eth, trie: fix tracer GC which accidentally pruned the metaroot

eth/api_tracer.go

@@ -297,7 +297,9 @@ func (api *PrivateDebugAPI) traceChain(ctx context.Context, start, end *types.Bl
 				database.TrieDB().Reference(root, common.Hash{})
 			}
 			// Dereference all past tries we ourselves are done working with
+			if proot != (common.Hash{}) {
 				database.TrieDB().Dereference(proot)
+			}
 			proot = root
 
 			// TODO(karalabe): Do we need the preimages? Won't they accumulate too much?
@@ -526,7 +528,9 @@ func (api *PrivateDebugAPI) computeStateDB(block *types.Block, reexec uint64) (*
 			return nil, err
 		}
 		database.TrieDB().Reference(root, common.Hash{})
+		if proot != (common.Hash{}) {
 			database.TrieDB().Dereference(proot)
+		}
 		proot = root
 	}
 	nodes, imgs := database.TrieDB().Size()

trie/database.go

@@ -431,6 +431,11 @@ func (db *Database) reference(child common.Hash, parent common.Hash) {
 
 // Dereference removes an existing reference from a root node.
 func (db *Database) Dereference(root common.Hash) {
+	// Sanity check to ensure that the meta-root is not removed
+	if root == (common.Hash{}) {
+		log.Error("Attempted to dereference the trie cache meta root")
+		return
+	}
 	db.lock.Lock()
 	defer db.lock.Unlock()