Fix WinHttpHandler when no proxy creds after 407 response

src/System.Net.Http.WinHttpHandler/src/System/Net/Http/WinHttpAuthHelper.cs

@@ -122,6 +122,15 @@ public void CheckResponseForAuthentication(
 
                     state.LastStatusCode = statusCode;
 
+                    // If we don't have any proxy credentials to try, then we end up with 407.
+                    ICredentials proxyCreds = state.Proxy == null ?
+                        state.DefaultProxyCredentials :
+                        state.Proxy.Credentials;
+                     if (proxyCreds == null)
+                     {
+                         break;
+                     }
+
                     // Determine authorization scheme to use. We ignore the firstScheme
                     // parameter which is included in the supportedSchemes flags already.
                     // We pass the schemes to ChooseAuthScheme which will pick the scheme

src/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.cs

@@ -953,7 +953,10 @@ public async Task GetAsync_SupportedSSLVersion_Succeeds(string name, string url)
         public void Proxy_BypassFalse_GetRequestGoesThroughCustomProxy(ICredentials creds)
         {
             int port;
-            Task<LoopbackGetRequestHttpProxy.ProxyResult> proxyTask = LoopbackGetRequestHttpProxy.StartAsync(out port, requireAuth: creds != null && creds != CredentialCache.DefaultCredentials);
+            Task<LoopbackGetRequestHttpProxy.ProxyResult> proxyTask = LoopbackGetRequestHttpProxy.StartAsync(
+                out port,
+                requireAuth: creds != null && creds != CredentialCache.DefaultCredentials,
+                expectCreds: true);
             Uri proxyUrl = new Uri($"http://localhost:{port}");
 
             using (var handler = new HttpClientHandler() { Proxy = new UseSpecifiedUriWebProxy(proxyUrl, creds) })
@@ -990,6 +993,26 @@ await response.Content.ReadAsStringAsync(),
             }
         }
 
+        [Fact]
+        public void Proxy_HaveNoCredsAndUseAuthenticatedCustomProxy_ProxyAuthenticationRequiredStatusCode()
+        {
+            int port;
+            Task<LoopbackGetRequestHttpProxy.ProxyResult> proxyTask = LoopbackGetRequestHttpProxy.StartAsync(
+                out port,
+                requireAuth: true,
+                expectCreds: false);
+            Uri proxyUrl = new Uri($"http://localhost:{port}");
+
+            using (var handler = new HttpClientHandler() { Proxy = new UseSpecifiedUriWebProxy(proxyUrl, null) })
+            using (var client = new HttpClient(handler))
+            {
+                Task<HttpResponseMessage> responseTask = client.GetAsync(HttpTestServers.RemoteEchoServer);
+                Task.WaitAll(proxyTask, responseTask);
+
+                Assert.Equal(HttpStatusCode.ProxyAuthenticationRequired, responseTask.Result.StatusCode);
+            }
+        }        
+
         private sealed class UseSpecifiedUriWebProxy : IWebProxy
         {
             private readonly Uri _uri;

src/System.Net.Http/tests/FunctionalTests/LoopbackGetRequestHttpProxy.cs

@@ -24,15 +24,15 @@ public struct ProxyResult
             public string AuthenticationHeaderValue;
         }
 
-        public static Task<ProxyResult> StartAsync(out int port, bool requireAuth)
+        public static Task<ProxyResult> StartAsync(out int port, bool requireAuth, bool expectCreds)
         {
             var listener = new TcpListener(IPAddress.Loopback, 0);
             listener.Start();
             port = ((IPEndPoint)listener.LocalEndpoint).Port;
-            return StartAsync(listener, requireAuth);
+            return StartAsync(listener, requireAuth, expectCreds);
         }
 
-        private static async Task<ProxyResult> StartAsync(TcpListener listener, bool requireAuth)
+        private static async Task<ProxyResult> StartAsync(TcpListener listener, bool requireAuth, bool expectCreds)
         {
             ProxyResult result = new ProxyResult();
             var headers = new Dictionary<string, string>();
@@ -60,20 +60,26 @@ private static async Task<ProxyResult> StartAsync(TcpListener listener, bool req
                 await getAndReadRequest().ConfigureAwait(false);
 
                 // If we're expecting credentials, look for them, and if we didn't get them, send back 
-                // a response and process a new request.
+                // a 407 response. Optionally, process a new request that would expect credentials.
                 if (requireAuth && !headers.ContainsKey("Proxy-Authorization"))
                 {
-                    Task<Socket> secondListen = listener.AcceptSocketAsync();
-
                     // Send back a 407
                     await clientSocket.SendAsync(
                         new ArraySegment<byte>(Encoding.ASCII.GetBytes("HTTP/1.1 407 Proxy Auth Required\r\nProxy-Authenticate: Basic\r\n\r\n")),
                         SocketFlags.None).ConfigureAwait(false);
                     clientSocket.Shutdown(SocketShutdown.Send);
                     clientSocket.Dispose();
 
-                    // Wait for a new connection that should have an auth header this time and parse it.
-                    await getAndReadRequest().ConfigureAwait(false);
+                    if (expectCreds)
+                    {
+                        // Wait for a new connection that should have an auth header this time and parse it.
+                        await getAndReadRequest().ConfigureAwait(false);
+                    }
+                    else
+                    {
+                        // No credentials will be coming in a subsequent request.
+                        return default(ProxyResult);
+                    }
                 }
 
                 // Store any auth header we may have for later comparison.