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

 rclone#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: golang/go#43521

cmd/serve/sftp/server.go

@@ -78,6 +78,39 @@ func (s *server) getVFS(what string, sshConn *ssh.ServerConn) (VFS *vfs.VFS) {
 	return VFS
 }
 
+// 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)
+		return
+	}
+
+	fs.Infof(what, "SSH login from %s using %s", sshConn.User(), sshConn.ClientVersion())
+
+	// Discard all global out-of-band Requests
+	go ssh.DiscardRequests(reqs)
+
+	c := &conn{
+		what: what,
+		vfs:  s.getVFS(what, sshConn),
+	}
+	if c.vfs == nil {
+		fs.Infof(what, "Closing unauthenticated connection (couldn't find VFS)")
+		_ = nConn.Close()
+		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()
@@ -88,33 +121,7 @@ func (s *server) acceptConnections() {
 			fs.Errorf(nil, "Failed to accept incoming connection: %v", err)
 			continue
 		}
-		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
-		}
-
-		fs.Infof(what, "SSH login from %s using %s", sshConn.User(), sshConn.ClientVersion())
-
-		// Discard all global out-of-band Requests
-		go ssh.DiscardRequests(reqs)
-
-		c := &conn{
-			what: what,
-			vfs:  s.getVFS(what, sshConn),
-		}
-		if c.vfs == nil {
-			fs.Infof(what, "Closing unauthenticated connection (couldn't find VFS)")
-			_ = nConn.Close()
-			continue
-		}
-		c.handlers = newVFSHandler(c.vfs)
-
-		// Accept all channels
-		go c.handleChannels(chans)
+		go s.acceptConnection(nConn)
 	}
 }