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/ssh.go

@@ -0,0 +1,123 @@
+package ssh
+
+import (
+	"crypto/subtle"
+	"net"
+
+	gossh "golang.org/x/crypto/ssh"
+)
+
+type Signal string
+
+// POSIX signals as listed in RFC 4254 Section 6.10.
+const (
+	SIGABRT Signal = "ABRT"
+	SIGALRM Signal = "ALRM"
+	SIGFPE  Signal = "FPE"
+	SIGHUP  Signal = "HUP"
+	SIGILL  Signal = "ILL"
+	SIGINT  Signal = "INT"
+	SIGKILL Signal = "KILL"
+	SIGPIPE Signal = "PIPE"
+	SIGQUIT Signal = "QUIT"
+	SIGSEGV Signal = "SEGV"
+	SIGTERM Signal = "TERM"
+	SIGUSR1 Signal = "USR1"
+	SIGUSR2 Signal = "USR2"
+)
+
+// DefaultHandler is the default Handler used by Serve.
+var DefaultHandler Handler
+
+// Option is a functional option handler for Server.
+type Option func(*Server) error
+
+// Handler is a callback for handling established SSH sessions.
+type Handler func(Session)
+
+// PublicKeyHandler is a callback for performing public key authentication.
+type PublicKeyHandler func(ctx Context, key PublicKey) bool
+
+// PasswordHandler is a callback for performing password authentication.
+type PasswordHandler func(ctx Context, password string) bool
+
+// KeyboardInteractiveHandler is a callback for performing keyboard-interactive authentication.
+type KeyboardInteractiveHandler func(ctx Context, challenger gossh.KeyboardInteractiveChallenge) bool
+
+// PtyCallback is a hook for allowing PTY sessions.
+type PtyCallback func(ctx Context, pty Pty) bool
+
+// SessionRequestCallback is a callback for allowing or denying SSH sessions.
+type SessionRequestCallback func(sess Session, requestType string) bool
+
+// ConnCallback is a hook for new connections before handling.
+// It allows wrapping for timeouts and limiting by returning
+// the net.Conn that will be used as the underlying connection.
+type ConnCallback func(conn net.Conn) net.Conn
+
+// LocalPortForwardingCallback is a hook for allowing port forwarding
+type LocalPortForwardingCallback func(ctx Context, destinationHost string, destinationPort uint32) bool
+
+// ReversePortForwardingCallback is a hook for allowing reverse port forwarding
+type ReversePortForwardingCallback func(ctx Context, bindHost string, bindPort uint32) bool
+
+// ServerConfigCallback is a hook for creating custom default server configs
+type ServerConfigCallback func(ctx Context) *gossh.ServerConfig
+
+// Window represents the size of a PTY window.
+type Window struct {
+	Width  int
+	Height int
+}
+
+// Pty represents a PTY request and configuration.
+type Pty struct {
+	Term   string
+	Window Window
+	// HELP WANTED: terminal modes!
+}
+
+// Serve accepts incoming SSH connections on the listener l, creating a new
+// connection goroutine for each. The connection goroutines read requests and
+// then calls handler to handle sessions. Handler is typically nil, in which
+// case the DefaultHandler is used.
+func Serve(l net.Listener, handler Handler, options ...Option) error {
+	srv := &Server{Handler: handler}
+	for _, option := range options {
+		if err := srv.SetOption(option); err != nil {
+			return err
+		}
+	}
+	return srv.Serve(l)
+}
+
+// ListenAndServe listens on the TCP network address addr and then calls Serve
+// with handler to handle sessions on incoming connections. Handler is typically
+// nil, in which case the DefaultHandler is used.
+func ListenAndServe(addr string, handler Handler, options ...Option) error {
+	srv := &Server{Addr: addr, Handler: handler}
+	for _, option := range options {
+		if err := srv.SetOption(option); err != nil {
+			return err
+		}
+	}
+	return srv.ListenAndServe()
+}
+
+// Handle registers the handler as the DefaultHandler.
+func Handle(handler Handler) {
+	DefaultHandler = handler
+}
+
+// KeysEqual is constant time compare of the keys to avoid timing attacks.
+func KeysEqual(ak, bk PublicKey) bool {
+
+	//avoid panic if one of the keys is nil, return false instead
+	if ak == nil || bk == nil {
+		return false
+	}
+
+	a := ak.Marshal()
+	b := bk.Marshal()
+	return (len(a) == len(b) && subtle.ConstantTimeCompare(a, b) == 1)
+}