make SendStatusFromServer independent on WriteAsync

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

@@ -69,6 +69,7 @@ namespace Grpc.Core.Internal
 
         protected AsyncCompletionDelegate<object> sendCompletionDelegate;  // Completion of a pending send or sendclose if not null.
         protected TaskCompletionSource<TRead> streamingReadTcs;  // Completion of a pending streaming read if not null.
+        protected TaskCompletionSource<object> sendStatusFromServerTcs;
 
         protected bool readingDone;  // True if last read (i.e. read with null payload) was already received.
         protected bool halfcloseRequested;  // True if send close have been initiated.
@@ -328,22 +329,18 @@ namespace Grpc.Core.Internal
         /// </summary>
         protected void HandleSendStatusFromServerFinished(bool success)
         {
-            AsyncCompletionDelegate<object> origCompletionDelegate = null;
             lock (myLock)
             {
-                origCompletionDelegate = sendCompletionDelegate;
-                sendCompletionDelegate = null;
-
                 ReleaseResourcesIfPossible();
             }
 
             if (!success)
             {
-                FireCompletion(origCompletionDelegate, null, new InvalidOperationException("Error sending status from server."));
+                sendStatusFromServerTcs.SetException(new InvalidOperationException("Error sending status from server."));
             }
             else
             {
-                FireCompletion(origCompletionDelegate, null, null);
+                sendStatusFromServerTcs.SetResult(null);
             }
         }
 

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

@@ -136,24 +136,24 @@ namespace Grpc.Core.Internal
         }
 
         /// <summary>
-        /// Sends call result status, also indicating server is done with streaming responses.
+        /// Sends call result status, indicating we are done with writes.
-        /// Only one pending send action is allowed at any given time.
+        /// Sending a status different from StatusCode.OK will also implicitly cancel the call.
-        /// completionDelegate is called when the operation finishes.
         /// </summary>
-        public void StartSendStatusFromServer(Status status, Metadata trailers, AsyncCompletionDelegate<object> completionDelegate)
+        public Task SendStatusFromServerAsync(Status status, Metadata trailers)
         {
             lock (myLock)
             {
-                GrpcPreconditions.CheckNotNull(completionDelegate, "Completion delegate cannot be null");
+                GrpcPreconditions.CheckState(started);
-                CheckSendingAllowed(allowFinished: false);
+                GrpcPreconditions.CheckState(!disposed);
+                GrpcPreconditions.CheckState(!halfcloseRequested, "Can only send status from server once.");
 
                 using (var metadataArray = MetadataArraySafeHandle.Create(trailers))
                 {
                     call.StartSendStatusFromServer(HandleSendStatusFromServerFinished, status, metadataArray, !initialMetadataSent);
                 }
                 halfcloseRequested = true;
-                readingDone = true;
+                sendStatusFromServerTcs = new TaskCompletionSource<object>();
-                sendCompletionDelegate = completionDelegate;
+                return sendStatusFromServerTcs.Task;
             }
         }
 
@@ -198,12 +198,13 @@ namespace Grpc.Core.Internal
         /// </summary>
         private void HandleFinishedServerside(bool success, bool cancelled)
         {
+            // NOTE: because this event is a result of batch containing GRPC_OP_RECV_CLOSE_ON_SERVER,
+            // success will be always set to true.
             lock (myLock)
             {
                 finished = true;
                 ReleaseResourcesIfPossible();
             }
-            // TODO(jtattermusch): handle error
 
             if (cancelled)
             {

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

@@ -93,7 +93,7 @@ namespace Grpc.Core.Internal
             }
             try
             {
-                await responseStream.WriteStatusAsync(status, context.ResponseTrailers).ConfigureAwait(false);
+                await asyncCall.SendStatusFromServerAsync(status, context.ResponseTrailers).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -149,7 +149,7 @@ namespace Grpc.Core.Internal
 
             try
             {
-                await responseStream.WriteStatusAsync(status, context.ResponseTrailers).ConfigureAwait(false);
+                await asyncCall.SendStatusFromServerAsync(status, context.ResponseTrailers).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -209,7 +209,7 @@ namespace Grpc.Core.Internal
 
             try
             {
-                await responseStream.WriteStatusAsync(status, context.ResponseTrailers).ConfigureAwait(false);
+                await asyncCall.SendStatusFromServerAsync(status, context.ResponseTrailers).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -260,7 +260,7 @@ namespace Grpc.Core.Internal
             }
             try
             {
-                await responseStream.WriteStatusAsync(status, context.ResponseTrailers).ConfigureAwait(false);
+                await asyncCall.SendStatusFromServerAsync(status, context.ResponseTrailers).ConfigureAwait(false);
             }
             catch (OperationCanceledException)
             {
@@ -282,9 +282,7 @@ namespace Grpc.Core.Internal
             
             asyncCall.Initialize(newRpc.Call);
             var finishedTask = asyncCall.ServerSideCallAsync();
-            var responseStream = new ServerResponseStream<byte[], byte[]>(asyncCall);
+            await asyncCall.SendStatusFromServerAsync(new Status(StatusCode.Unimplemented, ""), Metadata.Empty).ConfigureAwait(false);
-
-            await responseStream.WriteStatusAsync(new Status(StatusCode.Unimplemented, ""), Metadata.Empty).ConfigureAwait(false);
             await finishedTask.ConfigureAwait(false);
         }
     }

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

@@ -57,13 +57,6 @@ namespace Grpc.Core.Internal
             return taskSource.Task;
         }
 
-        public Task WriteStatusAsync(Status status, Metadata trailers)
-        {
-            var taskSource = new AsyncCompletionTaskSource<object>();
-            call.StartSendStatusFromServer(status, trailers, taskSource.CompletionDelegate);
-            return taskSource.Task;
-        }
-
         public Task WriteResponseHeadersAsync(Metadata responseHeaders)
         {
             var taskSource = new AsyncCompletionTaskSource<object>();