agent: refactor sync loop to linear flow of control

agent/ae/ae.go

@@ -93,12 +93,10 @@ func (s *StateSyncer) Run() {
 
 Sync:
 	for {
-		// update the sync status
-		err := s.State.UpdateSyncState()
-		if err == nil {
-			break
-		}
+		switch err := s.State.UpdateSyncState(); {
 
+		// update sync status failed
+		case err != nil:
 			s.Logger.Printf("[ERR] agent: failed to sync remote state: %v", err)
 
 			// retry updating sync status after some time or when a consul
@@ -106,7 +104,7 @@ Sync:
 			select {
 
 			// consul server added to cluster.
-		// retry sooner than retryFailIntv to converge cluster quicker
+			// retry sooner than retryFailIntv to converge cluster sooner
 			// but stagger delay to avoid thundering herd
 			case <-s.ServerUpCh:
 				select {
@@ -122,12 +120,13 @@ Sync:
 			case <-s.ShutdownCh:
 				return
 			}
-	}
 
-	// Force-trigger sync to pickup any changes
+		// update sync status OK
+		default:
+			// force-trigger sync to pickup any changes
 			s.triggerSync()
 
-	// Wait for sync events
+			// do partial syncs until it is time for a full sync again
 			for {
 				select {
 				// todo(fs): why don't we honor the ServerUpCh here as well?
@@ -141,7 +140,7 @@ Sync:
 				// 	}
 
 				case <-time.After(s.Interval + stagger(s.Interval)):
-			goto Sync
+					continue Sync
 
 				case <-s.TriggerCh:
 					if s.Paused() {
@@ -156,6 +155,8 @@ Sync:
 				}
 			}
 		}
+	}
+}
 
 // Pause temporarily disables sync runs.
 func (s *StateSyncer) Pause() {