discovery: transit all inactive syncers when needed

discovery/sync_manager.go

@@ -398,6 +398,13 @@ func (m *SyncManager) syncerHandler() {
 				continue
 			}
 
+			// We may not even have enough inactive syncers to be
+			// transitted. In that case, we will transit all the
+			// inactive syncers.
+			if len(m.inactiveSyncers) < numActiveLeft {
+				numActiveLeft = len(m.inactiveSyncers)
+			}
+
 			log.Debugf("Attempting to transition %v passive "+
 				"GossipSyncers to active", numActiveLeft)