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

ethdb/memorydb/memorydb.go

@@ -35,6 +35,10 @@ var (
 	// errMemorydbNotFound is returned if a key is requested that is not found in
 	// the provided memory database.
 	errMemorydbNotFound = errors.New("not found")
+
+	// errSnapshotReleased is returned if callers want to retrieve data from a
+	// released snapshot.
+	errSnapshotReleased = errors.New("snapshot released")
 )
 
 // Database is an ephemeral key-value store. Apart from basic data storage
@@ -53,7 +57,7 @@ func New() *Database {
 	}
 }
 
-// NewWithCap returns a wrapped map pre-allocated to the provided capcity with
+// NewWithCap returns a wrapped map pre-allocated to the provided capacity with
 // all the required database interface methods implemented.
 func NewWithCap(size int) *Database {
 	return &Database{
@@ -170,6 +174,13 @@ func (db *Database) NewIterator(prefix []byte, start []byte) ethdb.Iterator {
 	}
 }
 
+// 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.
+func (db *Database) NewSnapshot() (ethdb.Snapshot, error) {
+	return newSnapshot(db), nil
+}
+
 // Stat returns a particular internal stat of the database.
 func (db *Database) Stat(property string) (string, error) {
 	return "", errors.New("unknown property")
@@ -320,3 +331,59 @@ func (it *iterator) Value() []byte {
 func (it *iterator) Release() {
 	it.keys, it.values = nil, nil
 }
+
+// snapshot wraps a batch of key-value entries deep copied from the in-memory
+// database for implementing the Snapshot interface.
+type snapshot struct {
+	db   map[string][]byte
+	lock sync.RWMutex
+}
+
+// newSnapshot initializes the snapshot with the given database instance.
+func newSnapshot(db *Database) *snapshot {
+	db.lock.RLock()
+	defer db.lock.RUnlock()
+
+	copied := make(map[string][]byte)
+	for key, val := range db.db {
+		copied[key] = common.CopyBytes(val)
+	}
+	return &snapshot{db: copied}
+}
+
+// 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) {
+	snap.lock.RLock()
+	defer snap.lock.RUnlock()
+
+	if snap.db == nil {
+		return false, errSnapshotReleased
+	}
+	_, ok := snap.db[string(key)]
+	return ok, 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) {
+	snap.lock.RLock()
+	defer snap.lock.RUnlock()
+
+	if snap.db == nil {
+		return nil, errSnapshotReleased
+	}
+	if entry, ok := snap.db[string(key)]; ok {
+		return common.CopyBytes(entry), nil
+	}
+	return nil, errMemorydbNotFound
+}
+
+// Release releases associated resources. Release should always succeed and can
+// be called multiple times without causing error.
+func (snap *snapshot) Release() {
+	snap.lock.Lock()
+	defer snap.lock.Unlock()
+
+	snap.db = nil
+}