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Remove calls to Socket.Poll and use SocketShutdown.Both #1706

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Merged
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Oct 1, 2025
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Remove calls to Socket.Poll and use SocketShutdown.Both #1706

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28 changes: 0 additions & 28 deletions src/Renci.SshNet/Abstractions/SocketAbstraction.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,34 +12,6 @@ namespace Renci.SshNet.Abstractions
{
internal static partial class SocketAbstraction
{
public static bool CanRead(Socket socket)
{
if (socket.Connected)
{
return socket.Poll(-1, SelectMode.SelectRead) && socket.Available > 0;
}

return false;
}

/// <summary>
/// Returns a value indicating whether the specified <see cref="Socket"/> can be used
/// to send data.
/// </summary>
/// <param name="socket">The <see cref="Socket"/> to check.</param>
/// <returns>
/// <see langword="true"/> if <paramref name="socket"/> can be written to; otherwise, <see langword="false"/>.
/// </returns>
public static bool CanWrite(Socket socket)
{
if (socket != null && socket.Connected)
{
return socket.Poll(-1, SelectMode.SelectWrite);
}

return false;
}

public static Socket Connect(IPEndPoint remoteEndpoint, TimeSpan connectTimeout)
{
var socket = new Socket(remoteEndpoint.AddressFamily, SocketType.Stream, ProtocolType.Tcp) { NoDelay = true };
Expand Down
11 changes: 0 additions & 11 deletions src/Renci.SshNet/Common/Extensions.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -10,7 +10,6 @@
using System.Runtime.CompilerServices;
using System.Threading;

using Renci.SshNet.Abstractions;
using Renci.SshNet.Messages;

namespace Renci.SshNet.Common
Expand Down Expand Up @@ -319,16 +318,6 @@ public static byte[] Concat(this byte[] first, byte[] second)
return concat;
}

internal static bool CanRead(this Socket socket)
{
return SocketAbstraction.CanRead(socket);
}

internal static bool CanWrite(this Socket socket)
{
return SocketAbstraction.CanWrite(socket);
}

internal static bool IsConnected(this Socket socket)
{
if (socket is null)
Expand Down
179 changes: 18 additions & 161 deletions src/Renci.SshNet/Session.cs
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whitespace off: https://github.com/sshnet/SSH.NET/pull/1706/files?w=1

Original file line number Diff line number Diff line change
Expand Up @@ -81,12 +81,6 @@ public sealed class Session : ISession
private readonly ISocketFactory _socketFactory;
private readonly ILogger _logger;

/// <summary>
/// Holds an object that is used to ensure only a single thread can read from
/// <see cref="_socket"/> at any given time.
/// </summary>
private readonly Lock _socketReadLock = new Lock();
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no longer needed


/// <summary>
/// Holds an object that is used to ensure only a single thread can write to
/// <see cref="_socket"/> at any given time.
Expand All @@ -105,7 +99,7 @@ public sealed class Session : ISession
/// This is also used to ensure that <see cref="_socket"/> will not be disposed
/// while performing a given operation or set of operations on <see cref="_socket"/>.
/// </remarks>
private readonly SemaphoreSlim _socketDisposeLock = new SemaphoreSlim(1, 1);
private readonly Lock _socketDisposeLock = new Lock();

/// <summary>
/// Holds an object that is used to ensure only a single thread can connect
Expand Down Expand Up @@ -279,17 +273,11 @@ public bool IsConnected
{
get
{
if (_disposed || _isDisconnectMessageSent || !_isAuthenticated)
{
return false;
}

if (_messageListenerCompleted is null || _messageListenerCompleted.WaitOne(0))
{
return false;
}

return IsSocketConnected();
return !_disposed &&
!_isDisconnectMessageSent &&
_isAuthenticated &&
_messageListenerCompleted?.WaitOne(0) == false &&
_socket.IsConnected();
}
}

Expand Down Expand Up @@ -1046,7 +1034,7 @@ internal void WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
/// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
internal void SendMessage(Message message)
{
if (!_socket.CanWrite())
if (!_socket.IsConnected())
{
throw new SshConnectionException("Client not connected.");
}
Expand Down Expand Up @@ -1161,9 +1149,7 @@ internal void SendMessage(Message message)
/// </remarks>
private void SendPacket(byte[] packet, int offset, int length)
{
_socketDisposeLock.Wait();

try
lock (_socketDisposeLock)
{
if (!_socket.IsConnected())
{
Expand All @@ -1172,10 +1158,6 @@ private void SendPacket(byte[] packet, int offset, int length)

SocketAbstraction.Send(_socket, packet, offset, length);
}
finally
{
_ = _socketDisposeLock.Release();
}
}

/// <summary>
Expand Down Expand Up @@ -1259,11 +1241,6 @@ private Message ReceiveMessage(Socket socket)
byte[] data;
uint packetLength;

// avoid reading from socket while IsSocketConnected is attempting to determine whether the
// socket is still connected by invoking Socket.Poll(...) and subsequently verifying value of
// Socket.Available
lock (_socketReadLock)
{
// Read first block - which starts with the packet length
var firstBlock = new byte[blockSize];
if (TrySocketRead(socket, firstBlock, 0, blockSize) == 0)
Expand Down Expand Up @@ -1330,7 +1307,6 @@ private Message ReceiveMessage(Socket socket)
return null;
}
}
}

// validate encrypted message against MAC
if (_serverMac != null && _serverEtm)
Expand Down Expand Up @@ -1888,84 +1864,6 @@ private static string ToHex(byte[] bytes)
#endif
}

/// <summary>
/// Gets a value indicating whether the socket is connected.
/// </summary>
/// <returns>
/// <see langword="true"/> if the socket is connected; otherwise, <see langword="false"/>.
/// </returns>
/// <remarks>
/// <para>
/// As a first check we verify whether <see cref="Socket.Connected"/> is
/// <see langword="true"/>. However, this only returns the state of the socket as of
/// the last I/O operation.
/// </para>
/// <para>
/// Therefore we use the combination of <see cref="Socket.Poll(int, SelectMode)"/> with mode <see cref="SelectMode.SelectRead"/>
/// and <see cref="Socket.Available"/> to verify if the socket is still connected.
/// </para>
/// <para>
/// The MSDN doc mention the following on the return value of <see cref="Socket.Poll(int, SelectMode)"/>
/// with mode <see cref="SelectMode.SelectRead"/>:
/// <list type="bullet">
/// <item>
/// <description><see langword="true"/> if data is available for reading;</description>
/// </item>
/// <item>
/// <description><see langword="true"/> if the connection has been closed, reset, or terminated; otherwise, returns <see langword="false"/>.</description>
/// </item>
/// </list>
/// </para>
/// <para>
/// <c>Conclusion:</c> when the return value is <see langword="true"/> - but no data is available for reading - then
/// the socket is no longer connected.
/// </para>
/// <para>
/// When a <see cref="Socket"/> is used from multiple threads, there's a race condition
/// between the invocation of <see cref="Socket.Poll(int, SelectMode)"/> and the moment
/// when the value of <see cref="Socket.Available"/> is obtained. To workaround this issue
/// we synchronize reads from the <see cref="Socket"/>.
/// </para>
/// <para>
/// We assume the socket is still connected if the read lock cannot be acquired immediately.
/// In this case, we just return <see langword="true"/> without actually waiting to acquire
/// the lock. We don't want to wait for the read lock if another thread already has it because
/// there are cases where the other thread holding the lock can be waiting indefinitely for
/// a socket read operation to complete.
/// </para>
/// </remarks>
private bool IsSocketConnected()
{
_socketDisposeLock.Wait();

try
{
if (!_socket.IsConnected())
{
return false;
}

if (!_socketReadLock.TryEnter())
{
return true;
}

try
{
var connectionClosedOrDataAvailable = _socket.Poll(0, SelectMode.SelectRead);
return !(connectionClosedOrDataAvailable && _socket.Available == 0);
}
finally
{
_socketReadLock.Exit();
}
}
finally
{
_ = _socketDisposeLock.Release();
}
}

/// <summary>
/// Performs a blocking read on the socket until <paramref name="length"/> bytes are received.
/// </summary>
Expand All @@ -1988,16 +1886,13 @@ private static int TrySocketRead(Socket socket, byte[] buffer, int offset, int l
/// </summary>
private void SocketDisconnectAndDispose()
{
if (_socket != null)
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not a hot path, so simplify the locking and save some indentation

{
_socketDisposeLock.Wait();

try
lock (_socketDisposeLock)
{
#pragma warning disable CA1508 // Avoid dead conditional code; Value could have been changed by another thread.
if (_socket != null)
#pragma warning restore CA1508 // Avoid dead conditional code
if (_socket is null)
{
return;
}

if (_socket.Connected)
{
try
Expand All @@ -2010,7 +1905,7 @@ private void SocketDisconnectAndDispose()
// This may result in a SocketException (eg. An existing connection was forcibly
// closed by the remote host) which we'll log and ignore as it means the socket
// was already shut down.
_socket.Shutdown(SocketShutdown.Send);
_socket.Shutdown(SocketShutdown.Both);
}
catch (SocketException ex)
{
Expand All @@ -2024,12 +1919,6 @@ private void SocketDisconnectAndDispose()
_socket = null;
}
}
finally
{
_ = _socketDisposeLock.Release();
}
}
}

/// <summary>
/// Listens for incoming message from the server and handles them. This method run as a task on separate thread.
Expand All @@ -2048,25 +1937,6 @@ private void MessageListener()
break;
}

try
{
// Block until either data is available or the socket is closed
var connectionClosedOrDataAvailable = socket.Poll(-1, SelectMode.SelectRead);
if (connectionClosedOrDataAvailable && socket.Available == 0)
{
// connection with SSH server was closed or connection was reset
break;
}
}
catch (ObjectDisposedException)
{
// The socket was disposed by either:
// * a call to Disconnect()
// * a call to Dispose()
// * a SSH_MSG_DISCONNECT received from server
break;
}

var message = ReceiveMessage(socket);
if (message is null)
{
Expand Down Expand Up @@ -2102,37 +1972,24 @@ private void MessageListener()
/// <param name="exp">The <see cref="Exception"/>.</param>
private void RaiseError(Exception exp)
{
var connectionException = exp as SshConnectionException;

_logger.LogInformation(exp, "[{SessionId}] Raised exception", SessionIdHex);

if (_isDisconnecting)
{
// a connection exception which is raised while isDisconnecting is normal and
// should be ignored
if (connectionException != null)
{
return;
}

// any timeout while disconnecting can be caused by loss of connectivity
// altogether and should be ignored
if (exp is SocketException socketException && socketException.SocketErrorCode == SocketError.TimedOut)
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dead code, we always wrap SocketException in SshConnection

if (_isDisconnecting && exp is SshConnectionException or ObjectDisposedException)
{
// Such an exception raised while isDisconnecting is expected and can be ignored.
return;
}
}

// "save" exception and set exception wait handle to ensure any waits are interrupted
_exception = exp;
_ = _exceptionWaitHandle.Set();

ErrorOccured?.Invoke(this, new ExceptionEventArgs(exp));

if (connectionException != null)
if (exp is SshConnectionException connectionException)
{
_logger.LogInformation(exp, "[{SessionId}] Disconnecting after exception", SessionIdHex);
Disconnect(connectionException.DisconnectReason, exp.ToString());
Disconnect(connectionException.DisconnectReason, exp.Message);
}
}

Expand Down
6 changes: 0 additions & 6 deletions test/Renci.SshNet.Tests/Classes/AbstractionsTest.cs
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Expand Up @@ -8,12 +8,6 @@ namespace Renci.SshNet.Tests.Classes
[TestClass]
public class AbstractionsTest
{
[TestMethod]
public void SocketAbstraction_CanWrite_ShouldReturnFalseWhenSocketIsNull()
{
Assert.IsFalse(SocketAbstraction.CanWrite(null));
}

[TestMethod]
public void CryptoAbstraction_GenerateRandom_ShouldPerformNoOpWhenDataIsZeroLength()
{
Expand Down
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