Merge pull request #45294 from liggitt/proto-slices

Automatic merge from submit-queue

Remove null -> [] slice hack

Closes #44593

When 1.6 added protobuf storage, the storage layer lost the ability to persist slice fields with empty but non-null values.

As a workaround, we tried to convert empty slice fields to `[]`, rather than `null`. Compressing `null` -> `[]` was just as much of an API breakage as `[]` -> `null`, but was hoped to cause fewer problems in clients that don't do null checks.

Because of conversion optimizations around converting lists of objects, the `null` -> `[]` hack was discovered to only apply to individual get requests, not to a list of objects. 1.6 and 1.7 was released with this behavior, and the world didn't explode. 1.7 documented the breaking API change that `null` and `[]` should be considered equivalent, unless otherwise noted on a particular field.

This PR:

* Reverts the earlier attempt (https://github.com/kubernetes/kubernetes/pull/43422) at ensuring non-null json slice output in conversion
* Makes results of `get` consistent with the results of `list` (which helps naive clients that do deepequal comparisons of objects obtained via list/watch and get), and allows empty slice fields to be returned as `null`

```release-note
Protobuf serialization does not distinguish between `[]` and `null`.
API fields previously capable of storing and returning either `[]` and `null` via JSON API requests (for example, the Endpoints `subsets` field) can now store only `null` when created using the protobuf content-type or stored in etcd using protobuf serialization (the default in 1.6+). JSON API clients should tolerate `null` values for such fields, and treat `null` and `[]` as equivalent in meaning unless specifically documented otherwise for a particular field.
```

plugin/pkg/admission/resourcequota/apis/resourcequota/v1alpha1/zz_generated.conversion.go

@@ -53,11 +53,7 @@ func Convert_v1alpha1_Configuration_To_resourcequota_Configuration(in *Configura
 }
 
 func autoConvert_resourcequota_Configuration_To_v1alpha1_Configuration(in *resourcequota.Configuration, out *Configuration, s conversion.Scope) error {
-	if in.LimitedResources == nil {
-		out.LimitedResources = make([]LimitedResource, 0)
-	} else {
-		out.LimitedResources = *(*[]LimitedResource)(unsafe.Pointer(&in.LimitedResources))
-	}
+	out.LimitedResources = *(*[]LimitedResource)(unsafe.Pointer(&in.LimitedResources))
 	return nil
 }
 

plugin/pkg/auth/authorizer/rbac/bootstrappolicy/testdata/cluster-role-bindings.yaml

@@ -121,7 +121,7 @@ items:
     apiGroup: rbac.authorization.k8s.io
     kind: ClusterRole
     name: system:node
-  subjects: []
+  subjects: null
 - apiVersion: rbac.authorization.k8s.io/v1
   kind: ClusterRoleBinding
   metadata: