Add flush() method to SSHSocket, SSHClient, and SSHChannel

CHANGELOG.md

@@ -1,7 +1,10 @@
-## [2.22.1] - 2026-07-14
+## [2.22.2] - 2026-07-14
+- Added `flush()` to `SSHSocket`, `SSHClient`, and `SSHChannel` to allow force flushing of buffered outgoing data [#183]. Thanks [@vicajilau].
+
+## [2.22.1] - 2026-07-13
 - Fixed a keepalive issue where overlapping pings could occur and caught errors during ping execution. Thanks [@vicajilau].
 
-## [2.22.0] - 2026-07-13
+## [2.22.0] - 2026-07-03
 - Added optional `handshakeTimeout` and `authTimeout` to `SSHClient` to limit connection negotiation and user authentication times [#182]. Thanks [@GT-610].
 
 ## [2.21.1] - 2026-07-02

lib/src/socket/ssh_socket.dart

@@ -28,4 +28,7 @@ abstract class SSHSocket {
   Future<void> close();
 
   void destroy();
+
+  /// Force flush any buffered outgoing data.
+  Future<void> flush() async {}
 }

lib/src/socket/ssh_socket_io.dart

@@ -37,6 +37,11 @@ class _SSHNativeSocket implements SSHSocket {
     _socket.destroy();
   }
 
+  @override
+  Future<void> flush() async {
+    await _socket.flush();
+  }
+
   @override
   String toString() {
     final address = '${_socket.remoteAddress.host}:${_socket.remotePort}';

lib/src/ssh_channel.dart

@@ -31,6 +31,8 @@ class SSHChannelController {
 
   SSHChannel get channel => SSHChannel(this);
 
+  final Future<void> Function()? onFlush;
+
   SSHChannelController({
     required this.localId,
     required this.localMaximumPacketSize,
@@ -39,6 +41,7 @@ class SSHChannelController {
     required this.remoteInitialWindowSize,
     required this.remoteMaximumPacketSize,
     required this.sendMessage,
+    this.onFlush,
     this.printDebug,
   }) {
     if (remoteInitialWindowSize > 0) {
@@ -404,6 +407,10 @@ class SSHChannelController {
       _remoteWindow -= data.bytes.length;
     }
   });
+
+  Future<void> flush() async {
+    await onFlush?.call();
+  }
 }
 
 class SSHChannel {
@@ -434,6 +441,9 @@ class SSHChannel {
     sink.add(SSHChannelData(data, type: type));
   }
 
+  /// Force flush any buffered outgoing data on this channel to the socket.
+  Future<void> flush() => _controller.flush();
+
   void setRequestHandler(SSHChannelRequestHandler handler) {
     _controller._requestHandler = handler;
   }

lib/src/ssh_client.dart

@@ -701,6 +701,11 @@ class SSHClient {
     _transport.close();
   }
 
+  /// Force flush any buffered outgoing data to the socket.
+  Future<void> flush() async {
+    await _transport.flush();
+  }
+
   /// Close all channels that are currently open.
   void _closeChannels() {
     for (final channel in _channels.values) {
@@ -1376,6 +1381,7 @@ class SSHClient {
       remoteInitialWindowSize: remoteInitialWindowSize,
       remoteMaximumPacketSize: remoteMaximumPacketSize,
       sendMessage: _sendMessage,
+      onFlush: flush,
       printDebug: printDebug,
     );
 

lib/src/ssh_forward.dart

@@ -76,6 +76,13 @@ class SSHForwardChannel implements SSHSocket {
   void destroy() {
     _channel.destroy();
   }
+
+  /// Force flush any buffered outgoing data.
+  @override
+  Future<void> flush() async {
+    await Future.microtask(() {});
+    await _channel.flush();
+  }
 }
 
 class SSHX11Channel extends SSHForwardChannel {

lib/src/ssh_session.dart

@@ -32,6 +32,9 @@ class SSHSession {
   /// be available on the [stdout] and [stderr] streams at this time.
   Future<void> get done => _channel.done;
 
+  /// The underlying SSH channel.
+  SSHChannel get channel => _channel;
+
   SSHSession(this._channel) {
     _channel.setRequestHandler(_handleRequest);
 
@@ -79,6 +82,12 @@ class SSHSession {
     stdin.add(data);
   }
 
+  /// Force flush any buffered stdin data to the remote process.
+  Future<void> flush() async {
+    await Future.microtask(() {});
+    await _channel.flush();
+  }
+
   /// Inform remote process of the current window size.
   void resizeTerminal(
     int width,

lib/src/ssh_transport.dart

@@ -464,6 +464,11 @@ class SSHTransport {
     socket.destroy();
   }
 
+  /// Force flush any buffered outgoing data to the socket.
+  Future<void> flush() async {
+    await socket.flush();
+  }
+
   /// Subscribes to the underlying socket stream to handle incoming data and status events.
   void _initSocket() {
     _socketSubscription = socket.stream.listen(

test/src/http/http_client_test.dart

@@ -300,4 +300,7 @@ class _FakeSocket implements SSHSocket {
     }
     unawaited(_sinkController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }

test/src/ssh_auth_abort_error_test.dart

@@ -75,6 +75,9 @@ class _FakeSSHSocket implements SSHSocket {
     }
     unawaited(_inputController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }
 
 class _RecordingSink implements StreamSink<List<int>> {

test/src/ssh_client_forward_dynamic_test.dart

@@ -64,6 +64,9 @@ class _FakeSSHSocket implements SSHSocket {
     }
     unawaited(_inputController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }
 
 class _NoopSink implements StreamSink<List<int>> {

test/src/ssh_client_ident_test.dart

@@ -100,6 +100,9 @@ class _FakeSSHSocket implements SSHSocket {
     }
     unawaited(_inputController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }
 
 class _RecordingSink implements StreamSink<List<int>> {

test/src/ssh_client_run_with_result_test.dart

@@ -284,6 +284,9 @@ class _FakeSSHSocket implements SSHSocket {
     }
     unawaited(_inputController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }
 
 class _NoopSink implements StreamSink<List<int>> {

test/src/ssh_client_test.dart

@@ -4,6 +4,7 @@ library;
 import 'dart:convert';
 
 import 'package:dartssh2/dartssh2.dart';
+import 'package:dartssh2/src/ssh_channel.dart';
 import 'package:test/test.dart';
 
 import '../test_utils.dart';
@@ -264,4 +265,32 @@ void main() {
       client.close();
     });
   });
+
+  group('SSHClient.flush', () {
+    test('can flush client, session, channel, and forward channel', () async {
+      final client = await getTestClient();
+      await client.authenticated;
+
+      await client.flush();
+
+      final session = await client.execute('echo flush_test');
+      await session.flush();
+
+      final controller = SSHChannelController(
+        localId: 1,
+        localMaximumPacketSize: 1024,
+        localInitialWindowSize: 1024,
+        remoteId: 2,
+        remoteMaximumPacketSize: 1024,
+        remoteInitialWindowSize: 1024,
+        sendMessage: (msg) {},
+        onFlush: () async {},
+      );
+      final forwardChannel = SSHForwardChannel(controller.channel);
+      await forwardChannel.flush();
+
+      await session.done;
+      client.close();
+    });
+  });
 }

test/src/ssh_client_timeout_test.dart

@@ -77,6 +77,9 @@ class _FakeSSHSocket implements SSHSocket {
     }
     unawaited(_inputController.close());
   }
+
+  @override
+  Future<void> flush() async {}
 }
 
 class _RecordingSink implements StreamSink<List<int>> {