add checkpoint file

Author: meiji163

go/logic/checkpoint.go

@@ -0,0 +1,17 @@
+package logic
+
+import (
+	"github.com/github/gh-ost/go/mysql"
+	"github.com/github/gh-ost/go/sql"
+	"time"
+)
+
+// Checkpoint holds state necessary to resume a migration.
+type Checkpoint struct {
+	Id                int64
+	Timestamp         time.Time
+	LastTrxCoords     mysql.BinlogCoordinates
+	IterationRangeMin *sql.ColumnValues
+	IterationRangeMax *sql.ColumnValues
+	Iteration         int64
+}

go/logic/migrator.go

@@ -1394,6 +1394,7 @@ func (this *Migrator) onApplyEventStruct(eventStruct *applyEventStruct) error {
 }
 
 func (this *Migrator) Checkpoint(ctx context.Context) (*Checkpoint, error) {
+	// TODO: doesn't work if no DML events come in
 	coords := this.eventsStreamer.GetCurrentBinlogCoordinates()
 	this.applier.LastIterationRangeMutex.Lock()
 	if this.applier.LastIterationRangeMaxValues == nil || this.applier.LastIterationRangeMinValues == nil {