SSHServer: Add support for the window-change request

With this patch, SSHServer is able to update the pty on the server when
the size of the client's window is updated.

Resizing your terminal emulator's window while being connected to
SSHServer no longer make you disconnect.

While the implementation is functional, it seems to have quite more
latency than with a regular SSH server, so visual artifacts may appear
when aggressively resizing the window.

Userland/Services/SSHServer/SSHClient.cpp

@@ -636,6 +636,11 @@ ErrorOr<void> SSHClient::handle_channel_request(GenericMessage& message)
         return {};
     }
 
+    if (request_type == "window-change"sv.bytes()) {
+        TRY(handle_channel_window_change(session, message));
+        return {};
+    }
+
     return Error::from_string_literal("Unsupported channel request");
 }
 
@@ -887,6 +892,38 @@ ErrorOr<void> SSHClient::handle_channel_pseudo_terminal(Session& session, Generi
     return TRY(send_channel_success_message(session));
 }
 
+// 6.7.  Window Dimension Change Message
+// https://datatracker.ietf.org/doc/html/rfc4254#section-6.7
+ErrorOr<void> SSHClient::handle_channel_window_change(Session& session, GenericMessage& message)
+{
+    u32 width_characters = TRY(message.payload.read_value<NetworkOrdered<u32>>());
+    u32 height_rows = TRY(message.payload.read_value<NetworkOrdered<u32>>());
+    u32 width_pixels = TRY(message.payload.read_value<NetworkOrdered<u32>>());
+    u32 height_pixels = TRY(message.payload.read_value<NetworkOrdered<u32>>());
+
+    dbgln_if(SSH_DEBUG, "PTY window change: w={}, h={}, w_px={}, h_px={}",
+        width_characters, height_rows, width_pixels, height_pixels);
+
+    if (!session.system.has<Shell>())
+        return Error::from_string_literal("Requested a window change while no shell is present");
+
+    winsize ws {
+        .ws_row = static_cast<u16>(height_rows),
+        .ws_col = static_cast<u16>(width_characters),
+        .ws_xpixel = static_cast<u16>(width_pixels),
+        .ws_ypixel = static_cast<u16>(height_pixels),
+    };
+
+    auto maybe_error = Core::System::tcsetwinsize(session.system.get<Shell>().stdin_->fd(), ws);
+
+    if (maybe_error.is_error()) {
+        dbgln("Unable to set pty dimensions: {}", maybe_error.error());
+    }
+
+    // "A response SHOULD NOT be sent to this message."
+    return {};
+}
+
 ErrorOr<void> SSHClient::send_channel_success_message(Session const& session)
 {
     AllocatingMemoryStream stream;

Userland/Services/SSHServer/SSHClient.h

@@ -85,6 +85,7 @@ private:
     ErrorOr<void> handle_channel_data(GenericMessage&);
     ErrorOr<void> handle_channel_subsystem(Session&, GenericMessage&);
     ErrorOr<void> handle_channel_pseudo_terminal(Session&, GenericMessage&);
+    ErrorOr<void> handle_channel_window_change(Session&, GenericMessage&);
     ErrorOr<void> handle_channel_exec(NonnullRefPtr<Session> const&, GenericMessage&);
     ErrorOr<void> handle_channel_shell(NonnullRefPtr<Session> const&);
     ErrorOr<void> send_channel_success_message(Session const&);