mpt: introduce DummySTTempStoragePrefix

Use this constant as a dummy contract storage item prefix for those
situations when Billet is used as an MPT search wrapper. Otherwise it's
confusing to see `0` passed as a storage prefix to the billet
constructor and then panic inside the Billet's code if this prefix is
`0`.

Ref. #2201.

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

Author: AnnaShaleva

pkg/core/mpt/billet.go

@@ -11,6 +11,13 @@ import (
 	"github.com/nspcc-dev/neo-go/pkg/util"
 )
 
+// DummySTTempStoragePrefix is a dummy contract storage item prefix that may be
+// passed to Billet constructor when Billet's restoring functionality is not
+// used, i.e. for those situations when only traversal functionality is used.
+// Note that using this prefix for MPT restoring is a no-op since Billet must
+// have the ability to save contract storage items to the underlying DB.
+const DummySTTempStoragePrefix = 0x00
+
 var (
 	// ErrRestoreFailed is returned when replacing HashNode by its "unhashed"
 	// candidate fails.
@@ -68,7 +75,7 @@ func (b *Billet) RestoreHashNode(path []byte, node Node) error {
 
 	// If it's a leaf, then put into temporary contract storage.
 	if leaf, ok := node.(*LeafNode); ok {
-		if b.TempStoragePrefix == 0 {
+		if b.TempStoragePrefix == DummySTTempStoragePrefix {
 			panic("invalid storage prefix")
 		}
 		k := append([]byte{byte(b.TempStoragePrefix)}, fromNibbles(path)...)

pkg/core/mpt/trie.go

@@ -612,7 +612,7 @@ func (t *Trie) Find(prefix, from []byte, maxNum int) ([]storage.KeyValue, error)
 		res   []storage.KeyValue
 		count int
 	)
-	b := NewBillet(t.root.Hash(), t.mode, 0, t.Store)
+	b := NewBillet(t.root.Hash(), t.mode, DummySTTempStoragePrefix, t.Store)
 	process := func(pathToNode []byte, node Node, _ []byte) bool {
 		if leaf, ok := node.(*LeafNode); ok {
 			if from == nil || !bytes.Equal(pathToNode, from) { // (*Billet).traverse includes `from` path into result if so. Need to filter out manually.

pkg/core/mpt/trie_store.go

@@ -94,7 +94,7 @@ func (m *TrieStore) Seek(rng storage.SeekRange, f func(k, v []byte) bool) {
 		}
 	}
 
-	b := NewBillet(m.trie.root.Hash(), m.trie.mode, 0, m.trie.Store)
+	b := NewBillet(m.trie.root.Hash(), m.trie.mode, DummySTTempStoragePrefix, m.trie.Store)
 	process := func(pathToNode []byte, node Node, _ []byte) bool {
 		if leaf, ok := node.(*LeafNode); ok {
 			// (*Billet).traverse includes `from` path into the result if so. It's OK for Seek, so shouldn't be filtered out.

pkg/core/statesync/module.go

@@ -551,7 +551,7 @@ func (s *Module) Traverse(root util.Uint256, process func(node mpt.Node, nodeByt
 	if s.bc.GetConfig().Ledger.KeepOnlyLatestState || s.bc.GetConfig().Ledger.RemoveUntraceableBlocks {
 		mode |= mpt.ModeLatest
 	}
-	b := mpt.NewBillet(root, mode, 0, storage.NewMemCachedStore(s.dao.Store))
+	b := mpt.NewBillet(root, mode, mpt.DummySTTempStoragePrefix, storage.NewMemCachedStore(s.dao.Store))
 	return b.Traverse(func(pathToNode []byte, node mpt.Node, nodeBytes []byte) bool {
 		return process(node, nodeBytes)
 	}, false)