fixing phantom throttle-control-replicas lag result

Author: Shlomi Noach

go/base/context.go

@@ -559,7 +559,11 @@ func (this *MigrationContext) GetControlReplicasLagResult() mysql.ReplicationLag
 func (this *MigrationContext) SetControlReplicasLagResult(lagResult *mysql.ReplicationLagResult) {
 	this.throttleMutex.Lock()
 	defer this.throttleMutex.Unlock()
-	this.controlReplicasLagResult = *lagResult
+	if lagResult == nil {
+		this.controlReplicasLagResult = *mysql.NewNoReplicationLagResult()
+	} else {
+		this.controlReplicasLagResult = *lagResult
+	}
 }
 
 func (this *MigrationContext) GetThrottleControlReplicaKeys() *mysql.InstanceKeyMap {

go/logic/throttler.go

@@ -158,9 +158,7 @@ func (this *Throttler) collectControlReplicasLag() {
 			// No need to read lag
 			return
 		}
-		if result := readControlReplicasLag(); result != nil {
-			this.migrationContext.SetControlReplicasLagResult(result)
-		}
+		this.migrationContext.SetControlReplicasLagResult(readControlReplicasLag())
 	}
 	aggressiveTicker := time.Tick(100 * time.Millisecond)
 	relaxedFactor := 10

go/mysql/utils.go

@@ -22,6 +22,14 @@ type ReplicationLagResult struct {
 	Err error
 }
 
+func NewNoReplicationLagResult() *ReplicationLagResult {
+	return &ReplicationLagResult{Lag: 0, Err: nil}
+}
+
+func (this *ReplicationLagResult) HasLag() bool {
+	return this.Lag > 0
+}
+
 // GetReplicationLag returns replication lag for a given connection config; either by explicit query
 // or via SHOW SLAVE STATUS
 func GetReplicationLag(connectionConfig *ConnectionConfig) (replicationLag time.Duration, err error) {