use internal get during walk to avoid blowing the compaction txn

otherwise the walk itself precludes purge... duh!

blockstore/splitstore/splitstore.go

@@ -263,6 +263,19 @@ func (s *SplitStore) Get(cid cid.Cid) (blocks.Block, error) {
 	}
 }
 
+// internal version used by walk so that we don't blow the txn
+func (s *SplitStore) get(cid cid.Cid) (blocks.Block, error) {
+	blk, err := s.hot.Get(cid)
+
+	switch err {
+	case bstore.ErrNotFound:
+		return s.cold.Get(cid)
+
+	default:
+		return blk, err
+	}
+}
+
 func (s *SplitStore) GetSize(cid cid.Cid) (int, error) {
 	s.txnLk.RLock()
 	defer s.txnLk.RUnlock()
@@ -986,7 +999,7 @@ func (s *SplitStore) walk(ts *types.TipSet, boundary abi.ChainEpoch, inclMsgs bo
 			return nil
 		}
 
-		blk, err := s.Get(c)
+		blk, err := s.get(c)
 		if err != nil {
 			return xerrors.Errorf("error retrieving block (cid: %s): %w", c, err)
 		}
@@ -1053,7 +1066,7 @@ func (s *SplitStore) walkLinks(c cid.Cid, walked *cid.Set, f func(cid.Cid) error
 		return nil
 	}
 
-	blk, err := s.Get(c)
+	blk, err := s.get(c)
 	if err != nil {
 		return xerrors.Errorf("error retrieving linked block (cid: %s): %w", c, err)
 	}