fix data race in gc manager

pkg/txnutil/gc/gc_manager.go

@@ -49,7 +49,7 @@ type gcManager struct {
 
 	lastUpdatedTime   time.Time
 	lastSucceededTime time.Time
-	lastSafePointTs   uint64
+	lastSafePointTs   atomic.Uint64
 	isTiCDCBlockGC    atomic.Bool
 }
 
@@ -101,7 +101,7 @@ func (m *gcManager) TryUpdateGCSafePoint(
 	// means that the service gc safe point set by TiCDC is the min service
 	// gc safe point
 	m.isTiCDCBlockGC.Store(actual == checkpointTs)
-	m.lastSafePointTs = actual
+	m.lastSafePointTs.Store(actual)
 	m.lastSucceededTime = time.Now()
 	minServiceGCSafePointGauge.Set(float64(oracle.ExtractPhysical(actual)))
 	cdcGCSafePointGauge.Set(float64(oracle.ExtractPhysical(checkpointTs)))
@@ -126,11 +126,11 @@ func (m *gcManager) CheckStaleCheckpointTs(
 	} else {
 		// if `isTiCDCBlockGC` is false, it means there is another service gc
 		// point less than the min checkpoint ts.
-		if gcSafepointUpperBound < m.lastSafePointTs {
+		if gcSafepointUpperBound < m.lastSafePointTs.Load() {
 			return cerror.ErrSnapshotLostByGC.
 				GenWithStackByArgs(
 					checkpointTs,
-					m.lastSafePointTs,
+					m.lastSafePointTs.Load(),
 				)
 		}
 	}

pkg/txnutil/gc/gc_manager_test.go

@@ -99,7 +99,7 @@ func TestCheckStaleCheckpointTs(t *testing.T) {
 	err := gcManager.CheckStaleCheckpointTs(ctx, cfID, oracle.GoTimeToTS(time.Now()))
 	require.Nil(t, err)
 
-	gcManager.lastSafePointTs = 20
+	gcManager.lastSafePointTs.Store(20)
 	err = gcManager.CheckStaleCheckpointTs(ctx, cfID, 10)
 	require.True(t, cerror.ErrSnapshotLostByGC.Equal(errors.Cause(err)))
 	require.True(t, cerror.IsChangefeedGCFastFailError(err))