sort cold objects before deleting

so that we can't shoot ourselves in the foot by deleting the constituents of a DAG while it is
still in the hotstore.

blockstore/splitstore/splitstore.go

@@ -5,6 +5,7 @@ import (
 	"context"
 	"encoding/binary"
 	"errors"
+	"sort"
 	"sync"
 	"sync/atomic"
 	"time"
@@ -1114,7 +1115,17 @@ func (s *SplitStore) doCompact(curTs *types.TipSet) error {
 		log.Infow("moving done", "took", time.Since(startMove))
 	}
 
-	// 4. purge cold objects from the hotstore, taking protected references into account
+	// 4. sort cold objects so that the dags with most references are deleted first
+	//    this ensures that we can't refer to a dag with its consituents already deleted
+	log.Info("sorting cold objects")
+	startSort := time.Now()
+	err = s.sortObjects(cold)
+	if err != nil {
+		return xerrors.Errorf("error sorting objects: %w", err)
+	}
+	log.Infow("sorting done", "took", time.Since(startSort))
+
+	// 5. purge cold objects from the hotstore, taking protected references into account
 	log.Info("purging cold objects from the hotstore")
 	startPurge := time.Now()
 	err = s.purge(curTs, cold)
@@ -1348,6 +1359,46 @@ func (s *SplitStore) moveColdBlocks(cold []cid.Cid) error {
 	return nil
 }
 
+func (s *SplitStore) sortObjects(cids []cid.Cid) error {
+	weight := make(map[cid.Cid]int)
+	for _, c := range cids {
+		if c.Prefix().Codec != cid.DagCBOR {
+			continue
+		}
+
+		w := 0
+		err := s.walkObject(c, cid.NewSet(),
+			func(c cid.Cid) error {
+				wc, ok := weight[c]
+				if ok {
+					w += wc
+					return errStopWalk
+				}
+
+				w++
+				return nil
+			})
+
+		if err != nil {
+			return xerrors.Errorf("error determining cold object weight: %w", err)
+		}
+
+		weight[c] = w
+	}
+
+	sort.Slice(cids, func(i, j int) bool {
+		wi := weight[cids[i]]
+		wj := weight[cids[j]]
+		if wi == wj {
+			return bytes.Compare(cids[i].Hash(), cids[j].Hash()) > 0
+		}
+
+		return wi > wj
+	})
+
+	return nil
+}
+
 func (s *SplitStore) purgeBatch(cids []cid.Cid, deleteBatch func([]cid.Cid) error) error {
 	if len(cids) == 0 {
 		return nil