Migration only starting after first replication lag metric collected

go/logic/migrator.go

@@ -940,7 +940,9 @@ func (this *Migrator) initiateThrottler() error {
 
 	go this.throttler.initiateThrottlerCollection(this.firstThrottlingCollected)
 	log.Infof("Waiting for first throttle metrics to be collected")
-	<-this.firstThrottlingCollected
+	<-this.firstThrottlingCollected // replication lag
+	<-this.firstThrottlingCollected // other metrics
+	log.Infof("First throttle metrics collected")
 	go this.throttler.initiateThrottlerChecks()
 
 	return nil

go/logic/throttler.go

@@ -85,10 +85,8 @@ func (this *Throttler) parseChangelogHeartbeat(heartbeatValue string) (err error
 }
 
 // collectReplicationLag reads the latest changelog heartbeat value
-func (this *Throttler) collectReplicationLag() {
-	ticker := time.Tick(time.Duration(this.migrationContext.HeartbeatIntervalMilliseconds) * time.Millisecond)
-	for range ticker {
-		go func() error {
+func (this *Throttler) collectReplicationLag(firstThrottlingCollected chan<- bool) {
+	collectFunc := func() error {
 		if atomic.LoadInt64(&this.migrationContext.CleanupImminentFlag) > 0 {
 			return nil
 		}
@@ -110,7 +108,14 @@ func (this *Throttler) collectReplicationLag() {
 			}
 		}
 		return nil
-		}()
+	}
+
+	collectFunc()
+	firstThrottlingCollected <- true
+
+	ticker := time.Tick(time.Duration(this.migrationContext.HeartbeatIntervalMilliseconds) * time.Millisecond)
+	for range ticker {
+		go collectFunc()
 	}
 }
 
@@ -285,13 +290,14 @@ func (this *Throttler) collectGeneralThrottleMetrics() error {
 // that may affect throttling. There are several components, all running independently,
 // that collect such metrics.
 func (this *Throttler) initiateThrottlerCollection(firstThrottlingCollected chan<- bool) {
-	go this.collectReplicationLag()
+	go this.collectReplicationLag(firstThrottlingCollected)
 	go this.collectControlReplicasLag()
 
 	go func() {
-		throttlerMetricsTick := time.Tick(1 * time.Second)
 		this.collectGeneralThrottleMetrics()
 		firstThrottlingCollected <- true
+
+		throttlerMetricsTick := time.Tick(1 * time.Second)
 		for range throttlerMetricsTick {
 			this.collectGeneralThrottleMetrics()
 		}

go/mysql/utils.go

@@ -32,9 +32,11 @@ func GetReplicationLag(connectionConfig *ConnectionConfig) (replicationLag time.
 	}
 
 	err = sqlutils.QueryRowsMap(db, `show slave status`, func(m sqlutils.RowMap) error {
+		slaveIORunning := m.GetString("Slave_IO_Running")
+		slaveSQLRunning := m.GetString("Slave_SQL_Running")
 		secondsBehindMaster := m.GetNullInt64("Seconds_Behind_Master")
 		if !secondsBehindMaster.Valid {
-			return fmt.Errorf("replication not running")
+			return fmt.Errorf("replication not running; Slave_IO_Running=%+v, Slave_SQL_Running=", slaveIORunning, slaveSQLRunning)
 		}
 		replicationLag = time.Duration(secondsBehindMaster.Int64) * time.Second
 		return nil