CA: don't error out in HintingSimulator if a hinted Node is gone

If a hinted Node is no longer in the cluster snapshot (e.g. it was
a fake upcoming Node and the real one appeared).

This was introduced in the recent PredicateChecker->PredicateSnapshot
refactor. Previously, PredicateChecker.CheckPredicates() would return
an error if the hinted Node was gone, and HintingSimulator treated
this error the same as failing predicates - it would move on to the
non-hinting logic. After the refactor, HintingSimulator explicitly
errors out if it can't retrieve the hinted Node from the snapshot,
so the behavior changed.

I checked other CheckPredicates()/SchedulePod() callsites, and this is
the only one when ignoring the missing Node makes sense. For the others,
the Node is added to the snapshot just before the call, so it being
missing should cause an error.

cluster-autoscaler/simulator/scheduling/hinting_simulator.go

@@ -91,7 +91,8 @@ func (s *HintingSimulator) tryScheduleUsingHints(clusterSnapshot clustersnapshot
 
 	nodeInfo, err := clusterSnapshot.GetNodeInfo(hintedNode)
 	if err != nil {
-		return "", err
+		// The hinted Node is no longer in the cluster. No need to error out, we can just look for another one.
+		return "", nil
 	}
 	if !isNodeAcceptable(nodeInfo) {
 		return "", nil

cluster-autoscaler/simulator/scheduling/hinting_simulator_test.go

@@ -35,6 +35,7 @@ func TestTrySchedulePods(t *testing.T) {
 		nodes           []*apiv1.Node
 		pods            []*apiv1.Pod
 		newPods         []*apiv1.Pod
+		hints           map[*apiv1.Pod]string
 		acceptableNodes func(*framework.NodeInfo) bool
 		wantStatuses    []Status
 		wantErr         bool
@@ -58,6 +59,27 @@ func TestTrySchedulePods(t *testing.T) {
 				{Pod: BuildTestPod("p3", 500, 500000), NodeName: "n1"},
 			},
 		},
+
+		{
+			desc: "hinted Node no longer in the cluster doesn't cause an error",
+			nodes: []*apiv1.Node{
+				buildReadyNode("n1", 1000, 2000000),
+				buildReadyNode("n2", 1000, 2000000),
+			},
+			pods: []*apiv1.Pod{
+				buildScheduledPod("p1", 300, 500000, "n1"),
+			},
+			newPods: []*apiv1.Pod{
+				BuildTestPod("p2", 800, 500000),
+				BuildTestPod("p3", 500, 500000),
+			},
+			hints:           map[*apiv1.Pod]string{BuildTestPod("p2", 800, 500000): "non-existing-node"},
+			acceptableNodes: ScheduleAnywhere,
+			wantStatuses: []Status{
+				{Pod: BuildTestPod("p2", 800, 500000), NodeName: "n2"},
+				{Pod: BuildTestPod("p3", 500, 500000), NodeName: "n1"},
+			},
+		},
 		{
 			desc: "three new pods, two nodes, no fit",
 			nodes: []*apiv1.Node{
@@ -136,6 +158,11 @@ func TestTrySchedulePods(t *testing.T) {
 			clusterSnapshot := testsnapshot.NewTestSnapshotOrDie(t)
 			clustersnapshot.InitializeClusterSnapshotOrDie(t, clusterSnapshot, tc.nodes, tc.pods)
 			s := NewHintingSimulator()
+
+			for pod, nodeName := range tc.hints {
+				s.hints.Set(HintKeyFromPod(pod), nodeName)
+			}
+
 			statuses, _, err := s.TrySchedulePods(clusterSnapshot, tc.newPods, tc.acceptableNodes, false)
 			if tc.wantErr {
 				assert.Error(t, err)