core/state: small trie prefetcher nits (#28183)

Small trie prefetcher nits

core/state/trie_prefetcher.go

@@ -36,8 +36,8 @@ var (
 // Note, the prefetcher's API is not thread safe.
 type triePrefetcher struct {
 	db       Database                    // Database to fetch trie nodes through
-	root     common.Hash                 // Root hash of theaccount trie for metrics
-	fetches  map[common.Hash]Trie        // Partially or fully fetcher tries
+	root     common.Hash                 // Root hash of the account trie for metrics
+	fetches  map[common.Hash]Trie        // Partially or fully fetched tries. Only populated for inactive copies.
 	fetchers map[common.Hash]*subfetcher // Subfetchers for each trie
 
 	deliveryMissMeter metrics.Meter
@@ -191,6 +191,14 @@ func (p *triePrefetcher) used(root common.Hash, used [][]byte) {
 	}
 }
 
+// trieID returns an unique trie identifier consists the trie owner and root hash.
+func (p *triePrefetcher) trieID(owner common.Hash, root common.Hash) string {
+	trieID := make([]byte, common.HashLength*2)
+	copy(trieID, owner.Bytes())
+	copy(trieID[common.HashLength:], root.Bytes())
+	return string(trieID)
+}
+
 // subfetcher is a trie fetcher goroutine responsible for pulling entries for a
 // single trie. It is spawned when a new root is encountered and lives until the
 // main prefetcher is paused and either all requested items are processed or if