allow return value from native callbacks

src/csharp/Grpc.Core/Internal/NativeCallbackDispatcher.cs

@@ -27,9 +27,9 @@ using Grpc.Core.Logging;
 
 namespace Grpc.Core.Internal
 {
-    internal delegate void UniversalNativeCallback(IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5);
+    internal delegate int UniversalNativeCallback(IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5);
 
-    internal delegate void NativeCallbackDispatcherCallback(IntPtr tag, IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5);
+    internal delegate int NativeCallbackDispatcherCallback(IntPtr tag, IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5);
 
     internal class NativeCallbackDispatcher
     {
@@ -81,7 +81,7 @@ namespace Grpc.Core.Internal
         }
 
         [MonoPInvokeCallback(typeof(NativeCallbackDispatcherCallback))]
-        private static void HandleDispatcherCallback(IntPtr tag, IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5)
+        private static int HandleDispatcherCallback(IntPtr tag, IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5)
         {
             try
             {
@@ -89,13 +89,15 @@ namespace Grpc.Core.Internal
                 if (!TryGetCallback(tag, out callback))
                 {
                     Logger.Error("No native callback handler registered for tag {0}.", tag);
+                    return 0;
                 }
-                callback(arg0, arg1, arg2, arg3, arg4, arg5);
+                return callback(arg0, arg1, arg2, arg3, arg4, arg5);
             }
             catch (Exception e)
             {
                 // eat the exception, we must not throw when inside callback from native code.
                 Logger.Error(e, "Caught exception inside callback from native callback.");
+                return 0;
             }
         }
     }
@@ -103,9 +105,8 @@ namespace Grpc.Core.Internal
     internal class NativeCallbackRegistration : IDisposable
     {
         readonly IntPtr tag;
-        readonly Action disposeAction;
 
-        public NativeCallbackRegistration(IntPtr tag, Action disposeAction)
+        public NativeCallbackRegistration(IntPtr tag)
         {
             this.tag = tag;
         }

src/csharp/Grpc.Core/Internal/NativeMetadataCredentialsPlugin.cs

@@ -46,9 +46,10 @@ namespace Grpc.Core.Internal
             get { return credentials; }
         }
 
-        private void HandleUniversalCallback(IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5)
+        private int HandleUniversalCallback(IntPtr arg0, IntPtr arg1, IntPtr arg2, IntPtr arg3, IntPtr arg4, IntPtr arg5)
         {
             NativeMetadataInterceptorHandler(arg0, arg1, arg2, arg3, arg4 != IntPtr.Zero);
+            return 0;
         }
 
         private void NativeMetadataInterceptorHandler(IntPtr serviceUrlPtr, IntPtr methodNamePtr, IntPtr callbackPtr, IntPtr userDataPtr, bool isDestroy)

src/csharp/ext/grpc_csharp_ext.c

@@ -1012,7 +1012,7 @@ grpcsharp_composite_call_credentials_create(grpc_call_credentials* creds1,
 
 /* Native callback dispatcher */
 
-typedef void(GPR_CALLTYPE* grpcsharp_native_callback_dispatcher_func)(
+typedef int(GPR_CALLTYPE* grpcsharp_native_callback_dispatcher_func)(
     void* tag, void* arg0, void* arg1, void* arg2, void* arg3, void* arg4, void *arg5);
 
 static grpcsharp_native_callback_dispatcher_func native_callback_dispatcher = NULL;