[IMPROVED] Optimize locking for consumer info API (#4615)

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

server/jetstream_api.go

@@ -4164,9 +4164,20 @@ func (s *Server) jsConsumerInfoRequest(sub *subscription, c *client, _ *Account,
 		js.mu.RLock()
 		isLeader, sa, ca := cc.isLeader(), js.streamAssignment(acc.Name, streamName), js.consumerAssignment(acc.Name, streamName, consumerName)
 		ourID := cc.meta.ID()
-		var offline bool
+		var rg *raftGroup
+		var offline, isMember bool
 		if ca != nil {
-			offline = s.allPeersOffline(ca.Group)
+			if rg = ca.Group; rg != nil {
+				offline = s.allPeersOffline(rg)
+				isMember = rg.isMember(ourID)
+			}
+		}
+		// Capture consumer leader here.
+		isConsumerLeader := cc.isConsumerLeader(acc.Name, streamName, consumerName)
+		// Also capture if we think there is no meta leader.
+		var isLeaderLess bool
+		if !isLeader {
+			isLeaderLess = cc.meta.GroupLeader() == _EMPTY_ && time.Since(cc.meta.Created()) > lostQuorumIntervalDefault
 		}
 		js.mu.RUnlock()
 
@@ -4189,7 +4200,7 @@ func (s *Server) jsConsumerInfoRequest(sub *subscription, c *client, _ *Account,
 			s.sendAPIErrResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(&resp))
 			return
 		} else if ca == nil {
-			if js.isLeaderless() {
+			if isLeaderLess {
 				resp.Error = NewJSClusterNotAvailError()
 				// Delaying an error response gives the leader a chance to respond before us
 				s.sendDelayedAPIErrResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(&resp), nil)
@@ -4202,38 +4213,35 @@ func (s *Server) jsConsumerInfoRequest(sub *subscription, c *client, _ *Account,
 		}
 
 		// Check to see if we are a member of the group and if the group has no leader.
-		if js.isGroupLeaderless(ca.Group) {
+		if isMember && js.isGroupLeaderless(ca.Group) {
 			resp.Error = NewJSClusterNotAvailError()
 			s.sendAPIErrResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(&resp))
 			return
 		}
 
 		// We have the consumer assigned and a leader, so only the consumer leader should answer.
-		if !acc.JetStreamIsConsumerLeader(streamName, consumerName) {
-			if js.isLeaderless() {
+		if !isConsumerLeader {
+			if isLeaderLess {
 				resp.Error = NewJSClusterNotAvailError()
 				// Delaying an error response gives the leader a chance to respond before us
 				s.sendDelayedAPIErrResponse(ci, acc, subject, reply, string(msg), s.jsonResponse(&resp), ca.Group)
 				return
 			}
+
+			var node RaftNode
+			var leaderNotPartOfGroup bool
+
 			// We have a consumer assignment.
-			js.mu.RLock()
-			var (
-				node                 RaftNode
-				leaderNotPartOfGroup bool
-				isMember             bool
-			)
-			rg := ca.Group
-			if rg != nil && rg.isMember(ourID) {
-				isMember = true
+			if isMember {
+				js.mu.RLock()
 				if rg.node != nil {
 					node = rg.node
 					if gl := node.GroupLeader(); gl != _EMPTY_ && !rg.isMember(gl) {
 						leaderNotPartOfGroup = true
 					}
 				}
+				js.mu.RUnlock()
 			}
-			js.mu.RUnlock()
 
 			// Check if we should ignore all together.
 			if node == nil {