dont allow new writes if theres a write with delayed completion

src/csharp/Grpc.Core.Tests/Internal/AsyncCallTest.cs

@@ -223,6 +223,34 @@ namespace Grpc.Core.Internal.Tests
             AssertUnaryResponseError(asyncCall, fakeCall, resultTask, StatusCode.Internal);
         }
 
+        [Test]
+        public void ClientStreaming_WriteFailureThrowsRpcException3()
+        {
+            var resultTask = asyncCall.ClientStreamingCallAsync();
+            var requestStream = new ClientRequestStream<string, string>(asyncCall);
+
+            var writeTask = requestStream.WriteAsync("request1");
+            fakeCall.SendCompletionHandler(false);
+
+            // Until the delayed write completion has been triggered,
+            // we still act as if there was an active write.
+            Assert.Throws(typeof(InvalidOperationException), () => requestStream.WriteAsync("request2"));
+
+            fakeCall.UnaryResponseClientHandler(true,
+                CreateClientSideStatus(StatusCode.Internal),
+                null,
+                new Metadata());
+
+            var ex = Assert.ThrowsAsync<RpcException>(async () => await writeTask);
+            Assert.AreEqual(StatusCode.Internal, ex.Status.StatusCode);
+
+            // Following attempts to write keep delivering the same status
+            var ex2 = Assert.ThrowsAsync<RpcException>(async () => await requestStream.WriteAsync("after call has finished"));
+            Assert.AreEqual(StatusCode.Internal, ex2.Status.StatusCode);
+
+            AssertUnaryResponseError(asyncCall, fakeCall, resultTask, StatusCode.Internal);
+        }
+
         [Test]
         public void ClientStreaming_WriteAfterReceivingStatusThrowsRpcException()
         {

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

@@ -372,7 +372,8 @@ namespace Grpc.Core.Internal
         private Task CheckSendPreconditionsClientSide()
         {
             GrpcPreconditions.CheckState(!halfcloseRequested, "Request stream has already been completed.");
-            GrpcPreconditions.CheckState(streamingWriteTcs == null, "Only one write can be pending at a time.");
+            // if there is a delayed streaming write, we will treat that as if the write was still in progress until the call finishes.
+            GrpcPreconditions.CheckState(streamingWriteTcs == null && (finished || delayedStreamingWriteTcs == null), "Only one write can be pending at a time.");
 
             if (cancelRequested)
             {

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

@@ -208,7 +208,7 @@ namespace Grpc.Core.Internal
         {
             GrpcPreconditions.CheckState(!halfcloseRequested, "Response stream has already been completed.");
             GrpcPreconditions.CheckState(!finished, "Already finished.");
-            GrpcPreconditions.CheckState(streamingWriteTcs == null, "Only one write can be pending at a time");
+            GrpcPreconditions.CheckState(streamingWriteTcs == null && delayedStreamingWriteTcs == null, "Only one write can be pending at a time");
             GrpcPreconditions.CheckState(!disposed);
 
             return null;