Update

Author: jkotalik

src/Servers/IIS/IIS/src/Core/IISHttpContext.FeatureCollection.cs

@@ -206,7 +206,7 @@ protected Task CompleteResponseBodyAsync()
                 return ResponsePipeWrapper.CompleteAsync().AsTask();
             }
 
-            return Task.CompletedTask;
+            return AsyncIO.FlushAsync(moreData: false).AsTask();
         }
 
         bool IHttpUpgradeFeature.IsUpgradableRequest => true;

src/Servers/IIS/IIS/test/Common.FunctionalTests/Inprocess/HelloWorldTests.cs

@@ -26,5 +26,11 @@ public async Task HelloWorld_InProcess()
 
             Assert.Equal("?query", await _fixture.Client.GetStringAsync("/HelloWorld/Query%3F%3F?query"));
         }
+
+        [ConditionalFact]
+        public async Task CheckCompleteAsync()
+        {
+            await _fixture.Client.GetStringAsync("/CompleteAsync");
+        }
     }
 }

src/Servers/IIS/IIS/test/testassets/InProcessWebSite/Startup.cs

@@ -1315,6 +1315,25 @@ public async Task Reset_DuringRequestBody_Resets(HttpContext httpContext)
             }
         }
 
+        public Task Goaway(HttpContext httpContext)
+        {
+            httpContext.Response.Headers["Connection"] = "close";
+            return Task.CompletedTask;
+        }
+
+        private TaskCompletionSource<object> _completeAsync = new TaskCompletionSource<object>();
+        public async Task CompleteAsync(HttpContext httpContext)
+        {
+            await httpContext.Response.CompleteAsync();
+            await _completeAsync.Task;
+        }
+
+        public Task CompleteAsync_Completed(HttpContext httpContext)
+        {
+            _completeAsync.TrySetResult(null);
+            return Task.CompletedTask;
+        }
+
         public async Task Reset_DuringRequestBody_Resets_Complete(HttpContext httpContext)
         {
             await _resetDuringRequestBodyResetsCts.Task;