Skip recalculating the rate in MaxReplicationLagModule when it can't be done

go/vt/throttler/max_replication_lag_module.go

@@ -302,6 +302,12 @@ func (m *MaxReplicationLagModule) recalculateRate(lagRecordNow replicationLagRec
 	if lagRecordNow.isZero() {
 		panic("rate recalculation was triggered with a zero replication lag record")
 	}
+
+	// Protect against nil stats
+	if lagRecordNow.Stats == nil {
+		return
+	}
+
 	now := lagRecordNow.time
 	lagNow := lagRecordNow.lag()
 

go/vt/throttler/max_replication_lag_module_test.go

@@ -22,6 +22,8 @@ import (
 	"testing"
 	"time"
 
+	"github.com/stretchr/testify/assert"
+
 	"vitess.io/vitess/go/vt/log"
 
 	"vitess.io/vitess/go/vt/discovery"
@@ -83,6 +85,12 @@ func (tf *testFixture) process(lagRecord replicationLagRecord) {
 	tf.m.processRecord(lagRecord)
 }
 
+// recalculateRate does the same thing as MaxReplicationLagModule.recalculateRate() does
+// for a new "lagRecord".
+func (tf *testFixture) recalculateRate(lagRecord replicationLagRecord) {
+	tf.m.recalculateRate(lagRecord)
+}
+
 func (tf *testFixture) checkState(state state, rate int64, lastRateChange time.Time) error {
 	if got, want := tf.m.currentState, state; got != want {
 		return fmt.Errorf("module in wrong state. got = %v, want = %v", got, want)
@@ -96,6 +104,47 @@ func (tf *testFixture) checkState(state state, rate int64, lastRateChange time.T
 	return nil
 }
 
+func TestNewMaxReplicationLagModule_recalculateRate(t *testing.T) {
+	testCases := []struct {
+		name        string
+		lagRecord   replicationLagRecord
+		expectPanic bool
+	}{
+		{
+			name: "Zero lag",
+			lagRecord: replicationLagRecord{
+				time:         time.Time{},
+				TabletHealth: discovery.TabletHealth{Stats: nil},
+			},
+			expectPanic: true,
+		},
+		{
+			name: "nil lag record stats",
+			lagRecord: replicationLagRecord{
+				time:         time.Now(),
+				TabletHealth: discovery.TabletHealth{Stats: nil},
+			},
+			expectPanic: false,
+		},
+	}
+
+	for _, aTestCase := range testCases {
+		theCase := aTestCase
+
+		t.Run(theCase.name, func(t *testing.T) {
+			t.Parallel()
+
+			fixture, err := newTestFixtureWithMaxReplicationLag(5)
+			assert.NoError(t, err)
+
+			if theCase.expectPanic {
+				assert.Panics(t, func() { fixture.recalculateRate(theCase.lagRecord) })
+			}
+		},
+		)
+	}
+}
+
 func TestMaxReplicationLagModule_RateNotZeroWhenDisabled(t *testing.T) {
 	tf, err := newTestFixtureWithMaxReplicationLag(ReplicationLagModuleDisabled)
 	if err != nil {