Refactor scheduler preempt interface

- replace error with NodeToStatusMap in Preempt() signature
- eliminate podPreemptor interface and expose its functions statelessly
- move logic in scheduler.go#preempt to generic_scheduler.go#Preempt()

pkg/scheduler/util/utils_test.go

@@ -23,6 +23,9 @@ import (
 
 	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	clientsetfake "k8s.io/client-go/kubernetes/fake"
+	clienttesting "k8s.io/client-go/testing"
 	extenderv1 "k8s.io/kube-scheduler/extender/v1"
 )
 
@@ -114,3 +117,57 @@ func TestMoreImportantPod(t *testing.T) {
 		}
 	}
 }
+
+func TestRemoveNominatedNodeName(t *testing.T) {
+	tests := []struct {
+		name                     string
+		currentNominatedNodeName string
+		newNominatedNodeName     string
+		expectedPatchRequests    int
+		expectedPatchData        string
+	}{
+		{
+			name:                     "Should make patch request to clear node name",
+			currentNominatedNodeName: "node1",
+			expectedPatchRequests:    1,
+			expectedPatchData:        `{"status":{"nominatedNodeName":null}}`,
+		},
+		{
+			name:                     "Should not make patch request if nominated node is already cleared",
+			currentNominatedNodeName: "",
+			expectedPatchRequests:    0,
+		},
+	}
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			actualPatchRequests := 0
+			var actualPatchData string
+			cs := &clientsetfake.Clientset{}
+			cs.AddReactor("patch", "pods", func(action clienttesting.Action) (bool, runtime.Object, error) {
+				actualPatchRequests++
+				patch := action.(clienttesting.PatchAction)
+				actualPatchData = string(patch.GetPatch())
+				// For this test, we don't care about the result of the patched pod, just that we got the expected
+				// patch request, so just returning &v1.Pod{} here is OK because scheduler doesn't use the response.
+				return true, &v1.Pod{}, nil
+			})
+
+			pod := &v1.Pod{
+				ObjectMeta: metav1.ObjectMeta{Name: "foo"},
+				Status:     v1.PodStatus{NominatedNodeName: test.currentNominatedNodeName},
+			}
+
+			if err := ClearNominatedNodeName(cs, pod); err != nil {
+				t.Fatalf("Error calling removeNominatedNodeName: %v", err)
+			}
+
+			if actualPatchRequests != test.expectedPatchRequests {
+				t.Fatalf("Actual patch requests (%d) dos not equal expected patch requests (%d)", actualPatchRequests, test.expectedPatchRequests)
+			}
+
+			if test.expectedPatchRequests > 0 && actualPatchData != test.expectedPatchData {
+				t.Fatalf("Patch data mismatch: Actual was %v, but expected %v", actualPatchData, test.expectedPatchData)
+			}
+		})
+	}
+}