ftp: adapt to library changes to fix connection errors #6426

In https://github.com/jlaffaye/ftp/commit/212daf295f the upstream FTP
library changed the way adding your own dialer works which meant that
connections when using explicit FTP were failing.

This patch reworks our connection code to bring it into the
expectations of the library.

backend/ftp/ftp.go

@@ -336,14 +336,44 @@ func (f *Fs) ftpConnection(ctx context.Context) (c *ftp.ServerConn, err error) {
 	fs.Debugf(f, "Connecting to FTP server")
 
 	// Make ftp library dial with fshttp dialer optionally using TLS
+	initialConnection := true
 	dial := func(network, address string) (conn net.Conn, err error) {
+		fs.Debugf(f, "dial(%q,%q)", network, address)
+		defer func() {
+			fs.Debugf(f, "> dial: conn=%T, err=%v", conn, err)
+		}()
 		conn, err = fshttp.NewDialer(ctx).Dial(network, address)
-		if f.tlsConf != nil && err == nil {
-			conn = tls.Client(conn, f.tlsConf)
+		if err != nil {
+			return nil, err
 		}
-		return
+		// Connect using cleartext only for non TLS
+		if f.tlsConf == nil {
+			return conn, nil
+		}
+		// Initial connection only needs to be cleartext for explicit TLS
+		if f.opt.ExplicitTLS && initialConnection {
+			initialConnection = false
+			return conn, nil
+		}
+		// Upgrade connection to TLS
+		tlsConn := tls.Client(conn, f.tlsConf)
+		// Do the initial handshake - tls.Client doesn't do it for us
+		// If we do this then connections to proftpd/pureftpd lock up
+		// See: https://github.com/rclone/rclone/issues/6426
+		// See: https://github.com/jlaffaye/ftp/issues/282
+		if false {
+			err = tlsConn.HandshakeContext(ctx)
+			if err != nil {
+				_ = conn.Close()
+				return nil, err
+			}
+		}
+		return tlsConn, nil
+	}
+	ftpConfig := []ftp.DialOption{
+		ftp.DialWithContext(ctx),
+		ftp.DialWithDialFunc(dial),
 	}
-	ftpConfig := []ftp.DialOption{ftp.DialWithDialFunc(dial)}
 
 	if f.opt.TLS {
 		// Our dialer takes care of TLS but ftp library also needs tlsConf
@@ -351,12 +381,6 @@ func (f *Fs) ftpConnection(ctx context.Context) (c *ftp.ServerConn, err error) {
 		ftpConfig = append(ftpConfig, ftp.DialWithTLS(f.tlsConf))
 	} else if f.opt.ExplicitTLS {
 		ftpConfig = append(ftpConfig, ftp.DialWithExplicitTLS(f.tlsConf))
-		// Initial connection needs to be cleartext for explicit TLS
-		conn, err := fshttp.NewDialer(ctx).Dial("tcp", f.dialAddr)
-		if err != nil {
-			return nil, err
-		}
-		ftpConfig = append(ftpConfig, ftp.DialWithNetConn(conn))
 	}
 	if f.opt.DisableEPSV {
 		ftpConfig = append(ftpConfig, ftp.DialWithDisabledEPSV(true))