WinHttpTrailersHelper.cs

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Diagnostics;
using System.Net.Http.Headers;
using System.Runtime.InteropServices;
using SafeWinHttpHandle = Interop.WinHttp.SafeWinHttpHandle;

namespace System.Net.Http
{
    internal static class WinHttpTrailersHelper
    {
        // UNITTEST is true when building against WinHttpHandler.Unit.Tests, which includes the source file.
#if !NETSTANDARD2_1 && !NET && !UNITTEST
        // Trailer property name was chosen to be descriptive and be unlikely to collide with a user set property.
        // Apps and libraries will use this key so it shouldn't change.
        private const string RequestMessagePropertyName = "__ResponseTrailers";
        private sealed class HttpResponseTrailers : HttpHeaders
        {
        }
#endif
        private static readonly Lazy<bool> s_trailersSupported = new Lazy<bool>(GetTrailersSupported);
        public static bool OsSupportsTrailers => s_trailersSupported.Value;

        public static HttpHeaders GetResponseTrailers(HttpResponseMessage response)
        {
#if NETSTANDARD2_1 || NET || UNITTEST
            return response.TrailingHeaders;
#else
            HttpResponseTrailers responseTrailers = new HttpResponseTrailers();
            response.RequestMessage.Properties[RequestMessagePropertyName] = responseTrailers;
            return responseTrailers;
#endif
        }

        // There is no way to verify if WINHTTP_QUERY_FLAG_TRAILERS is supported by the OS without creating a request.
        // Instead, the WinHTTP team recommended to check if WINHTTP_OPTION_STREAM_ERROR_CODE is recognized by the OS.
        // Both features were introduced in Manganese and are planned to be backported to older Windows versions together.
        private static bool GetTrailersSupported()
        {
            using SafeWinHttpHandle sessionHandle = Interop.WinHttp.WinHttpOpen(
                    IntPtr.Zero,
                    Interop.WinHttp.WINHTTP_ACCESS_TYPE_DEFAULT_PROXY,
                    Interop.WinHttp.WINHTTP_NO_PROXY_NAME,
                    Interop.WinHttp.WINHTTP_NO_PROXY_BYPASS,
                    (int)Interop.WinHttp.WINHTTP_FLAG_ASYNC);

            if (sessionHandle.IsInvalid) return false;
            uint buffer = 0;
            uint bufferSize = sizeof(uint);
            if (Interop.WinHttp.WinHttpQueryOption(sessionHandle, Interop.WinHttp.WINHTTP_OPTION_STREAM_ERROR_CODE, ref buffer, ref bufferSize))
            {
                Debug.Fail("Querying WINHTTP_OPTION_STREAM_ERROR_CODE on a session handle should never succeed.");
                return false;
            }

            int lastError = Marshal.GetLastWin32Error();

            // New Windows builds are expected to fail with ERROR_WINHTTP_INCORRECT_HANDLE_TYPE,
            // when querying WINHTTP_OPTION_STREAM_ERROR_CODE on a session handle.
            return lastError != Interop.WinHttp.ERROR_INVALID_PARAMETER;
        }
    }
}