migrate: migration callbacks

Callback enables interrupting the migration process and executing code
while migrating.

migrate/callback.go

@@ -0,0 +1,28 @@
+// Copyright (C) 2017 ScyllaDB
+// Use of this source code is governed by a ALv2-style
+// license that can be found in the LICENSE file.
+
+package migrate
+
+import (
+	"context"
+
+	"github.com/gocql/gocql"
+)
+
+// CallbackEvent specifies type of the event when calling CallbackFunc.
+type CallbackEvent uint8
+
+// enumeration of CallbackEvents
+const (
+	BeforeMigration CallbackEvent = iota
+	AfterMigration
+)
+
+// CallbackFunc enables interrupting the migration process and executing code
+// while migrating. If error is returned the migration is aborted.
+type CallbackFunc func(ctx context.Context, session *gocql.Session, ev CallbackEvent, name string) error
+
+// Callback is called before and after each migration.
+// See CallbackFunc for details.
+var Callback CallbackFunc