switch to use gliderlabs/ssh for builtin server (#7250)

resolves git conflicts from #3896 (credit to @belak, in case github doesn't keep original author during squash)

Co-Authored-By: Matti Ranta <techknowlogick@gitea.io>

vendor/github.com/gliderlabs/ssh/server.go

@@ -0,0 +1,394 @@
+package ssh
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"sync"
+	"time"
+
+	gossh "golang.org/x/crypto/ssh"
+)
+
+// ErrServerClosed is returned by the Server's Serve, ListenAndServe,
+// and ListenAndServeTLS methods after a call to Shutdown or Close.
+var ErrServerClosed = errors.New("ssh: Server closed")
+
+type RequestHandler func(ctx Context, srv *Server, req *gossh.Request) (ok bool, payload []byte)
+
+var DefaultRequestHandlers = map[string]RequestHandler{}
+
+type ChannelHandler func(srv *Server, conn *gossh.ServerConn, newChan gossh.NewChannel, ctx Context)
+
+var DefaultChannelHandlers = map[string]ChannelHandler{
+	"session": DefaultSessionHandler,
+}
+
+// Server defines parameters for running an SSH server. The zero value for
+// Server is a valid configuration. When both PasswordHandler and
+// PublicKeyHandler are nil, no client authentication is performed.
+type Server struct {
+	Addr        string   // TCP address to listen on, ":22" if empty
+	Handler     Handler  // handler to invoke, ssh.DefaultHandler if nil
+	HostSigners []Signer // private keys for the host key, must have at least one
+	Version     string   // server version to be sent before the initial handshake
+
+	KeyboardInteractiveHandler    KeyboardInteractiveHandler    // keyboard-interactive authentication handler
+	PasswordHandler               PasswordHandler               // password authentication handler
+	PublicKeyHandler              PublicKeyHandler              // public key authentication handler
+	PtyCallback                   PtyCallback                   // callback for allowing PTY sessions, allows all if nil
+	ConnCallback                  ConnCallback                  // optional callback for wrapping net.Conn before handling
+	LocalPortForwardingCallback   LocalPortForwardingCallback   // callback for allowing local port forwarding, denies all if nil
+	ReversePortForwardingCallback ReversePortForwardingCallback // callback for allowing reverse port forwarding, denies all if nil
+	ServerConfigCallback          ServerConfigCallback          // callback for configuring detailed SSH options
+	SessionRequestCallback        SessionRequestCallback        // callback for allowing or denying SSH sessions
+
+	IdleTimeout time.Duration // connection timeout when no activity, none if empty
+	MaxTimeout  time.Duration // absolute connection timeout, none if empty
+
+	// ChannelHandlers allow overriding the built-in session handlers or provide
+	// extensions to the protocol, such as tcpip forwarding. By default only the
+	// "session" handler is enabled.
+	ChannelHandlers map[string]ChannelHandler
+
+	// RequestHandlers allow overriding the server-level request handlers or
+	// provide extensions to the protocol, such as tcpip forwarding. By default
+	// no handlers are enabled.
+	RequestHandlers map[string]RequestHandler
+
+	listenerWg sync.WaitGroup
+	mu         sync.Mutex
+	listeners  map[net.Listener]struct{}
+	conns      map[*gossh.ServerConn]struct{}
+	connWg     sync.WaitGroup
+	doneChan   chan struct{}
+}
+
+func (srv *Server) ensureHostSigner() error {
+	if len(srv.HostSigners) == 0 {
+		signer, err := generateSigner()
+		if err != nil {
+			return err
+		}
+		srv.HostSigners = append(srv.HostSigners, signer)
+	}
+	return nil
+}
+
+func (srv *Server) ensureHandlers() {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	if srv.RequestHandlers == nil {
+		srv.RequestHandlers = map[string]RequestHandler{}
+		for k, v := range DefaultRequestHandlers {
+			srv.RequestHandlers[k] = v
+		}
+	}
+	if srv.ChannelHandlers == nil {
+		srv.ChannelHandlers = map[string]ChannelHandler{}
+		for k, v := range DefaultChannelHandlers {
+			srv.ChannelHandlers[k] = v
+		}
+	}
+}
+
+func (srv *Server) config(ctx Context) *gossh.ServerConfig {
+	var config *gossh.ServerConfig
+	if srv.ServerConfigCallback == nil {
+		config = &gossh.ServerConfig{}
+	} else {
+		config = srv.ServerConfigCallback(ctx)
+	}
+	for _, signer := range srv.HostSigners {
+		config.AddHostKey(signer)
+	}
+	if srv.PasswordHandler == nil && srv.PublicKeyHandler == nil {
+		config.NoClientAuth = true
+	}
+	if srv.Version != "" {
+		config.ServerVersion = "SSH-2.0-" + srv.Version
+	}
+	if srv.PasswordHandler != nil {
+		config.PasswordCallback = func(conn gossh.ConnMetadata, password []byte) (*gossh.Permissions, error) {
+			applyConnMetadata(ctx, conn)
+			if ok := srv.PasswordHandler(ctx, string(password)); !ok {
+				return ctx.Permissions().Permissions, fmt.Errorf("permission denied")
+			}
+			return ctx.Permissions().Permissions, nil
+		}
+	}
+	if srv.PublicKeyHandler != nil {
+		config.PublicKeyCallback = func(conn gossh.ConnMetadata, key gossh.PublicKey) (*gossh.Permissions, error) {
+			applyConnMetadata(ctx, conn)
+			if ok := srv.PublicKeyHandler(ctx, key); !ok {
+				return ctx.Permissions().Permissions, fmt.Errorf("permission denied")
+			}
+			ctx.SetValue(ContextKeyPublicKey, key)
+			return ctx.Permissions().Permissions, nil
+		}
+	}
+	if srv.KeyboardInteractiveHandler != nil {
+		config.KeyboardInteractiveCallback = func(conn gossh.ConnMetadata, challenger gossh.KeyboardInteractiveChallenge) (*gossh.Permissions, error) {
+			if ok := srv.KeyboardInteractiveHandler(ctx, challenger); !ok {
+				return ctx.Permissions().Permissions, fmt.Errorf("permission denied")
+			}
+			return ctx.Permissions().Permissions, nil
+		}
+	}
+	return config
+}
+
+// Handle sets the Handler for the server.
+func (srv *Server) Handle(fn Handler) {
+	srv.Handler = fn
+}
+
+// Close immediately closes all active listeners and all active
+// connections.
+//
+// Close returns any error returned from closing the Server's
+// underlying Listener(s).
+func (srv *Server) Close() error {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	srv.closeDoneChanLocked()
+	err := srv.closeListenersLocked()
+	for c := range srv.conns {
+		c.Close()
+		delete(srv.conns, c)
+	}
+	return err
+}
+
+// Shutdown gracefully shuts down the server without interrupting any
+// active connections. Shutdown works by first closing all open
+// listeners, and then waiting indefinitely for connections to close.
+// If the provided context expires before the shutdown is complete,
+// then the context's error is returned.
+func (srv *Server) Shutdown(ctx context.Context) error {
+	srv.mu.Lock()
+	lnerr := srv.closeListenersLocked()
+	srv.closeDoneChanLocked()
+	srv.mu.Unlock()
+
+	finished := make(chan struct{}, 1)
+	go func() {
+		srv.listenerWg.Wait()
+		srv.connWg.Wait()
+		finished <- struct{}{}
+	}()
+
+	select {
+	case <-ctx.Done():
+		return ctx.Err()
+	case <-finished:
+		return lnerr
+	}
+}
+
+// Serve accepts incoming connections on the Listener l, creating a new
+// connection goroutine for each. The connection goroutines read requests and then
+// calls srv.Handler to handle sessions.
+//
+// Serve always returns a non-nil error.
+func (srv *Server) Serve(l net.Listener) error {
+	srv.ensureHandlers()
+	defer l.Close()
+	if err := srv.ensureHostSigner(); err != nil {
+		return err
+	}
+	if srv.Handler == nil {
+		srv.Handler = DefaultHandler
+	}
+	var tempDelay time.Duration
+
+	srv.trackListener(l, true)
+	defer srv.trackListener(l, false)
+	for {
+		conn, e := l.Accept()
+		if e != nil {
+			select {
+			case <-srv.getDoneChan():
+				return ErrServerClosed
+			default:
+			}
+			if ne, ok := e.(net.Error); ok && ne.Temporary() {
+				if tempDelay == 0 {
+					tempDelay = 5 * time.Millisecond
+				} else {
+					tempDelay *= 2
+				}
+				if max := 1 * time.Second; tempDelay > max {
+					tempDelay = max
+				}
+				time.Sleep(tempDelay)
+				continue
+			}
+			return e
+		}
+		go srv.handleConn(conn)
+	}
+}
+
+func (srv *Server) handleConn(newConn net.Conn) {
+	if srv.ConnCallback != nil {
+		cbConn := srv.ConnCallback(newConn)
+		if cbConn == nil {
+			newConn.Close()
+			return
+		}
+		newConn = cbConn
+	}
+	ctx, cancel := newContext(srv)
+	conn := &serverConn{
+		Conn:          newConn,
+		idleTimeout:   srv.IdleTimeout,
+		closeCanceler: cancel,
+	}
+	if srv.MaxTimeout > 0 {
+		conn.maxDeadline = time.Now().Add(srv.MaxTimeout)
+	}
+	defer conn.Close()
+	sshConn, chans, reqs, err := gossh.NewServerConn(conn, srv.config(ctx))
+	if err != nil {
+		// TODO: trigger event callback
+		return
+	}
+
+	srv.trackConn(sshConn, true)
+	defer srv.trackConn(sshConn, false)
+
+	ctx.SetValue(ContextKeyConn, sshConn)
+	applyConnMetadata(ctx, sshConn)
+	//go gossh.DiscardRequests(reqs)
+	go srv.handleRequests(ctx, reqs)
+	for ch := range chans {
+		handler := srv.ChannelHandlers[ch.ChannelType()]
+		if handler == nil {
+			handler = srv.ChannelHandlers["default"]
+		}
+		if handler == nil {
+			ch.Reject(gossh.UnknownChannelType, "unsupported channel type")
+			continue
+		}
+		go handler(srv, sshConn, ch, ctx)
+	}
+}
+
+func (srv *Server) handleRequests(ctx Context, in <-chan *gossh.Request) {
+	for req := range in {
+		handler := srv.RequestHandlers[req.Type]
+		if handler == nil {
+			handler = srv.RequestHandlers["default"]
+		}
+		if handler == nil {
+			req.Reply(false, nil)
+			continue
+		}
+		/*reqCtx, cancel := context.WithCancel(ctx)
+		defer cancel() */
+		ret, payload := handler(ctx, srv, req)
+		req.Reply(ret, payload)
+	}
+}
+
+// ListenAndServe listens on the TCP network address srv.Addr and then calls
+// Serve to handle incoming connections. If srv.Addr is blank, ":22" is used.
+// ListenAndServe always returns a non-nil error.
+func (srv *Server) ListenAndServe() error {
+	addr := srv.Addr
+	if addr == "" {
+		addr = ":22"
+	}
+	ln, err := net.Listen("tcp", addr)
+	if err != nil {
+		return err
+	}
+	return srv.Serve(ln)
+}
+
+// AddHostKey adds a private key as a host key. If an existing host key exists
+// with the same algorithm, it is overwritten. Each server config must have at
+// least one host key.
+func (srv *Server) AddHostKey(key Signer) {
+	// these are later added via AddHostKey on ServerConfig, which performs the
+	// check for one of every algorithm.
+	srv.HostSigners = append(srv.HostSigners, key)
+}
+
+// SetOption runs a functional option against the server.
+func (srv *Server) SetOption(option Option) error {
+	return option(srv)
+}
+
+func (srv *Server) getDoneChan() <-chan struct{} {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	return srv.getDoneChanLocked()
+}
+
+func (srv *Server) getDoneChanLocked() chan struct{} {
+	if srv.doneChan == nil {
+		srv.doneChan = make(chan struct{})
+	}
+	return srv.doneChan
+}
+
+func (srv *Server) closeDoneChanLocked() {
+	ch := srv.getDoneChanLocked()
+	select {
+	case <-ch:
+		// Already closed. Don't close again.
+	default:
+		// Safe to close here. We're the only closer, guarded
+		// by srv.mu.
+		close(ch)
+	}
+}
+
+func (srv *Server) closeListenersLocked() error {
+	var err error
+	for ln := range srv.listeners {
+		if cerr := ln.Close(); cerr != nil && err == nil {
+			err = cerr
+		}
+		delete(srv.listeners, ln)
+	}
+	return err
+}
+
+func (srv *Server) trackListener(ln net.Listener, add bool) {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	if srv.listeners == nil {
+		srv.listeners = make(map[net.Listener]struct{})
+	}
+	if add {
+		// If the *Server is being reused after a previous
+		// Close or Shutdown, reset its doneChan:
+		if len(srv.listeners) == 0 && len(srv.conns) == 0 {
+			srv.doneChan = nil
+		}
+		srv.listeners[ln] = struct{}{}
+		srv.listenerWg.Add(1)
+	} else {
+		delete(srv.listeners, ln)
+		srv.listenerWg.Done()
+	}
+}
+
+func (srv *Server) trackConn(c *gossh.ServerConn, add bool) {
+	srv.mu.Lock()
+	defer srv.mu.Unlock()
+	if srv.conns == nil {
+		srv.conns = make(map[*gossh.ServerConn]struct{})
+	}
+	if add {
+		srv.conns[c] = struct{}{}
+		srv.connWg.Add(1)
+	} else {
+		delete(srv.conns, c)
+		srv.connWg.Done()
+	}
+}