backedn: protect backend access with lock

storage/backend/backend.go

@@ -201,6 +201,18 @@ func (b *backend) Commits() int64 {
 }
 
 func (b *backend) Defrag() error {
+	err := b.defrag()
+	if err != nil {
+		return err
+	}
+
+	// commit to update metadata like db.size
+	b.batchTx.Commit()
+
+	return nil
+}
+
+func (b *backend) defrag() error {
 	// TODO: make this non-blocking?
 	// lock batchTx to ensure nobody is using previous tx, and then
 	// close previous ongoing tx.
@@ -251,8 +263,6 @@ func (b *backend) Defrag() error {
 	if err != nil {
 		log.Fatalf("backend: cannot begin tx (%s)", err)
 	}
-	// commit to update metadata like db.size
-	b.batchTx.commit(false)
 
 	return nil
 }

storage/backend/batch_tx.go

@@ -160,6 +160,8 @@ func (t *batchTx) commit(stop bool) {
 	// commit the last tx
 	if t.tx != nil {
 		if t.pending == 0 && !stop {
+			t.backend.mu.RLock()
+			defer t.backend.mu.RUnlock()
 			atomic.StoreInt64(&t.backend.size, t.tx.Size())
 			return
 		}