Prevent racy access to session parties

Prefer using session.getParties instead of using session.parties
directly to prevent races when new parties are added. This also
prevents holding the mutex locking the parties when performing
external io operations.

lib/srv/sess.go

@@ -573,7 +573,7 @@ func (s *SessionRegistry) NotifyFileTransferRequest(req *FileTransferRequest, re
 		return trace.Wrap(err)
 	}
 
-	for _, p := range session.parties {
+	for _, p := range session.getParties() {
 		// Send the message as a global request.
 		_, _, err = p.sconn.SendRequest(teleport.SessionEvent, false, eventPayload)
 		if err != nil {
@@ -1090,7 +1090,7 @@ func (s *session) emitSessionJoinEvent(ctx *ServerContext) {
 
 	// Notify all members of the party that a new member has joined over the
 	// "x-teleport-event" channel.
-	for _, p := range s.parties {
+	for _, p := range s.getParties() {
 		if len(notifyPartyPayload) == 0 {
 			s.logger.WarnContext(ctx.srv.Context(), "No session join event to send to party.", "party", p)
 			continue
@@ -1136,7 +1136,7 @@ func (s *session) emitSessionLeaveEvent(ctx *ServerContext) {
 
 	// Notify all members of the party that a new member has left over the
 	// "x-teleport-event" channel.
-	for _, p := range s.parties {
+	for _, p := range s.getParties() {
 		eventPayload, err := utils.FastMarshal(sessionLeaveEvent)
 		if err != nil {
 			s.logger.WarnContext(ctx.srv.Context(), "Unable to marshal session leave event for party.", "error", err, "party", p)
@@ -1588,11 +1588,8 @@ func (s *session) startExec(ctx context.Context, channel ssh.Channel, scx *Serve
 }
 
 func (s *session) broadcastResult(r ExecResult) {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-
 	payload := ssh.Marshal(struct{ C uint32 }{C: uint32(r.Code)})
-	for _, p := range s.parties {
+	for _, p := range s.getParties() {
 		if _, err := p.ch.SendRequest("exit-status", false, payload); err != nil {
 			s.logger.InfoContext(
 				s.serverCtx, "Failed to send exit status",
@@ -1604,7 +1601,7 @@ func (s *session) broadcastResult(r ExecResult) {
 }
 
 func (s *session) String() string {
-	return fmt.Sprintf("session(id=%v, parties=%v)", s.id, len(s.parties))
+	return fmt.Sprintf("session(id=%v, parties=%v)", s.id, len(s.getParties()))
 }
 
 // removePartyUnderLock removes the party from the in-memory map that holds all party members
@@ -1950,7 +1947,7 @@ func (s *session) denyFileTransferRequest(params *rsession.FileTransferDecisionP
 func (s *session) checkIfStart() (bool, auth.PolicyOptions, error) {
 	var participants []auth.SessionAccessContext
 
-	for _, party := range s.parties {
+	for _, party := range s.getParties() {
 		if party.ctx.Identity.TeleportUser == s.initiator {
 			continue
 		}