Add IHttpResponseTrailersFeature and extensions

src/Microsoft.AspNetCore.Http.Abstractions/Extensions/ResponseTrailerExtensions.cs

@@ -0,0 +1,53 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+using System;
+using Microsoft.AspNetCore.Http.Features;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.AspNetCore.Http
+{
+    public static class ResponseTrailerExtensions
+    {
+        private const string Trailer = "Trailer";
+
+        /// <summary>
+        /// Adds the given trailer name to the 'Trailer' response header. This must happen before the response headers are sent.
+        /// </summary>
+        /// <param name="response"></param>
+        /// <param name="trailerName"></param>
+        public static void DeclareTrailer(this HttpResponse response, string trailerName)
+        {
+            response.Headers.AppendCommaSeparatedValues(Trailer, trailerName);
+        }
+
+        /// <summary>
+        /// Indicates if the server supports sending trailer headers for this response.
+        /// </summary>
+        /// <param name="response"></param>
+        /// <returns></returns>
+        public static bool SupportsTrailers(this HttpResponse response)
+        {
+            var feature = response.HttpContext.Features.Get<IHttpResponseTrailersFeature>();
+            return feature?.Trailers != null && !feature.Trailers.IsReadOnly;
+        }
+
+        /// <summary>
+        /// Adds the given trailer header to the trailers collection to be sent at the end of the response body.
+        /// Check <see cref="SupportsTrailers" /> or an InvalidOperationException may be thrown.
+        /// </summary>
+        /// <param name="response"></param>
+        /// <param name="trailerName"></param>
+        /// <param name="trailerValues"></param>
+        public static void AppendTrailer(this HttpResponse response, string trailerName, StringValues trailerValues)
+        {
+            var feature = response.HttpContext.Features.Get<IHttpResponseTrailersFeature>();
+            if (feature?.Trailers == null || feature.Trailers.IsReadOnly)
+            {
+                throw new InvalidOperationException("Trailers are not supported for this response.");
+            }
+
+            feature.Trailers.Append(trailerName, trailerValues);
+        }
+    }
+}

src/Microsoft.AspNetCore.Http.Features/IHttpResponseTrailersFeature.cs

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+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Http.Features
+{
+    public interface IHttpResponseTrailersFeature
+    {
+        IHeaderDictionary Trailers { get; set; }
+    }
+}