fix some residual purge races

blockstore/splitstore/splitstore.go

@@ -236,17 +236,19 @@ func (s *SplitStore) Has(c cid.Cid) (bool, error) {
 	}
 
 	if has {
-		// treat it as an implicit Write, absence options -- the vm uses this check to avoid duplicate
-		// writes on Flush. When we have options in the API, the vm can explicitly signal that this is
-		// an implicit Write.
+		// treat it as an implicit (recursive) Write, absence options -- the vm uses this check to avoid
+		// duplicate writes on Copy.
+		// When we have options in the API, the vm can explicitly signal that this is an implicit Write.
 		s.trackWrite(c, true)
 
 		// also make sure the object is considered live during compaction in case we have already
-		// flushed pending writes and started compaction
+		// flushed pending writes and started compaction.
+		// in case of a race with purge, this will return a track error, which we can use to
+		// signal to the vm that the object is not fully present.
 		trackErr := s.trackTxnRef(c, true)
 
 		// if we failed to track the object and all its dependencies, then return false so as
-		// to cause the vm to recompute
+		// to cause the vm to copy
 		return trackErr == nil, nil
 	}
 
@@ -673,6 +675,16 @@ func (s *SplitStore) trackTxnRef(c cid.Cid, implicit bool) error {
 		err = s.txnProtect.Mark(c)
 	} else {
 		err = s.walkLinks(c, cid.NewSet(), func(c cid.Cid) error {
+			// this check is necessary to avoid races because objects are purged in random order
+			has, err := s.hot.Has(c)
+			if err != nil {
+				return err
+			}
+
+			if !has {
+				return xerrors.Errorf("object (cid: %s) has been purged", c)
+			}
+
 			return s.txnProtect.Mark(c)
 		})
 	}
@@ -949,28 +961,8 @@ func (s *SplitStore) doCompact(curTs *types.TipSet) error {
 		return xerrors.Errorf("error creating mark set: %w", err)
 	}
 	defer markSet.Close() //nolint:errcheck
-
-	// create the pruge protect filter
-	s.txnLk.Lock()
-	s.txnProtect, err = s.txnEnv.Create("protected", s.markSetSize)
-	if err != nil {
-		s.txnLk.Unlock()
-		return xerrors.Errorf("error creating transactional mark set: %w", err)
-	}
-	s.txnLk.Unlock()
-
-	defer func() {
-		s.txnLk.Lock()
-		_ = s.txnProtect.Close()
-		s.txnProtect = nil
-		s.txnLk.Unlock()
-	}()
-
 	defer s.debug.Flush()
 
-	// flush pending writes to update the tracker
-	s.flushPendingWrites(false)
-
 	// 1. mark reachable objects by walking the chain from the current epoch to the boundary epoch
 	log.Infow("marking reachable blocks", "currentEpoch", currentEpoch, "boundaryEpoch", boundaryEpoch)
 	startMark := time.Now()
@@ -992,6 +984,28 @@ func (s *SplitStore) doCompact(curTs *types.TipSet) error {
 
 	log.Infow("marking done", "took", time.Since(startMark), "marked", count)
 
+	// create the transaction protect filter
+	s.txnLk.Lock()
+	s.txnProtect, err = s.txnEnv.Create("protected", s.markSetSize)
+	if err != nil {
+		s.txnLk.Unlock()
+		return xerrors.Errorf("error creating transactional mark set: %w", err)
+	}
+	s.txnLk.Unlock()
+
+	defer func() {
+		s.txnLk.Lock()
+		_ = s.txnProtect.Close()
+		s.txnProtect = nil
+		s.txnLk.Unlock()
+	}()
+
+	// flush pending writes to update the tracker
+	log.Info("flushing pending writes")
+	startFlush := time.Now()
+	s.flushPendingWrites(false)
+	log.Infow("flushing done", "took", time.Since(startFlush))
+
 	// 2. move cold unreachable objects to the coldstore
 	log.Info("collecting cold objects")
 	startCollect := time.Now()
@@ -1071,6 +1085,7 @@ func (s *SplitStore) doCompact(curTs *types.TipSet) error {
 		}
 		log.Infow("moving done", "took", time.Since(startMove))
 	}
+
 	// 2.3 purge cold objects from the hotstore
 	log.Info("purging cold objects from the hotstore")
 	startPurge := time.Now()
@@ -1293,7 +1308,7 @@ func (s *SplitStore) purge(curTs *types.TipSet, cids []cid.Cid) error {
 	deadCids := make([]cid.Cid, 0, batchSize)
 	var purgeCnt, liveCnt int
 	defer func() {
-		log.Infow("purged objects", "purged", purgeCnt, "live", liveCnt)
+		log.Infow("purged cold objects", "purged", purgeCnt, "live", liveCnt)
 	}()
 
 	return s.purgeBatch(cids,