IsListType uses reflection and is expensive for hot paths

IsListType was causing ~100 allocations for a non list object. It is
used in a wide range of code, so perform a more targeted check.

The use of `%#v` in a hot return path for `fmt.Errorf()` was the main
victim.

Replace `%#v` with a typed error and create a cache of types that are
lists with a bounded size (probably not necessary, but safer).

```
BenchmarkGet-12          	  100000	    119635 ns/op	   20110 B/op	     206 allocs/op
BenchmarkWatchHTTP-12    	  100000	     65761 ns/op	    7296 B/op	     139 allocs/op

BenchmarkGet-12          	  100000	    109085 ns/op	   17831 B/op	     152 allocs/op
BenchmarkWatchHTTP-12    	  200000	     33966 ns/op	    1913 B/op	      30 allocs/op
```

staging/src/k8s.io/apimachinery/pkg/api/meta/help.go

@@ -17,30 +17,76 @@ limitations under the License.
 package meta
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
+	"sync"
 
 	"k8s.io/apimachinery/pkg/conversion"
 	"k8s.io/apimachinery/pkg/runtime"
 )
 
-// IsListType returns true if the provided Object has a slice called Items
+var (
+	// isListCache maintains a cache of types that are checked for lists
+	// which is used by IsListType.
+	// TODO: remove and replace with an interface check
+	isListCache = struct {
+		lock   sync.RWMutex
+		byType map[reflect.Type]bool
+	}{
+		byType: make(map[reflect.Type]bool, 1024),
+	}
+)
+
+// IsListType returns true if the provided Object has a slice called Items.
+// TODO: Replace the code in this check with an interface comparison by
+//   creating and enforcing that lists implement a list accessor.
 func IsListType(obj runtime.Object) bool {
-	// if we're a runtime.Unstructured, check whether this is a list.
+	switch t := obj.(type) {
-	// TODO: refactor GetItemsPtr to use an interface that returns []runtime.Object
+	case runtime.Unstructured:
-	if unstructured, ok := obj.(runtime.Unstructured); ok {
+		return t.IsList()
-		return unstructured.IsList()
+	}
+	t := reflect.TypeOf(obj)
+
+	isListCache.lock.RLock()
+	ok, exists := isListCache.byType[t]
+	isListCache.lock.RUnlock()
+
+	if !exists {
+		_, err := getItemsPtr(obj)
+		ok = err == nil
+
+		// cache only the first 1024 types
+		isListCache.lock.Lock()
+		if len(isListCache.byType) < 1024 {
+			isListCache.byType[t] = ok
+		}
+		isListCache.lock.Unlock()
 	}
 
-	_, err := GetItemsPtr(obj)
+	return ok
-	return err == nil
 }
 
+var (
+	errExpectFieldItems = errors.New("no Items field in this object")
+	errExpectSliceItems = errors.New("Items field must be a slice of objects")
+)
+
 // GetItemsPtr returns a pointer to the list object's Items member.
 // If 'list' doesn't have an Items member, it's not really a list type
 // and an error will be returned.
 // This function will either return a pointer to a slice, or an error, but not both.
+// TODO: this will be replaced with an interface in the future
 func GetItemsPtr(list runtime.Object) (interface{}, error) {
+	obj, err := getItemsPtr(list)
+	if err != nil {
+		return nil, fmt.Errorf("%T is not a list: %v", err)
+	}
+	return obj, nil
+}
+
+// getItemsPtr returns a pointer to the list object's Items member or an error.
+func getItemsPtr(list runtime.Object) (interface{}, error) {
 	v, err := conversion.EnforcePtr(list)
 	if err != nil {
 		return nil, err
@@ -48,19 +94,19 @@ func GetItemsPtr(list runtime.Object) (interface{}, error) {
 
 	items := v.FieldByName("Items")
 	if !items.IsValid() {
-		return nil, fmt.Errorf("no Items field in %#v", list)
+		return nil, errExpectFieldItems
 	}
 	switch items.Kind() {
 	case reflect.Interface, reflect.Ptr:
 		target := reflect.TypeOf(items.Interface()).Elem()
 		if target.Kind() != reflect.Slice {
-			return nil, fmt.Errorf("items: Expected slice, got %s", target.Kind())
+			return nil, errExpectSliceItems
 		}
 		return items.Interface(), nil
 	case reflect.Slice:
 		return items.Addr().Interface(), nil
 	default:
-		return nil, fmt.Errorf("items: Expected slice, got %s", items.Kind())
+		return nil, errExpectSliceItems
 	}
 }
 

staging/src/k8s.io/apiserver/pkg/endpoints/handlers/rest.go

@@ -292,7 +292,12 @@ func checkName(obj runtime.Object, name, namespace string, namer ScopeNamer) err
 }
 
 // setObjectSelfLink sets the self link of an object as needed.
+// TODO: remove the need for the namer LinkSetters by requiring objects implement either Object or List
+//   interfaces
 func setObjectSelfLink(ctx context.Context, obj runtime.Object, req *http.Request, namer ScopeNamer) error {
+	// We only generate list links on objects that implement ListInterface - historically we duck typed this
+	// check via reflection, but as we move away from reflection we require that you not only carry Items but
+	// ListMeta into order to be identified as a list.
 	if !meta.IsListType(obj) {
 		requestInfo, ok := request.RequestInfoFrom(ctx)
 		if !ok {