[FIXED] Prevent a server from panicking when shutdown immediately aft…

…er starting. (#5663)

Signed-off-by: Derek Collison <derek@nats.io>

server/events.go

@@ -1066,51 +1066,64 @@ func (s *Server) Node() string {
 // Tradeoff is subscription and interest graph events vs connect and
 // disconnect events, etc.
 func (s *Server) initEventTracking() {
-	if !s.EventsEnabled() {
+	// Capture sys in case we are shutdown while setting up.
+	s.mu.RLock()
+	sys := s.sys
+	s.mu.RUnlock()
+
+	if sys == nil || sys.client == nil || sys.account == nil {
 		return
 	}
 	// Create a system hash which we use for other servers to target us specifically.
-	s.sys.shash = getHash(s.info.Name)
+	sys.shash = getHash(s.info.Name)
 
 	// This will be for all inbox responses.
-	subject := fmt.Sprintf(inboxRespSubj, s.sys.shash, "*")
+	subject := fmt.Sprintf(inboxRespSubj, sys.shash, "*")
 	if _, err := s.sysSubscribe(subject, s.inboxReply); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
-	s.sys.inboxPre = subject
+	sys.inboxPre = subject
 	// This is for remote updates for connection accounting.
 	subject = fmt.Sprintf(accConnsEventSubjOld, "*")
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteConnsUpdate)); err != nil {
 		s.Errorf("Error setting up internal tracking for %s: %v", subject, err)
+		return
 	}
 	// This will be for responses for account info that we send out.
 	subject = fmt.Sprintf(connsRespSubj, s.info.ID)
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteConnsUpdate)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	// Listen for broad requests to respond with number of subscriptions for a given subject.
 	if _, err := s.sysSubscribe(accNumSubsReqSubj, s.noInlineCallback(s.nsubsRequest)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	// Listen for statsz from others.
 	subject = fmt.Sprintf(serverStatsSubj, "*")
 	if sub, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteServerUpdate)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	} else {
 		// Keep track of this one.
-		s.sys.remoteStatsSub = sub
+		sys.remoteStatsSub = sub
 	}
+
 	// Listen for all server shutdowns.
 	subject = fmt.Sprintf(shutdownEventSubj, "*")
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteServerShutdown)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	// Listen for servers entering lame-duck mode.
 	// NOTE: This currently is handled in the same way as a server shutdown, but has
 	// a different subject in case we need to handle differently in future.
 	subject = fmt.Sprintf(lameDuckEventSubj, "*")
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteServerShutdown)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	// Listen for account claims updates.
 	subscribeToUpdate := true
@@ -1121,13 +1134,15 @@ func (s *Server) initEventTracking() {
 		for _, sub := range []string{accUpdateEventSubjOld, accUpdateEventSubjNew} {
 			if _, err := s.sysSubscribe(fmt.Sprintf(sub, "*"), s.noInlineCallback(s.accountClaimUpdate)); err != nil {
 				s.Errorf("Error setting up internal tracking: %v", err)
+				return
 			}
 		}
 	}
 	// Listen for ping messages that will be sent to all servers for statsz.
 	// This subscription is kept for backwards compatibility. Got replaced by ...PING.STATZ from below
 	if _, err := s.sysSubscribe(serverStatsPingReqSubj, s.noInlineCallback(s.statszReq)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	monSrvc := map[string]sysMsgHandler{
 		"IDZ":    s.idzReq,
@@ -1181,10 +1196,12 @@ func (s *Server) initEventTracking() {
 		subject = fmt.Sprintf(serverDirectReqSubj, s.info.ID, name)
 		if _, err := s.sysSubscribe(subject, s.noInlineCallback(req)); err != nil {
 			s.Errorf("Error setting up internal tracking: %v", err)
+			return
 		}
 		subject = fmt.Sprintf(serverPingReqSubj, name)
 		if _, err := s.sysSubscribe(subject, s.noInlineCallback(req)); err != nil {
 			s.Errorf("Error setting up internal tracking: %v", err)
+			return
 		}
 	}
 	extractAccount := func(subject string) (string, error) {
@@ -1277,6 +1294,7 @@ func (s *Server) initEventTracking() {
 	for name, req := range monAccSrvc {
 		if _, err := s.sysSubscribe(fmt.Sprintf(accDirectReqSubj, "*", name), s.noInlineCallback(req)); err != nil {
 			s.Errorf("Error setting up internal tracking: %v", err)
+			return
 		}
 	}
 
@@ -1285,6 +1303,7 @@ func (s *Server) initEventTracking() {
 	// is only one that will answer. This breaks tests since we still forward on remote server connect.
 	if _, err := s.sysSubscribe(fmt.Sprintf(userDirectReqSubj, "*"), s.userInfoReq); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 
 	// For now only the STATZ subject has an account specific ping equivalent.
@@ -1302,39 +1321,46 @@ func (s *Server) initEventTracking() {
 			})
 		})); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 
 	// Listen for updates when leaf nodes connect for a given account. This will
 	// force any gateway connections to move to `modeInterestOnly`
 	subject = fmt.Sprintf(leafNodeConnectEventSubj, "*")
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.leafNodeConnected)); err != nil {
 		s.Errorf("Error setting up internal tracking: %v", err)
+		return
 	}
 	// For tracking remote latency measurements.
-	subject = fmt.Sprintf(remoteLatencyEventSubj, s.sys.shash)
+	subject = fmt.Sprintf(remoteLatencyEventSubj, sys.shash)
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.remoteLatencyUpdate)); err != nil {
 		s.Errorf("Error setting up internal latency tracking: %v", err)
+		return
 	}
 	// This is for simple debugging of number of subscribers that exist in the system.
 	if _, err := s.sysSubscribeInternal(accSubsSubj, s.noInlineCallback(s.debugSubscribers)); err != nil {
 		s.Errorf("Error setting up internal debug service for subscribers: %v", err)
+		return
 	}
 
 	// Listen for requests to reload the server configuration.
 	subject = fmt.Sprintf(serverReloadReqSubj, s.info.ID)
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.reloadConfig)); err != nil {
 		s.Errorf("Error setting up server reload handler: %v", err)
+		return
 	}
 
 	// Client connection kick
 	subject = fmt.Sprintf(clientKickReqSubj, s.info.ID)
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.kickClient)); err != nil {
 		s.Errorf("Error setting up client kick service: %v", err)
+		return
 	}
 	// Client connection LDM
 	subject = fmt.Sprintf(clientLDMReqSubj, s.info.ID)
 	if _, err := s.sysSubscribe(subject, s.noInlineCallback(s.ldmClient)); err != nil {
 		s.Errorf("Error setting up client LDM service: %v", err)
+		return
 	}
 }