apiserver/storage/cacher: cache supports pagination

Author: p0lyn0mial

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher.go

@@ -762,12 +762,26 @@ func shouldDelegateList(opts storage.ListOptions) bool {
 	consistentReadFromStorage := resourceVersion == "" && !(consistentListFromCacheEnabled && requestWatchProgressSupported)
 	// Watch cache doesn't support continuations, so serve them from etcd.
 	hasContinuation := len(pred.Continue) > 0
-	// Serve paginated requests about revision "0" from watch cache to avoid overwhelming etcd.
-	hasLimit := pred.Limit > 0 && resourceVersion != "0"
 	// Watch cache only supports ResourceVersionMatchNotOlderThan (default).
-	unsupportedMatch := match != "" && match != metav1.ResourceVersionMatchNotOlderThan
+	// see https://kubernetes.io/docs/reference/using-api/api-concepts/#semantics-for-get-and-list
+	isLegacyExactMatch := opts.Predicate.Limit > 0 && match == "" && len(resourceVersion) > 0 && resourceVersion != "0"
+	unsupportedMatch := match != "" && match != metav1.ResourceVersionMatchNotOlderThan || isLegacyExactMatch
 
-	return consistentReadFromStorage || hasContinuation || hasLimit || unsupportedMatch
+	return consistentReadFromStorage || hasContinuation || unsupportedMatch
+}
+
+// computeListLimit determines whether the cacher should
+// apply a limit to an incoming LIST request and returns its value.
+//
+// note that this function doesn't check RVM nor the Continuation token.
+// these parameters are validated by the shouldDelegateList function.
+//
+// as of today, the limit is ignored for requests that set RV == 0
+func computeListLimit(opts storage.ListOptions) int64 {
+	if opts.Predicate.Limit <= 0 || opts.ResourceVersion == "0" {
+		return 0
+	}
+	return opts.Predicate.Limit
 }
 
 func shouldDelegateListOnNotReadyCache(opts storage.ListOptions) bool {
@@ -877,13 +891,21 @@ func (c *Cacher) GetList(ctx context.Context, key string, opts storage.ListOptio
 	//   the elements in ListObject are Struct type, making slice will bring excessive memory consumption.
 	//   so we try to delay this action as much as possible
 	var selectedObjects []runtime.Object
-	for _, obj := range objs {
+	var lastSelectedObjectKey string
+	var hasMoreListItems bool
+	limit := computeListLimit(opts)
+	for i, obj := range objs {
 		elem, ok := obj.(*storeElement)
 		if !ok {
 			return fmt.Errorf("non *storeElement returned from storage: %v", obj)
 		}
 		if filter(elem.Key, elem.Labels, elem.Fields) {
 			selectedObjects = append(selectedObjects, elem.Object)
+			lastSelectedObjectKey = elem.Key
+		}
+		if limit > 0 && int64(len(selectedObjects)) >= limit {
+			hasMoreListItems = i < len(objs)-1
+			break
 		}
 	}
 	if len(selectedObjects) == 0 {
@@ -899,7 +921,12 @@ func (c *Cacher) GetList(ctx context.Context, key string, opts storage.ListOptio
 	}
 	span.AddEvent("Filtered items", attribute.Int("count", listVal.Len()))
 	if c.versioner != nil {
-		if err := c.versioner.UpdateList(listObj, readResourceVersion, "", nil); err != nil {
+		continueValue, remainingItemCount, err := storage.PrepareContinueToken(lastSelectedObjectKey, key, int64(readResourceVersion), int64(len(objs)), hasMoreListItems, opts)
+		if err != nil {
+			return err
+		}
+
+		if err = c.versioner.UpdateList(listObj, readResourceVersion, continueValue, remainingItemCount); err != nil {
 			return err
 		}
 	}

staging/src/k8s.io/apiserver/pkg/storage/cacher/cacher_whitebox_test.go

@@ -201,7 +201,6 @@ func TestGetListCacheBypass(t *testing.T) {
 		{opts: storage.ListOptions{ResourceVersion: "0", Predicate: storage.SelectionPredicate{Continue: "a"}}, expectBypass: true},
 		{opts: storage.ListOptions{ResourceVersion: "1", Predicate: storage.SelectionPredicate{Continue: "a"}}, expectBypass: true},
 
-		{opts: storage.ListOptions{ResourceVersion: "", Predicate: storage.SelectionPredicate{Limit: 500}}, expectBypass: true},
 		{opts: storage.ListOptions{ResourceVersion: "0", Predicate: storage.SelectionPredicate{Limit: 500}}, expectBypass: false},
 		{opts: storage.ListOptions{ResourceVersion: "1", Predicate: storage.SelectionPredicate{Limit: 500}}, expectBypass: true},
 
@@ -214,6 +213,7 @@ func TestGetListCacheBypass(t *testing.T) {
 		featuregatetesting.SetFeatureGateDuringTest(t, utilfeature.DefaultFeatureGate, features.ConsistentListFromCache, false)
 		testCases := append(commonTestCases,
 			testCase{opts: storage.ListOptions{ResourceVersion: ""}, expectBypass: true},
+			testCase{opts: storage.ListOptions{ResourceVersion: "", Predicate: storage.SelectionPredicate{Limit: 500}}, expectBypass: true},
 		)
 		for _, tc := range testCases {
 			testGetListCacheBypass(t, tc.opts, tc.expectBypass)
@@ -233,6 +233,7 @@ func TestGetListCacheBypass(t *testing.T) {
 
 		testCases := append(commonTestCases,
 			testCase{opts: storage.ListOptions{ResourceVersion: ""}, expectBypass: false},
+			testCase{opts: storage.ListOptions{ResourceVersion: "", Predicate: storage.SelectionPredicate{Limit: 500}}, expectBypass: false},
 		)
 		for _, tc := range testCases {
 			testGetListCacheBypass(t, tc.opts, tc.expectBypass)
@@ -2566,6 +2567,63 @@ func TestWatchStreamSeparation(t *testing.T) {
 	}
 }
 
+func TestComputeListLimit(t *testing.T) {
+	scenarios := []struct {
+		name          string
+		opts          storage.ListOptions
+		expectedLimit int64
+	}{
+		{
+			name: "limit is zero",
+			opts: storage.ListOptions{
+				Predicate: storage.SelectionPredicate{
+					Limit: 0,
+				},
+			},
+			expectedLimit: 0,
+		},
+		{
+			name: "limit is positive, RV is unset",
+			opts: storage.ListOptions{
+				Predicate: storage.SelectionPredicate{
+					Limit: 1,
+				},
+				ResourceVersion: "",
+			},
+			expectedLimit: 1,
+		},
+		{
+			name: "limit is positive, RV = 100",
+			opts: storage.ListOptions{
+				Predicate: storage.SelectionPredicate{
+					Limit: 1,
+				},
+				ResourceVersion: "100",
+			},
+			expectedLimit: 1,
+		},
+		{
+			name: "legacy case: limit is positive, RV = 0",
+			opts: storage.ListOptions{
+				Predicate: storage.SelectionPredicate{
+					Limit: 1,
+				},
+				ResourceVersion: "0",
+			},
+			expectedLimit: 0,
+		},
+	}
+
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			actualLimit := computeListLimit(scenario.opts)
+			if actualLimit != scenario.expectedLimit {
+				t.Errorf("computeListLimit returned = %v, expected %v", actualLimit, scenario.expectedLimit)
+			}
+		})
+	}
+}
+
 func watchAndWaitForBookmark(t *testing.T, ctx context.Context, etcdStorage storage.Interface) func() (resourceVersion uint64) {
 	opts := storage.ListOptions{ResourceVersion: "", Predicate: storage.Everything, Recursive: true}
 	opts.Predicate.AllowWatchBookmarks = true