trie: there's no point in retrieving the metaroot

trie/database.go

@@ -18,6 +18,7 @@ package trie
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"sync"
@@ -400,6 +401,10 @@ func (db *Database) node(hash common.Hash) node {
 // Node retrieves an encoded cached trie node from memory. If it cannot be found
 // cached, the method queries the persistent database for the content.
 func (db *Database) Node(hash common.Hash) ([]byte, error) {
+	// It doens't make sense to retrieve the metaroot
+	if hash == (common.Hash{}) {
+		return nil, errors.New("not found")
+	}
 	// Retrieve the node from the clean cache if available
 	if db.cleans != nil {
 		if enc, err := db.cleans.Get(string(hash[:])); err == nil && enc != nil {

trie/database_test.go

@@ -0,0 +1,33 @@
+// Copyright 2019 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+package trie
+
+import (
+	"testing"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/ethdb/memorydb"
+)
+
+// Tests that the trie database returns a missing trie node error if attempting
+// to retrieve the meta root.
+func TestDatabaseMetarootFetch(t *testing.T) {
+	db := NewDatabase(memorydb.New())
+	if _, err := db.Node(common.Hash{}); err == nil {
+		t.Fatalf("metaroot retrieval succeeded")
+	}
+}