Make trailers safer in TestHost (#32633)

Author: JamesNK

src/Hosting/TestHost/src/ClientHandler.cs

@@ -184,12 +184,16 @@ protected override async Task<HttpResponseMessage> SendAsync(
             // Copy trailers to the response message when the response stream is complete
             contextBuilder.RegisterResponseReadCompleteCallback(context =>
             {
-                var responseTrailersFeature = context.Features.Get<IHttpResponseTrailersFeature>()!;
+                var responseTrailersFeature = context.Features.Get<IHttpResponseTrailersFeature>();
 
-                foreach (var trailer in responseTrailersFeature.Trailers)
+                // Trailers collection is settable so double check the app hasn't set it to null.
+                if (responseTrailersFeature?.Trailers != null)
                 {
-                    bool success = response.TrailingHeaders.TryAddWithoutValidation(trailer.Key, (IEnumerable<string>)trailer.Value);
-                    Contract.Assert(success, "Bad trailer");
+                    foreach (var trailer in responseTrailersFeature.Trailers)
+                    {
+                        bool success = response.TrailingHeaders.TryAddWithoutValidation(trailer.Key, (IEnumerable<string>)trailer.Value);
+                        Contract.Assert(success, "Bad trailer");
+                    }
                 }
             });
 

src/Hosting/TestHost/src/HttpContextBuilder.cs

@@ -47,7 +47,7 @@ internal HttpContextBuilder(ApplicationWrapper application, bool allowSynchronou
             _requestPipe = new Pipe();
 
             var responsePipe = new Pipe();
-            _responseReaderStream = new ResponseBodyReaderStream(responsePipe, ClientInitiatedAbort, () => _responseReadCompleteCallback?.Invoke(_httpContext));
+            _responseReaderStream = new ResponseBodyReaderStream(responsePipe, ClientInitiatedAbort, ResponseBodyReadComplete);
             _responsePipeWriter = new ResponseBodyPipeWriter(responsePipe, ReturnResponseMessageAsync);
             _responseFeature.Body = new ResponseBodyWriterStream(_responsePipeWriter, () => AllowSynchronousIO);
             _responseFeature.BodyWriter = _responsePipeWriter;
@@ -180,6 +180,11 @@ internal void ClientInitiatedAbort()
             CancelRequestBody();
         }
 
+        private void ResponseBodyReadComplete()
+        {
+            _responseReadCompleteCallback?.Invoke(_httpContext);
+        }
+
         private bool RequestBodyReadInProgress()
         {
             try