Use write lock in memstore.LoadNextMsg

Signed-off-by: Waldemar Quevedo <wally@synadia.com>

server/memstore.go

@@ -913,8 +913,8 @@ func (ms *memStore) LoadLastMsg(subject string, smp *StoreMsg) (*StoreMsg, error
 // LoadNextMsg will find the next message matching the filter subject starting at the start sequence.
 // The filter subject can be a wildcard.
 func (ms *memStore) LoadNextMsg(filter string, wc bool, start uint64, smp *StoreMsg) (*StoreMsg, uint64, error) {
-	ms.mu.RLock()
-	defer ms.mu.RUnlock()
+	ms.mu.Lock()
+	defer ms.mu.Unlock()
 
 	if start < ms.state.FirstSeq {
 		start = ms.state.FirstSeq
@@ -952,7 +952,11 @@ func (ms *memStore) LoadNextMsg(filter string, wc bool, start uint64, smp *Store
 				continue
 			}
 			if ss.firstNeedsUpdate {
+				ms.mu.RUnlock()
+				ms.mu.Lock()
 				ms.recalculateFirstForSubj(subj, ss.First, ss)
+				ms.mu.Unlock()
+				ms.mu.RLock()
 			}
 			if ss.First < fseq {
 				fseq = ss.First
@@ -1063,6 +1067,7 @@ func (ms *memStore) removeSeqPerSubject(subj string, seq uint64) {
 }
 
 // Will recalulate the first sequence for this subject in this block.
+// Lock should be held.
 func (ms *memStore) recalculateFirstForSubj(subj string, startSeq uint64, ss *SimpleState) {
 	for tseq := startSeq + 1; tseq <= ss.Last; tseq++ {
 		if sm := ms.msgs[tseq]; sm != nil && sm.subj == subj {