[release/1.4.6] eth/downloader, trie: pull head state concurrently with chain

(cherry picked from commit 4f1d92b332)

trie/sync.go

@@ -17,6 +17,7 @@
 package trie
 
 import (
+	"errors"
 	"fmt"
 
 	"github.com/ethereum/go-ethereum/common"
@@ -24,6 +25,10 @@ import (
 	"gopkg.in/karalabe/cookiejar.v2/collections/prque"
 )
 
+// ErrNotRequested is returned by the trie sync when it's requested to process a
+// node it did not request.
+var ErrNotRequested = errors.New("not requested")
+
 // request represents a scheduled or already in-flight state retrieval request.
 type request struct {
 	hash   common.Hash // Hash of the node data content to retrieve
@@ -144,7 +149,7 @@ func (s *TrieSync) Process(results []SyncResult) (int, error) {
 		// If the item was not requested, bail out
 		request := s.requests[item.Hash]
 		if request == nil {
-			return i, fmt.Errorf("not requested: %x", item.Hash)
+			return i, ErrNotRequested
 		}
 		// If the item is a raw entry request, commit directly
 		if request.object == nil {