Fix for race in selectMsgBlock

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

server/filestore.go

@@ -1148,28 +1148,35 @@ func (fs *fileStore) syncBlocks() {
 // Return nil if not in the set.
 func (fs *fileStore) selectMsgBlock(seq uint64) *msgBlock {
 	fs.mu.RLock()
-	outOfRange := seq < fs.state.FirstSeq || seq > fs.state.LastSeq
-	blks := fs.blks
-	lmb := fs.lmb
-	fs.mu.RUnlock()
-
-	if outOfRange {
+	// Check for out of range.
+	if seq < fs.state.FirstSeq || seq > fs.state.LastSeq {
+		fs.mu.RUnlock()
 		return nil
 	}
 
+	var smb *msgBlock
+	var needsFlush bool
+
 	// blks are sorted in ascending order.
 	// TODO(dlc) - Can be smarter here, when lots of blks maybe use binary search.
 	// For now this is cache friendly for small to medium num blks.
-	for _, mb := range blks {
+	for _, mb := range fs.blks {
 		if seq <= atomic.LoadUint64(&mb.last.seq) {
 			// This detects if what we may be looking for is staged in the write buffer.
-			if mb == lmb {
-				fs.flushPendingWritesUnlocked()
+			if mb == fs.lmb {
+				needsFlush = true
 			}
-			return mb
+			smb = mb
+			break
 		}
 	}
-	return nil
+	fs.mu.RUnlock()
+
+	if needsFlush {
+		fs.flushPendingWritesUnlocked()
+	}
+
+	return smb
 }
 
 // Select the message block where this message should be found.