Fix race between CompleteAsync and call dispose (#1429)

Author: JamesNK

src/Grpc.Net.Client/Internal/HttpContentClientStreamWriter.cs

@@ -47,8 +47,15 @@ public HttpContentClientStreamWriter(GrpcCall<TRequest, TResponse> call)
         {
             _call = call;
 
+            // CompleteTcs doesn't use RunContinuationsAsynchronously because we want the caller of CompleteAsync
+            // to wait until the TCS's awaiter, PushStreamContent, finishes completing the request.
+            // This is required to avoid a race condition between the HttpContent completing, and sending an
+            // END_STREAM flag to the server, and app code disposing the call, which will trigger a RST_STREAM
+            // if HttpContent has finished.
+            // See https://github.com/grpc/grpc-dotnet/issues/1394 for an example.
+            CompleteTcs = new TaskCompletionSource<bool>(TaskCreationOptions.None);
+
             WriteStreamTcs = new TaskCompletionSource<Stream>(TaskCreationOptions.RunContinuationsAsynchronously);
-            CompleteTcs = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
             WriteOptions = _call.Options.WriteOptions;
         }