fix "client not connected" after SFTP reconnect

if the server closes the session and the client reconnects,
this currently leads to a broken state because the session
is re-created, but the SFTP subsession is not and still
references the old session.

This causes all operations to fail with "client not connected" or
even throwing the "An established connection was aborted by the server."
exception of the old session.

Always re-create the SFTP subsession to fix this.

fixes sshnet#1474

src/Renci.SshNet/SftpClient.cs

@@ -2496,15 +2496,8 @@ protected override void OnConnected()
         {
             base.OnConnected();
 
-            var sftpSession = _sftpSession;
-            if (sftpSession is null)
-            {
-                _sftpSession = CreateAndConnectToSftpSession();
-            }
-            else if (!sftpSession.IsOpen)
-            {
-                sftpSession.Connect();
-            }
+            _sftpSession?.Dispose();
+            _sftpSession = CreateAndConnectToSftpSession();
         }
 
         /// <summary>

test/Renci.SshNet.IntegrationTests/ConnectivityTests.cs

@@ -326,6 +326,54 @@ public async Task SftpClient_HandleSftpSessionCloseAsync()
             }
         }
 
+        [TestMethod]
+        public void SftpClient_HandleSftpSessionAbortByServer()
+        {
+            using (var client = new SftpClient(_connectionInfoFactory.Create()))
+            {
+                client.Connect();
+                Assert.IsTrue(client.IsConnected);
+
+                _sshConnectionDisruptor.BreakConnections();
+                WaitForConnectionInterruption(client);
+                Assert.IsFalse(client.IsConnected);
+
+                client.Connect();
+                Assert.IsTrue(client.IsConnected);
+
+                foreach (var file in client.ListDirectory("."))
+                {
+                }
+
+                client.Disconnect();
+                Assert.IsFalse(client.IsConnected);
+            }
+        }
+
+        [TestMethod]
+        public async Task SftpClient_HandleSftpSessionAbortByServerAsync()
+        {
+            using (var client = new SftpClient(_connectionInfoFactory.Create()))
+            {
+                await client.ConnectAsync(CancellationToken.None);
+                Assert.IsTrue(client.IsConnected);
+
+                _sshConnectionDisruptor.BreakConnections();
+                WaitForConnectionInterruption(client);
+                Assert.IsFalse(client.IsConnected);
+
+                await client.ConnectAsync(CancellationToken.None);
+                Assert.IsTrue(client.IsConnected);
+
+                await foreach (var file in client.ListDirectoryAsync(".", CancellationToken.None))
+                {
+                }
+
+                client.Disconnect();
+                Assert.IsFalse(client.IsConnected);
+            }
+        }
+
         [TestMethod]
         public void Common_DetectSessionKilledOnServer()
         {