add test for nested ssl authentication (dotnet#873)

* add test for nested ssl authentication

* feedback from review

src/libraries/System.Net.Security/tests/FunctionalTests/SslStreamNetworkStreamTest.cs

@@ -2,12 +2,13 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
+using System.IO;
 using System.Net.Sockets;
+using System.Net.Test.Common;
 using System.Security.Authentication;
 using System.Security.Cryptography.X509Certificates;
 using System.Text;
 using System.Threading.Tasks;
-
 using Xunit;
 
 namespace System.Net.Security.Tests
@@ -167,6 +168,22 @@ public async Task SslStream_NetworkStream_Renegotiation_Succeeds()
             }
         }
 
+        [Fact]
+        public async Task SslStream_NestedAuth_Throws()
+        {
+            VirtualNetwork network = new VirtualNetwork();
+
+            using (var clientStream = new VirtualNetworkStream(network, isServer: false))
+            using (var serverStream = new VirtualNetworkStream(network, isServer: true))
+            using (var ssl = new SslStream(clientStream))
+            {
+                // Start handshake.
+                Task task = ssl.AuthenticateAsClientAsync("foo.com", null, SslProtocols.Tls12, false);
+                // Do it again without waiting for previous one to finish.
+                await Assert.ThrowsAsync<InvalidOperationException>(() => ssl.AuthenticateAsClientAsync("foo.com", null, SslProtocols.Tls12, false));
+            }
+        }
+
         private static bool ValidateServerCertificate(
             object sender,
             X509Certificate retrievedServerPublicCertificate,