Merge pull request #19629 from jtattermusch/csharp_sync_completion_queue_atomic

dont use shutdownRef count for sync completion queue

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

@@ -29,7 +29,7 @@ namespace Grpc.Core.Internal
     {
         static readonly NativeMethods Native = NativeMethods.Get();
 
-        AtomicCounter shutdownRefcount = new AtomicCounter(1);
+        AtomicCounter shutdownRefcount;
         CompletionRegistry completionRegistry;
 
         private CompletionQueueSafeHandle()
@@ -51,6 +51,7 @@ namespace Grpc.Core.Internal
         {
             var cq = Native.grpcsharp_completion_queue_create_async();
             cq.completionRegistry = completionRegistry;
+            cq.shutdownRefcount = new AtomicCounter(1);
             return cq;
         }
 
@@ -95,7 +96,7 @@ namespace Grpc.Core.Internal
 
         private void DecrementShutdownRefcount()
         {
-            if (shutdownRefcount.Decrement() == 0)
+            if (shutdownRefcount == null || shutdownRefcount.Decrement() == 0)
             {
                 Native.grpcsharp_completion_queue_shutdown(this);
             }
@@ -103,6 +104,7 @@ namespace Grpc.Core.Internal
 
         private void BeginOp()
         {
+            GrpcPreconditions.CheckNotNull(shutdownRefcount, nameof(shutdownRefcount));
             bool success = false;
             shutdownRefcount.IncrementIfNonzero(ref success);
             GrpcPreconditions.CheckState(success, "Shutdown has already been called");
@@ -110,6 +112,7 @@ namespace Grpc.Core.Internal
 
         private void EndOp()
         {
+            GrpcPreconditions.CheckNotNull(shutdownRefcount, nameof(shutdownRefcount));
             DecrementShutdownRefcount();
         }