Document limitations of the modified scan

pump/storage/vlog.go

@@ -406,7 +406,11 @@ func (vlog *valueLog) scanRequests(start valuePointer, fn func(*request) error)
 	})
 }
 
-// currently we only use this in NewAppend** to scan the record which not write to KV but in the value log, so it's OK to hold the vlog.filesLock lock
+// scan visits binlogs in order starting from the specified position.
+// There are two limitations to the usage of scan:
+// 1. Binlogs added in new logFiles after scan starts are not visible, so don't assume
+//    that every single binlog added would be visited
+// 2. If GC is running concurrently, logFiles may be closed and deleted, thus breaking the scanning.
 func (vlog *valueLog) scan(start valuePointer, fn func(vp valuePointer, record *Record) error) error {
 	vlog.filesLock.RLock()
 	fids := vlog.sortedFids()