ns: Only time sync on absolute time

pkg/gatewayserver/io/io.go

@@ -655,6 +655,11 @@ func (c *Connection) ScheduleDown(path *ttnpb.DownlinkPath, msg *ttnpb.DownlinkM
 			settings.Time = request.AbsoluteTime
 		case ttnpb.Class_CLASS_C:
 			if request.AbsoluteTime != nil {
+				if !c.scheduler.IsGatewayTimeSynced() {
+					rxErrs = append(rxErrs, errNoGPSSync.New())
+					continue
+				}
+
 				f = c.scheduler.ScheduleAt
 				settings.Time = request.AbsoluteTime
 			} else {

pkg/gatewayserver/scheduling/clock.go

@@ -38,16 +38,19 @@
 
 // RolloverClock is a Clock that takes roll-over of a uint32 microsecond concentrator time into account.
 type RolloverClock struct {
-	synced   bool
-	relative uint32
-	absolute ConcentratorTime
-	server   *time.Time
-	gateway  *time.Time
+	synced         bool
+	relative       uint32
+	absolute       ConcentratorTime
+	server         *time.Time
+	gateway        *time.Time
+	absoluteSynced bool
 }
 
 // IsSynced implements Clock.
 func (c *RolloverClock) IsSynced() bool { return c.synced }
 
+func (c *RolloverClock) IsAbsoluteSynced() bool { return c.absoluteSynced }
+
 // SyncTime implements Clock.
 func (c *RolloverClock) SyncTime() (time.Time, bool) {
 	if !c.synced {
@@ -82,6 +85,7 @@
 func (c *RolloverClock) SyncWithGatewayAbsolute(timestamp uint32, server, gateway time.Time) ConcentratorTime {
 	ct := c.Sync(timestamp, server)
 	c.gateway = &gateway
+	c.absoluteSynced = true
 	return ct
 }
 

pkg/gatewayserver/scheduling/scheduler.go

@@ -296,7 +296,7 @@ func (s *Scheduler) ScheduleAt(ctx context.Context, opts Options) (res Emission,
 	if opts.UplinkToken != nil {
 		s.syncWithUplinkToken(opts.UplinkToken)
 	}
-	if !s.clock.IsSynced() {
+	if !s.clock.IsAbsoluteSynced() {
 		return Emission{}, 0, errNoClockSync.New()
 	}
 	minScheduleTime := ScheduleTimeShort
@@ -477,7 +477,7 @@ func (s *Scheduler) SyncWithGatewayConcentrator(timestamp uint32, server time.Ti
 func (s *Scheduler) IsGatewayTimeSynced() bool {
 	s.mu.RLock()
 	defer s.mu.RUnlock()
-	return s.clock.IsSynced() && s.clock.gateway != nil
+	return s.clock.IsAbsoluteSynced() && s.clock.gateway != nil
 }
 
 // Now returns an indication of the current concentrator time.