server: Implement schema migrations

server/storage/schema/changes.go

@@ -0,0 +1,50 @@
+// Copyright 2021 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package schema
+
+import "go.etcd.io/etcd/server/v3/storage/backend"
+
+type schemaChange interface {
+	upgradeAction() action
+	downgradeAction() action
+}
+
+// addNewField represents adding new field when upgrading. Downgrade will remove the field.
+func addNewField(bucket backend.Bucket, fieldName []byte, fieldValue []byte) schemaChange {
+	return simpleSchemaChange{
+		upgrade: setKeyAction{
+			Bucket:     bucket,
+			FieldName:  fieldName,
+			FieldValue: fieldValue,
+		},
+		downgrade: deleteKeyAction{
+			Bucket:    bucket,
+			FieldName: fieldName,
+		},
+	}
+}
+
+type simpleSchemaChange struct {
+	upgrade   action
+	downgrade action
+}
+
+func (c simpleSchemaChange) upgradeAction() action {
+	return c.upgrade
+}
+
+func (c simpleSchemaChange) downgradeAction() action {
+	return c.downgrade
+}