serve sftp: fix authentication on one connection blocking others - fixes #4882

Before this change, if one connection was authenticating this would
block any others from authenticating.

This was due to ssh.NewServerConn not being called in a go routine
after the Accept call.

This is fixed by running the ssh authentication in a go routine.

Thanks to @FiloSottile for advice on how to fix this.

See: https://github.com/golang/go/issues/43521

cmd/serve/sftp/server.go

@@ -78,23 +78,16 @@ func (s *server) getVFS(what string, sshConn *ssh.ServerConn) (VFS *vfs.VFS) {
 	return VFS
 }
 
-func (s *server) acceptConnections() {
-	for {
-		nConn, err := s.listener.Accept()
-		if err != nil {
-			if strings.Contains(err.Error(), "use of closed network connection") {
-				return
-			}
-			fs.Errorf(nil, "Failed to accept incoming connection: %v", err)
-			continue
-		}
+// Accept a single connection - run in a go routine as the ssh
+// authentication can block
+func (s *server) acceptConnection(nConn net.Conn) {
 	what := describeConn(nConn)
 
 	// Before use, a handshake must be performed on the incoming net.Conn.
 	sshConn, chans, reqs, err := ssh.NewServerConn(nConn, s.config)
 	if err != nil {
 		fs.Errorf(what, "SSH login failed: %v", err)
-			continue
+		return
 	}
 
 	fs.Infof(what, "SSH login from %s using %s", sshConn.User(), sshConn.ClientVersion())
@@ -109,13 +102,27 @@ func (s *server) acceptConnections() {
 	if c.vfs == nil {
 		fs.Infof(what, "Closing unauthenticated connection (couldn't find VFS)")
 		_ = nConn.Close()
-			continue
+		return
 	}
 	c.handlers = newVFSHandler(c.vfs)
 
 	// Accept all channels
 	go c.handleChannels(chans)
 }
+
+// Accept connections and call them in a go routine
+func (s *server) acceptConnections() {
+	for {
+		nConn, err := s.listener.Accept()
+		if err != nil {
+			if strings.Contains(err.Error(), "use of closed network connection") {
+				return
+			}
+			fs.Errorf(nil, "Failed to accept incoming connection: %v", err)
+			continue
+		}
+		go s.acceptConnection(nConn)
+	}
 }
 
 // Based on example server code from golang.org/x/crypto/ssh and server_standalone