Add aggregator backfill, pulling fine+coarse buckets from collectors

cmd/collector/store.go

@@ -154,6 +154,32 @@ func (s *Store) coarseView() st.RingView {
 	return st.RingView{Ring: ring, Head: s.coarseHead, Size: st.CoarseRingSize}
 }
 
+// DumpRings returns copies of all non-empty fine and coarse ring snapshots in
+// chronological order. The lock is held only for the duration of the copy.
+func (s *Store) DumpRings() (fine, coarse []st.Snapshot) {
+	s.mu.RLock()
+	fineRing := s.fineRing
+	fineHead := s.fineHead
+	fineFilled := s.fineFilled
+	coarseRing := s.coarseRing
+	coarseHead := s.coarseHead
+	coarseFilled := s.coarseFilled
+	s.mu.RUnlock()
+
+	fine = make([]st.Snapshot, 0, fineFilled)
+	for i := 0; i < fineFilled; i++ {
+		idx := (fineHead - fineFilled + i + st.FineRingSize) % st.FineRingSize
+		fine = append(fine, fineRing[idx])
+	}
+
+	coarse = make([]st.Snapshot, 0, coarseFilled)
+	for i := 0; i < coarseFilled; i++ {
+		idx := (coarseHead - coarseFilled + i + st.CoarseRingSize) % st.CoarseRingSize
+		coarse = append(coarse, coarseRing[idx])
+	}
+	return fine, coarse
+}
+
 func (s *Store) Subscribe() chan st.Snapshot {
 	ch := make(chan st.Snapshot, 4)
 	s.subMu.Lock()