core, ethdb, tests, trie: introduce database snapshot (#24486)

ethdb/leveldb/leveldb.go

@@ -228,6 +228,19 @@ func (db *Database) NewIterator(prefix []byte, start []byte) ethdb.Iterator {
 	return db.db.NewIterator(bytesPrefixRange(prefix, start), nil)
 }
 
+// NewSnapshot creates a database snapshot based on the current state.
+// The created snapshot will not be affected by all following mutations
+// happened on the database.
+// Note don't forget to release the snapshot once it's used up, otherwise
+// the stale data will never be cleaned up by the underlying compactor.
+func (db *Database) NewSnapshot() (ethdb.Snapshot, error) {
+	snap, err := db.db.GetSnapshot()
+	if err != nil {
+		return nil, err
+	}
+	return &snapshot{db: snap}, nil
+}
+
 // Stat returns a particular internal stat of the database.
 func (db *Database) Stat(property string) (string, error) {
 	return db.db.GetProperty(property)
@@ -527,3 +540,26 @@ func bytesPrefixRange(prefix, start []byte) *util.Range {
 	r.Start = append(r.Start, start...)
 	return r
 }
+
+// snapshot wraps a leveldb snapshot for implementing the Snapshot interface.
+type snapshot struct {
+	db *leveldb.Snapshot
+}
+
+// Has retrieves if a key is present in the snapshot backing by a key-value
+// data store.
+func (snap *snapshot) Has(key []byte) (bool, error) {
+	return snap.db.Has(key, nil)
+}
+
+// Get retrieves the given key if it's present in the snapshot backing by
+// key-value data store.
+func (snap *snapshot) Get(key []byte) ([]byte, error) {
+	return snap.db.Get(key, nil)
+}
+
+// Release releases associated resources. Release should always succeed and can
+// be called multiple times without causing error.
+func (snap *snapshot) Release() {
+	snap.db.Release()
+}