.GetNSObject<D2> (ptr), "c");
+				Assert.Throws<RuntimeException> (() => Runtime.GetNSObject<D2> (ptr), "c");
 			} finally {
 				Messaging.void_objc_msgSend (ptr, Selector.GetHandle ("release"));
 			}