[runtime] Throw a MarshalDirectiveException when returning an array from a pinvoke method. (mono#20425)

Fixes mono#20367.

Some light massaging done by @UnityAlex to cherry pick

Author: vargaz

mono/metadata/marshal.c

@@ -6822,10 +6822,12 @@ emit_marshal_array (EmitMarshalContext *m, int argnum, MonoType *t,
 			mono_mb_emit_ldloc (mb, conv_arg);
 		break;
 
-	case MARSHAL_ACTION_CONV_RESULT:
-		/* fixme: we need conversions here */
-		mono_mb_emit_stloc (mb, 3);
+	case MARSHAL_ACTION_CONV_RESULT: {
+		mono_mb_emit_byte (mb, CEE_POP);
+		char *msg = g_strdup_printf ("Cannot marshal 'return value': Invalid managed/unmanaged type combination.");
+		mono_mb_emit_exception_marshal_directive (mb, msg);
 		break;
+	}
 
 	case MARSHAL_ACTION_MANAGED_CONV_IN: {
 		MonoClass *eklass;

mono/tests/libtest.c

@@ -7556,3 +7556,12 @@ mono_test_native_to_managed_exception_rethrow (NativeToManagedExceptionRethrowFu
 	pthread_join (t, NULL);
 }
 #endif
+
+static guint8 static_arr[] = { 1, 2, 3, 4 };
+
+LIBTEST_API guint8*
+mono_test_marshal_return_array (void)
+{
+	return static_arr;
+}
+

mono/tests/pinvoke2.cs

@@ -1987,5 +1987,17 @@ public static unsafe int test_0_fixed_buffer_unicode () {
 			return 2;
 		return 0;
 	}
+
+	[DllImport ("libtest", EntryPoint="mono_test_marshal_return_array")]
+	public static extern int[] mono_test_marshal_return_array ();
+
+	public static int test_0_return_array () {
+		try {
+			var arr = mono_test_marshal_return_array ();
+			return 1;
+		} catch (MarshalDirectiveException) {
+			return 0;
+		}
+	}
 }