Sidecar: API changes

- Add SidecarContaienrs feature gate
- Add ContainerRestartPolicy type
- Add RestartPolicy field to the Container
- Drop RestartPolicy field if the feature is disabled
- Add validation for the SidecarContainers
- Allow restartable init containaers to have a startup probe

pkg/api/pod/util.go

@@ -510,6 +510,14 @@ func dropDisabledFields(
 			podSpec.EphemeralContainers[i].ResizePolicy = nil
 		}
 	}
+
+	if !utilfeature.DefaultFeatureGate.Enabled(features.SidecarContainers) && !restartableInitContainersInUse(oldPodSpec) {
+		// Drop the RestartPolicy field of init containers.
+		for i := range podSpec.InitContainers {
+			podSpec.InitContainers[i].RestartPolicy = nil
+		}
+		// For other types of containers, validateContainers will handle them.
+	}
 }
 
 // dropDisabledPodStatusFields removes disabled fields from the pod status
@@ -778,6 +786,23 @@ func schedulingGatesInUse(podSpec *api.PodSpec) bool {
 	return len(podSpec.SchedulingGates) != 0
 }
 
+// restartableInitContainersInUse returns true if the pod spec is non-nil and
+// it has any init container with ContainerRestartPolicyAlways.
+func restartableInitContainersInUse(podSpec *api.PodSpec) bool {
+	if podSpec == nil {
+		return false
+	}
+	var inUse bool
+	VisitContainers(podSpec, InitContainers, func(c *api.Container, containerType ContainerType) bool {
+		if c.RestartPolicy != nil && *c.RestartPolicy == api.ContainerRestartPolicyAlways {
+			inUse = true
+			return false
+		}
+		return true
+	})
+	return inUse
+}
+
 func hasInvalidLabelValueInAffinitySelector(spec *api.PodSpec) bool {
 	if spec.Affinity != nil {
 		if spec.Affinity.PodAffinity != nil {

pkg/api/pod/util_test.go

@@ -2052,6 +2052,109 @@ func TestDropInPlacePodVerticalScaling(t *testing.T) {
 	}
 }
 
+func TestDropSidecarContainers(t *testing.T) {
+	containerRestartPolicyAlways := api.ContainerRestartPolicyAlways
+
+	podWithSidecarContainers := func() *api.Pod {
+		return &api.Pod{
+			Spec: api.PodSpec{
+				InitContainers: []api.Container{
+					{
+						Name:          "c1",
+						Image:         "image",
+						RestartPolicy: &containerRestartPolicyAlways,
+					},
+				},
+			},
+		}
+	}
+
+	podWithoutSidecarContainers := func() *api.Pod {
+		return &api.Pod{
+			Spec: api.PodSpec{
+				InitContainers: []api.Container{
+					{
+						Name:  "c1",
+						Image: "image",
+					},
+				},
+			},
+		}
+	}
+
+	podInfo := []struct {
+		description         string
+		hasSidecarContainer bool
+		pod                 func() *api.Pod
+	}{
+		{
+			description:         "has a sidecar container",
+			hasSidecarContainer: true,
+			pod:                 podWithSidecarContainers,
+		},
+		{
+			description:         "does not have a sidecar container",
+			hasSidecarContainer: false,
+			pod:                 podWithoutSidecarContainers,
+		},
+		{
+			description:         "is nil",
+			hasSidecarContainer: false,
+			pod:                 func() *api.Pod { return nil },
+		},
+	}
+
+	for _, enabled := range []bool{true, false} {
+		for _, oldPodInfo := range podInfo {
+			for _, newPodInfo := range podInfo {
+				oldPodHasSidecarContainer, oldPod := oldPodInfo.hasSidecarContainer, oldPodInfo.pod()
+				newPodHasSidecarContainer, newPod := newPodInfo.hasSidecarContainer, newPodInfo.pod()
+				if newPod == nil {
+					continue
+				}
+
+				t.Run(fmt.Sprintf("feature enabled=%v, old pod %v, new pod %v", enabled, oldPodInfo.description, newPodInfo.description), func(t *testing.T) {
+					defer featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.SidecarContainers, enabled)()
+
+					var oldPodSpec *api.PodSpec
+					if oldPod != nil {
+						oldPodSpec = &oldPod.Spec
+					}
+					dropDisabledFields(&newPod.Spec, nil, oldPodSpec, nil)
+
+					// old pod should never be changed
+					if !reflect.DeepEqual(oldPod, oldPodInfo.pod()) {
+						t.Errorf("old pod changed: %v", cmp.Diff(oldPod, oldPodInfo.pod()))
+					}
+
+					switch {
+					case enabled || oldPodHasSidecarContainer:
+						// new pod shouldn't change if feature enabled or if old pod has
+						// any sidecar container
+						if !reflect.DeepEqual(newPod, newPodInfo.pod()) {
+							t.Errorf("new pod changed: %v", cmp.Diff(newPod, newPodInfo.pod()))
+						}
+					case newPodHasSidecarContainer:
+						// new pod should be changed
+						if reflect.DeepEqual(newPod, newPodInfo.pod()) {
+							t.Errorf("new pod was not changed")
+						}
+						// new pod should not have any sidecar container
+						if !reflect.DeepEqual(newPod, podWithoutSidecarContainers()) {
+							t.Errorf("new pod has a sidecar container: %v", cmp.Diff(newPod, podWithoutSidecarContainers()))
+						}
+					default:
+						// new pod should not need to be changed
+						if !reflect.DeepEqual(newPod, newPodInfo.pod()) {
+							t.Errorf("new pod changed: %v", cmp.Diff(newPod, newPodInfo.pod()))
+						}
+					}
+				})
+			}
+		}
+	}
+}
+
 func TestMarkPodProposedForResize(t *testing.T) {
 	testCases := []struct {
 		desc        string