When stores and load for last for subject where concurrent and competiting for the same msg, we could fail to retrieve a newly placed message.

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

server/filestore.go

@@ -1467,6 +1467,7 @@ func (fs *fileStore) FilteredState(sseq uint64, subj string) SimpleState {
 	wc := subjectHasWildcard(subj)
 	// Are we tracking multiple subject states?
 	if fs.tms {
+		fs.mu.RLock()
 		for _, mb := range fs.blks {
 			// Skip blocks that are less than our starting sequence.
 			if sseq > atomic.LoadUint64(&mb.last.seq) {
@@ -1481,6 +1482,7 @@ func (fs *fileStore) FilteredState(sseq uint64, subj string) SimpleState {
 				ss.Last = l
 			}
 		}
+		fs.mu.RUnlock()
 	} else {
 		// Fallback to linear scan.
 		var smv StoreMsg
@@ -3799,15 +3801,44 @@ func (fs *fileStore) LoadMsg(seq uint64, sm *StoreMsg) (*StoreMsg, error) {
 	return fs.msgForSeq(seq, sm)
 }
 
+// loadLast will load the last message for a subject. Subject should be non empty and not ">".
+func (fs *fileStore) loadLast(subj string, sm *StoreMsg) (lsm *StoreMsg, err error) {
+	fs.mu.RLock()
+	defer fs.mu.RUnlock()
+
+	wc := subjectHasWildcard(subj)
+	// Walk blocks backwards.
+	for i := len(fs.blks) - 1; i >= 0; i-- {
+		mb := fs.blks[i]
+		if mb == nil {
+			continue
+		}
+		mb.mu.Lock()
+		_, _, l := mb.filteredPendingLocked(subj, wc, mb.first.seq)
+		if l > 0 {
+			if mb.cacheNotLoaded() {
+				if err := mb.loadMsgsWithLock(); err != nil {
+					mb.mu.Unlock()
+					return nil, err
+				}
+			}
+			lsm, err = mb.cacheLookup(l, sm)
+		}
+		mb.mu.Unlock()
+		if l > 0 {
+			break
+		}
+	}
+	return lsm, err
+}
+
 // LoadLastMsg will return the last message we have that matches a given subject.
 // The subject can be a wildcard.
 func (fs *fileStore) LoadLastMsg(subject string, sm *StoreMsg) (*StoreMsg, error) {
 	if subject == _EMPTY_ || subject == fwcs {
 		sm, _ = fs.msgForSeq(fs.lastSeq(), sm)
-	} else if ss := fs.FilteredState(1, subject); ss.Msgs > 0 {
-		sm, _ = fs.msgForSeq(ss.Last, sm)
 	} else {
-		sm = nil
+		sm, _ = fs.loadLast(subject, sm)
 	}
 	if sm == nil {
 		return nil, ErrStoreMsgNotFound