feat: allocate real pty

This fills the gap of allocating a real PTY when a session asks for one.
It uses [go-pty](https://github.com/aymanbagabas/go-pty) to support PTYs
for both Unix and Windows OSs. It also adds two new PtyCallback options,
  `AllocatePty` opens a new PTY for the session when requested.
  `EmulatePty` preserves the current behavior of the library.

Signed-off-by: Ayman Bagabas <ayman.bagabas@gmail.com>

go.mod

@@ -4,6 +4,6 @@ go 1.12
 
 require (
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be
-	golang.org/x/crypto v0.0.0-20220826181053-bd7e27e6170d
-	golang.org/x/term v0.5.0 // indirect
+	github.com/aymanbagabas/go-pty v0.2.0
+	golang.org/x/crypto v0.14.0
 )

options.go

@@ -82,3 +82,27 @@ func WrapConn(fn ConnCallback) Option {
 		return nil
 	}
 }
+
+// EmulatePty returns a functional option that sets PtyCallback on the server
+// to emulate a "fake" PTY by using PtyWriter.
+func EmulatePty() Option {
+	return func(srv *Server) error {
+		srv.PtyCallback = func(_ Context, pty Pty) bool {
+			pty.emulate = true
+			return true
+		}
+		return nil
+	}
+}
+
+// AllocatePty returns a functional option that sets PtyCallback on the server
+// to allocate a PTY for sessions that request it.
+func AllocatePty() Option {
+	return func(srv *Server) error {
+		srv.PtyCallback = func(_ Context, pty Pty) bool {
+			err := pty.allocate()
+			return err == nil
+		}
+		return nil
+	}
+}

pty.go

@@ -3,6 +3,8 @@ package ssh
 import (
 	"bytes"
 	"io"
+
+	"github.com/aymanbagabas/go-pty"
 )
 
 // NewPtyWriter creates a writer that handles when the session has a active
@@ -55,3 +57,23 @@ func (rw readWriterDelegate) Read(p []byte) (n int, err error) {
 func (rw readWriterDelegate) Write(p []byte) (n int, err error) {
 	return rw.w.Write(p)
 }
+
+// allocate is a helper function to allocate a pty session.
+// It returns a Pty object that can be used to run a command on a tty.
+// If this fails, use NewPtyReadWriter to _emulate_ a tty.
+func (p *Pty) allocate() error {
+	tty, err := pty.New()
+	if err != nil {
+		return err
+	}
+
+	p.Pty = tty
+
+	if err := pty.ApplyTerminalModes(int(tty.Fd()),
+		p.Window.Width, p.Window.Height, p.Modes,
+	); err != nil {
+		return err
+	}
+
+	return nil
+}

session.go

@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"log"
 	"net"
 	"sync"
 
@@ -128,14 +129,14 @@ type session struct {
 }
 
 func (sess *session) Stderr() io.ReadWriter {
-	if sess.pty != nil {
+	if sess.pty != nil && sess.pty.emulate {
 		return NewPtyReadWriter(sess.Channel.Stderr())
 	}
 	return sess.Channel.Stderr()
 }
 
 func (sess *session) Write(p []byte) (int, error) {
-	if sess.pty != nil {
+	if sess.pty != nil && sess.pty.emulate {
 		return NewPtyWriter(sess.Channel).Write(p)
 	}
 	return sess.Channel.Write(p)
@@ -331,6 +332,21 @@ func (sess *session) handleRequests(reqs <-chan *gossh.Request) {
 					continue
 				}
 			}
+			if err := ptyReq.allocate(); err != nil {
+				req.Reply(false, nil)
+				continue
+			}
+			if ptyReq.Pty != nil {
+				log.Printf("pty allocated: %s", ptyReq.Pty.Name())
+				sess.env = append(sess.env,
+					fmt.Sprintf("TERM=%s", ptyReq.Term),
+					fmt.Sprintf("SSH_TTY=%s", ptyReq.Pty.Name()),
+				)
+				// input to pty
+				go io.Copy(ptyReq.Pty, sess) // nolint: errcheck
+				// pty to output
+				go io.Copy(sess, ptyReq.Pty) // nolint: errcheck
+			}
 			sess.pty = &ptyReq
 			sess.winch = make(chan Window, 1)
 			sess.winch <- ptyReq.Window
@@ -348,6 +364,9 @@ func (sess *session) handleRequests(reqs <-chan *gossh.Request) {
 			if ok {
 				sess.pty.Window = win
 				sess.winch <- win
+				if sess.pty.Pty != nil {
+					sess.pty.Pty.Resize(win.Width, win.Height)
+				}
 			}
 			req.Reply(ok, nil)
 		case agentRequestType:

ssh.go

@@ -4,6 +4,7 @@ import (
 	"crypto/subtle"
 	"net"
 
+	"github.com/aymanbagabas/go-pty"
 	gossh "golang.org/x/crypto/ssh"
 )
 
@@ -91,6 +92,14 @@ type Window struct {
 
 // Pty represents a PTY request and configuration.
 type Pty struct {
+	// If this is true, the server will emulate a "fake" Pty by using
+	// PtyWriter.
+	emulate bool
+
+	// Pty is the actual pty allocated. It is nil if the request did not
+	// allocated a pty.
+	pty.Pty
+
 	// Term is the TERM environment variable value.
 	Term string