Backwards compatibility test

Author: JamesNK

src/libraries/Common/tests/System/Net/Http/Http2LoopbackConnection.cs

@@ -100,6 +100,8 @@ private async Task ReadPrefixAsync()
                 // The contents of what we send don't really matter, as long as it is interpreted by SocketsHttpHandler as an invalid response.
                 await _connectionStream.WriteAsync(Encoding.ASCII.GetBytes("HTTP/2.0 400 Bad Request\r\n\r\n"));
                 _connectionSocket.Shutdown(SocketShutdown.Send);
+
+                throw new Exception("HTTP/1.1 request sent to HTTP/2 connection.");
             }
         }
 

src/libraries/System.Net.Http.WinHttpHandler/tests/FunctionalTests/BidirectionStreamingTest.cs

@@ -26,6 +26,8 @@ public BidirectionStreamingTest(ITestOutputHelper output) : base(output)
 
         public static bool TestsEnabled => OsSupportsWinHttpBidirectionalStreaming && PlatformDetection.SupportsAlpn;
 
+        public static bool TestsBackwardsCompatibilityEnabled => !OsSupportsWinHttpBidirectionalStreaming && PlatformDetection.SupportsAlpn;
+
         protected override Version UseVersion => new Version(2, 0);
 
         protected static byte[] DataBytes = Encoding.ASCII.GetBytes("data");
@@ -311,6 +313,32 @@ async Task RunServer()
             }
         }
 
+        [ConditionalFact(nameof(TestsBackwardsCompatibilityEnabled))]
+        public async Task BackwardsCompatibility_DowngradeToHttp11()
+        {
+            TaskCompletionSource<object> completeStreamTcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+            using (Http2LoopbackServer server = Http2LoopbackServer.CreateServer())
+            using (HttpClient client = CreateHttpClient())
+            {
+                HttpRequestMessage message = new HttpRequestMessage(HttpMethod.Post, server.Address);
+                message.Version = new Version(2, 0);
+                message.Content = new StreamingContent(async s =>
+                {
+                    await completeStreamTcs.Task;
+                });
+
+                Task<HttpResponseMessage> sendTask = client.SendAsync(message, HttpCompletionOption.ResponseHeadersRead);
+
+                // If WinHTTP doesn't support streaming a request without a length then it will fallback
+                // to HTTP/1.1. This is pretty weird behavior but we keep it for backwards compatibility.
+                Exception ex = await Assert.ThrowsAsync<Exception>(async () => await server.EstablishConnectionAsync());
+                Assert.Equal("HTTP/1.1 request sent to HTTP/2 connection.", ex.Message);
+
+                completeStreamTcs.SetResult(null);
+            }
+        }
+
         private class StreamingContent : HttpContent
         {
             private readonly Func<Stream, Task> _writeFunc;