Check if buffering is still happening before flushing (#522)

Fixes #521

Author: twsouthwick

src/Microsoft.AspNetCore.SystemWebAdapters.CoreServices/RegisterAdapterFeaturesMiddleware.cs

@@ -30,7 +30,11 @@ public async Task InvokeAsync(HttpContextCore context)
             }
             finally
             {
-                await context.Features.GetRequiredFeature<IHttpResponseBufferingFeature>().FlushAsync();
+                // The buffering feature may be removed if the response has ended i.e in usage with YARP
+                if (context.Features.Get<IHttpResponseBufferingFeature>() is { } buffer)
+                {
+                    await buffer.FlushAsync();
+                }
             }
         }
     }

test/Microsoft.AspNetCore.SystemWebAdapters.CoreServices.Tests/ResponseStreamTests.cs

@@ -378,6 +378,18 @@ public async Task BufferingCanBeDisabledAndFlushesUsingPipe()
         Assert.Equal("before after", result);
     }
 
+    [Fact]
+    public async Task BufferIsNotFlushedIfNotAvailable()
+    {
+        var result = await RunAsync(middleware: (ctx, next) =>
+        {
+            ctx.Features.Set<IHttpResponseBufferingFeature>(null);
+            return next(ctx);
+        });
+
+        Assert.Equal(string.Empty, result);
+    }
+
     private static Task<string> RunAsync(Action<HttpContext> action, Action<IEndpointConventionBuilder>? builder = null, Func<Http.HttpContext, RequestDelegate, Task>? middleware = null)
         => RunAsync(ctx =>
         {