scaleutils: fix least busy selector on clusters older than v1.0.0 (#508)

This adds additional API calls when performing the least busy node
selector as a best effort to work on cluster that running Nomad
versions before v1.0.0.

v1.0.0 including API functionality which allows node resources to
be include in the stub list node object which is used when
performing least busy calculations. In version prior to this, the
information is only available within the node info object.

sdk/helper/scaleutils/nodeselector/least_busy.go

@@ -48,6 +48,30 @@ func (l *leastBusyClusterScaleInNodeSelector) Select(nodes []*api.NodeListStub,
 
 	for i, node := range nodes {
 
+		// The NodeResources object is only available within the Nomad
+		// api.NodeListStub from v1.0.0 onwards. Therefore in clusters running
+		// clients on lower versions, we need additional API calls to discover
+		// this information.
+		if node.NodeResources == nil {
+
+			nodeInfo, _, err := l.client.Nodes().Info(node.ID, nil)
+			if err != nil {
+				l.log.Error("failed to call node info for resource detail", "error", err)
+				return nil
+			}
+			if nodeInfo.NodeResources == nil {
+				l.log.Error("node object does not contain resource info", "node_id", node.ID)
+				return nil
+			}
+
+			// Update the node to include the resource object.
+			node.NodeResources = nodeInfo.NodeResources
+
+			if node.ReservedResources == nil && nodeInfo.ReservedResources != nil {
+				node.ReservedResources = nodeInfo.ReservedResources
+			}
+		}
+
 		// In the event of an API error, we are unable to ensure the selected
 		// nodes are the least busy. Therefore we must exit. In the future we
 		// could consider adding a threshold, configurable by the operator to