Merge pull request #102171 from wojtek-t/pf_watch_initialization_support

Implement support for watch initialization in P&F

staging/src/k8s.io/apiserver/pkg/server/filters/maxinflight.go

@@ -47,7 +47,10 @@ const (
 	observationMaintenancePeriod = 10 * time.Second
 )
 
-var nonMutatingRequestVerbs = sets.NewString("get", "list", "watch")
+var (
+	nonMutatingRequestVerbs = sets.NewString("get", "list", "watch")
+	watchVerbs              = sets.NewString("watch")
+)
 
 func handleError(w http.ResponseWriter, r *http.Request, err error) {
 	errorMsg := fmt.Sprintf("Internal Server Error: %#v", r.RequestURI)

staging/src/k8s.io/apiserver/pkg/server/filters/priority-and-fairness.go

@@ -17,9 +17,9 @@ limitations under the License.
 package filters
 
 import (
-	"context"
 	"fmt"
 	"net/http"
+	"runtime"
 	"sync/atomic"
 
 	flowcontrol "k8s.io/api/flowcontrol/v1beta1"
@@ -31,10 +31,6 @@ import (
 	"k8s.io/klog/v2"
 )
 
-type priorityAndFairnessKeyType int
-
-const priorityAndFairnessKey priorityAndFairnessKeyType = iota
-
 // PriorityAndFairnessClassification identifies the results of
 // classification for API Priority and Fairness
 type PriorityAndFairnessClassification struct {
@@ -44,12 +40,6 @@ type PriorityAndFairnessClassification struct {
 	PriorityLevelUID  apitypes.UID
 }
 
-// GetClassification returns the classification associated with the
-// given context, if any, otherwise nil
-func GetClassification(ctx context.Context) *PriorityAndFairnessClassification {
-	return ctx.Value(priorityAndFairnessKey).(*PriorityAndFairnessClassification)
-}
-
 // waitingMark tracks requests waiting rather than being executed
 var waitingMark = &requestWatermark{
 	phase:            epmetrics.WaitingPhase,
@@ -60,6 +50,9 @@ var waitingMark = &requestWatermark{
 var atomicMutatingExecuting, atomicReadOnlyExecuting int32
 var atomicMutatingWaiting, atomicReadOnlyWaiting int32
 
+// newInitializationSignal is defined for testing purposes.
+var newInitializationSignal = utilflowcontrol.NewInitializationSignal
+
 // WithPriorityAndFairness limits the number of in-flight
 // requests in a fine-grained way.
 func WithPriorityAndFairness(
@@ -84,8 +77,10 @@ func WithPriorityAndFairness(
 			return
 		}
 
-		// Skip tracking long running requests.
-		if longRunningRequestCheck != nil && longRunningRequestCheck(r, requestInfo) {
+		isWatchRequest := watchVerbs.Has(requestInfo.Verb)
+
+		// Skip tracking long running non-watch requests.
+		if longRunningRequestCheck != nil && longRunningRequestCheck(r, requestInfo) && !isWatchRequest {
 			klog.V(6).Infof("Serving RequestInfo=%#+v, user.Info=%#+v as longrunning\n", requestInfo, user)
 			handler.ServeHTTP(w, r)
 			return
@@ -116,15 +111,53 @@ func WithPriorityAndFairness(
 				waitingMark.recordReadOnly(int(atomic.AddInt32(&atomicReadOnlyWaiting, delta)))
 			}
 		}
+		var resultCh chan interface{}
+		if isWatchRequest {
+			resultCh = make(chan interface{})
+		}
 		execute := func() {
 			noteExecutingDelta(1)
 			defer noteExecutingDelta(-1)
 			served = true
-			innerCtx := context.WithValue(ctx, priorityAndFairnessKey, classification)
-			innerReq := r.Clone(innerCtx)
+
+			innerCtx := ctx
+			innerReq := r
+
+			var watchInitializationSignal utilflowcontrol.InitializationSignal
+			if isWatchRequest {
+				watchInitializationSignal = newInitializationSignal()
+				innerCtx = utilflowcontrol.WithInitializationSignal(ctx, watchInitializationSignal)
+				innerReq = r.Clone(innerCtx)
+			}
 			setResponseHeaders(classification, w)
 
-			handler.ServeHTTP(w, innerReq)
+			if isWatchRequest {
+				go func() {
+					defer func() {
+						err := recover()
+						// do not wrap the sentinel ErrAbortHandler panic value
+						if err != nil && err != http.ErrAbortHandler {
+							// Same as stdlib http server code. Manually allocate stack
+							// trace buffer size to prevent excessively large logs
+							const size = 64 << 10
+							buf := make([]byte, size)
+							buf = buf[:runtime.Stack(buf, false)]
+							err = fmt.Sprintf("%v\n%s", err, buf)
+						}
+						resultCh <- err
+					}()
+
+					// Protect from the situations when request will not reach storage layer
+					// and the initialization signal will not be send.
+					defer watchInitializationSignal.Signal()
+
+					handler.ServeHTTP(w, innerReq)
+				}()
+
+				watchInitializationSignal.Wait()
+			} else {
+				handler.ServeHTTP(w, innerReq)
+			}
 		}
 		digest := utilflowcontrol.RequestDigest{RequestInfo: requestInfo, User: user}
 		fcIfc.Handle(ctx, digest, note, func(inQueue bool) {
@@ -143,9 +176,23 @@ func WithPriorityAndFairness(
 				epmetrics.DroppedRequests.WithContext(ctx).WithLabelValues(epmetrics.ReadOnlyKind).Inc()
 			}
 			epmetrics.RecordRequestTermination(r, requestInfo, epmetrics.APIServerComponent, http.StatusTooManyRequests)
+			if isWatchRequest {
+				close(resultCh)
+			}
 			tooManyRequests(r, w)
 		}
 
+		// For watch requests, from the APF point of view the request is already
+		// finished at this point. However, that doesn't mean it is already finished
+		// from the non-APF point of view. So we need to wait here until the request is:
+		// 1) finished being processed or
+		// 2) rejected
+		if isWatchRequest {
+			err := <-resultCh
+			if err != nil {
+				panic(err)
+			}
+		}
 	})
 }
 

staging/src/k8s.io/apiserver/pkg/server/filters/priority-and-fairness_test.go

@@ -112,7 +112,7 @@ func (t fakeApfFilter) Run(stopCh <-chan struct{}) error {
 func (t fakeApfFilter) Install(c *mux.PathRecorderMux) {
 }
 
-func newApfServerWithSingleRequest(decision mockDecision, t *testing.T) *httptest.Server {
+func newApfServerWithSingleRequest(t *testing.T, decision mockDecision) *httptest.Server {
 	onExecuteFunc := func() {
 		if decision == decisionCancelWait {
 			t.Errorf("execute should not be invoked")
@@ -134,20 +134,30 @@ func newApfServerWithSingleRequest(decision mockDecision, t *testing.T) *httptes
 			t.Errorf("Wanted %d requests in queue, got %d", 0, atomicReadOnlyWaiting)
 		}
 	}
-	return newApfServerWithHooks(decision, onExecuteFunc, postExecuteFunc, postEnqueueFunc, postDequeueFunc, t)
+	return newApfServerWithHooks(t, decision, onExecuteFunc, postExecuteFunc, postEnqueueFunc, postDequeueFunc)
 }
 
-func newApfServerWithHooks(decision mockDecision, onExecute, postExecute, postEnqueue, postDequeue func(), t *testing.T) *httptest.Server {
+func newApfServerWithHooks(t *testing.T, decision mockDecision, onExecute, postExecute, postEnqueue, postDequeue func()) *httptest.Server {
+	fakeFilter := fakeApfFilter{
+		mockDecision: decision,
+		postEnqueue:  postEnqueue,
+		postDequeue:  postDequeue,
+	}
+	return newApfServerWithFilter(t, fakeFilter, onExecute, postExecute)
+}
+
+func newApfServerWithFilter(t *testing.T, flowControlFilter utilflowcontrol.Interface, onExecute, postExecute func()) *httptest.Server {
+	apfServer := httptest.NewServer(newApfHandlerWithFilter(t, flowControlFilter, onExecute, postExecute))
+	return apfServer
+}
+
+func newApfHandlerWithFilter(t *testing.T, flowControlFilter utilflowcontrol.Interface, onExecute, postExecute func()) http.Handler {
 	requestInfoFactory := &apirequest.RequestInfoFactory{APIPrefixes: sets.NewString("apis", "api"), GrouplessAPIPrefixes: sets.NewString("api")}
 	longRunningRequestCheck := BasicLongRunningRequestCheck(sets.NewString("watch"), sets.NewString("proxy"))
 
 	apfHandler := WithPriorityAndFairness(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		onExecute()
-	}), longRunningRequestCheck, fakeApfFilter{
-		mockDecision: decision,
-		postEnqueue:  postEnqueue,
-		postDequeue:  postDequeue,
-	})
+	}), longRunningRequestCheck, flowControlFilter)
 
 	handler := apifilters.WithRequestInfo(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		r = r.WithContext(apirequest.WithUser(r.Context(), &user.DefaultInfo{
@@ -160,14 +170,13 @@ func newApfServerWithHooks(decision mockDecision, onExecute, postExecute, postEn
 		}
 	}), requestInfoFactory)
 
-	apfServer := httptest.NewServer(handler)
-	return apfServer
+	return handler
 }
 
 func TestApfSkipLongRunningRequest(t *testing.T) {
 	epmetrics.Register()
 
-	server := newApfServerWithSingleRequest(decisionSkipFilter, t)
+	server := newApfServerWithSingleRequest(t, decisionSkipFilter)
 	defer server.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -175,7 +184,7 @@ func TestApfSkipLongRunningRequest(t *testing.T) {
 	StartPriorityAndFairnessWatermarkMaintenance(ctx.Done())
 
 	// send a watch request to test skipping long running request
-	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces?watch=true", server.URL), http.StatusOK); err != nil {
+	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/foos/foo/proxy", server.URL), http.StatusOK); err != nil {
 		// request should not be rejected
 		t.Error(err)
 	}
@@ -184,7 +193,7 @@ func TestApfSkipLongRunningRequest(t *testing.T) {
 func TestApfRejectRequest(t *testing.T) {
 	epmetrics.Register()
 
-	server := newApfServerWithSingleRequest(decisionReject, t)
+	server := newApfServerWithSingleRequest(t, decisionReject)
 	defer server.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -209,7 +218,7 @@ func TestApfExemptRequest(t *testing.T) {
 	// so that an observation will cause some data to go into the Prometheus metrics.
 	time.Sleep(time.Millisecond * 50)
 
-	server := newApfServerWithSingleRequest(decisionNoQueuingExecute, t)
+	server := newApfServerWithSingleRequest(t, decisionNoQueuingExecute)
 	defer server.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -235,7 +244,7 @@ func TestApfExecuteRequest(t *testing.T) {
 	// so that an observation will cause some data to go into the Prometheus metrics.
 	time.Sleep(time.Millisecond * 50)
 
-	server := newApfServerWithSingleRequest(decisionQueuingExecute, t)
+	server := newApfServerWithSingleRequest(t, decisionQueuingExecute)
 	defer server.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -307,7 +316,7 @@ func TestApfExecuteMultipleRequests(t *testing.T) {
 		finishExecute.Wait()
 	}
 
-	server := newApfServerWithHooks(decisionQueuingExecute, onExecuteFunc, postExecuteFunc, postEnqueueFunc, postDequeueFunc, t)
+	server := newApfServerWithHooks(t, decisionQueuingExecute, onExecuteFunc, postExecuteFunc, postEnqueueFunc, postDequeueFunc)
 	defer server.Close()
 
 	ctx, cancel := context.WithCancel(context.Background())
@@ -334,10 +343,212 @@ func TestApfExecuteMultipleRequests(t *testing.T) {
 	})
 }
 
+type fakeWatchApfFilter struct {
+	lock     sync.Mutex
+	inflight int
+	capacity int
+}
+
+func (f *fakeWatchApfFilter) Handle(ctx context.Context,
+	requestDigest utilflowcontrol.RequestDigest,
+	_ func(fs *flowcontrol.FlowSchema, pl *flowcontrol.PriorityLevelConfiguration),
+	_ fq.QueueNoteFn,
+	execFn func(),
+) {
+	canExecute := false
+	func() {
+		f.lock.Lock()
+		defer f.lock.Unlock()
+		if f.inflight < f.capacity {
+			f.inflight++
+			canExecute = true
+		}
+	}()
+	if !canExecute {
+		return
+	}
+
+	execFn()
+
+	f.lock.Lock()
+	defer f.lock.Unlock()
+	f.inflight--
+}
+
+func (f *fakeWatchApfFilter) MaintainObservations(stopCh <-chan struct{}) {
+}
+
+func (f *fakeWatchApfFilter) Run(stopCh <-chan struct{}) error {
+	return nil
+}
+
+func (t *fakeWatchApfFilter) Install(c *mux.PathRecorderMux) {
+}
+
+func (f *fakeWatchApfFilter) wait() error {
+	return wait.Poll(100*time.Millisecond, wait.ForeverTestTimeout, func() (bool, error) {
+		f.lock.Lock()
+		defer f.lock.Unlock()
+		return f.inflight == 0, nil
+	})
+}
+
+func TestApfExecuteWatchRequestsWithInitializationSignal(t *testing.T) {
+	signalsLock := sync.Mutex{}
+	signals := []utilflowcontrol.InitializationSignal{}
+	sendSignals := func() {
+		signalsLock.Lock()
+		defer signalsLock.Unlock()
+		for i := range signals {
+			signals[i].Signal()
+		}
+		signals = signals[:0]
+	}
+
+	newInitializationSignal = func() utilflowcontrol.InitializationSignal {
+		signalsLock.Lock()
+		defer signalsLock.Unlock()
+		signal := utilflowcontrol.NewInitializationSignal()
+		signals = append(signals, signal)
+		return signal
+	}
+	defer func() {
+		newInitializationSignal = utilflowcontrol.NewInitializationSignal
+	}()
+
+	// We test if initialization after receiving initialization signal the
+	// new requests will be allowed to run by:
+	// - sending N requests that will occupy the whole capacity
+	// - sending initialiation signals for them
+	// - ensuring that number of inflight requests will get to zero
+	concurrentRequests := 5
+	firstRunning := sync.WaitGroup{}
+	firstRunning.Add(concurrentRequests)
+	allRunning := sync.WaitGroup{}
+	allRunning.Add(2 * concurrentRequests)
+
+	fakeFilter := &fakeWatchApfFilter{
+		capacity: concurrentRequests,
+	}
+
+	onExecuteFunc := func() {
+		firstRunning.Done()
+		firstRunning.Wait()
+
+		sendSignals()
+		fakeFilter.wait()
+
+		allRunning.Done()
+		allRunning.Wait()
+	}
+
+	postExecuteFunc := func() {}
+
+	server := newApfServerWithFilter(t, fakeFilter, onExecuteFunc, postExecuteFunc)
+	defer server.Close()
+
+	var wg sync.WaitGroup
+	wg.Add(2 * concurrentRequests)
+	for i := 0; i < concurrentRequests; i++ {
+		go func() {
+			defer wg.Done()
+			if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default/pods?watch=true", server.URL), http.StatusOK); err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+
+	firstRunning.Wait()
+	fakeFilter.wait()
+
+	firstRunning.Add(concurrentRequests)
+	for i := 0; i < concurrentRequests; i++ {
+		go func() {
+			defer wg.Done()
+			if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default/pods?watch=true", server.URL), http.StatusOK); err != nil {
+				t.Error(err)
+			}
+		}()
+	}
+	wg.Wait()
+}
+
+func TestApfRejectWatchRequestsWithInitializationSignal(t *testing.T) {
+	fakeFilter := &fakeWatchApfFilter{
+		capacity: 0,
+	}
+
+	onExecuteFunc := func() {
+		t.Errorf("Request unexepectedly executing")
+	}
+	postExecuteFunc := func() {}
+
+	server := newApfServerWithFilter(t, fakeFilter, onExecuteFunc, postExecuteFunc)
+	defer server.Close()
+
+	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default/pods?watch=true", server.URL), http.StatusTooManyRequests); err != nil {
+		t.Error(err)
+	}
+}
+
+func TestApfWatchPanic(t *testing.T) {
+	fakeFilter := &fakeWatchApfFilter{
+		capacity: 1,
+	}
+
+	onExecuteFunc := func() {
+		panic("test panic")
+	}
+	postExecuteFunc := func() {}
+
+	apfHandler := newApfHandlerWithFilter(t, fakeFilter, onExecuteFunc, postExecuteFunc)
+	handler := func(w http.ResponseWriter, r *http.Request) {
+		defer func() {
+			if err := recover(); err == nil {
+				t.Errorf("expected panic, got %v", err)
+			}
+		}()
+		apfHandler.ServeHTTP(w, r)
+	}
+	server := httptest.NewServer(http.HandlerFunc(handler))
+	defer server.Close()
+
+	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default/pods?watch=true", server.URL), http.StatusOK); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+}
+
+// TestContextClosesOnRequestProcessed ensures that the request context is cancelled
+// automatically even if the server doesn't cancel is explicitly.
+// This is required to ensure we won't be leaking goroutines that wait for context
+// cancelling (e.g. in queueset::StartRequest method).
+// Even though in production we are not using httptest.Server, this logic is shared
+// across these two.
+func TestContextClosesOnRequestProcessed(t *testing.T) {
+	wg := sync.WaitGroup{}
+	wg.Add(1)
+	handler := func(w http.ResponseWriter, r *http.Request) {
+		ctx := r.Context()
+		// asynchronously wait for context being closed
+		go func() {
+			<-ctx.Done()
+			wg.Done()
+		}()
+	}
+	server := httptest.NewServer(http.HandlerFunc(handler))
+	defer server.Close()
+
+	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default/pods?watch=true", server.URL), http.StatusOK); err != nil {
+		t.Errorf("unexpected error: %v", err)
+	}
+
+	wg.Wait()
+}
+
 func TestApfCancelWaitRequest(t *testing.T) {
 	epmetrics.Register()
 
-	server := newApfServerWithSingleRequest(decisionCancelWait, t)
+	server := newApfServerWithSingleRequest(t, decisionCancelWait)
 	defer server.Close()
 
 	if err := expectHTTPGet(fmt.Sprintf("%s/api/v1/namespaces/default", server.URL), http.StatusTooManyRequests); err != nil {

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

@@ -38,6 +38,7 @@ import (
 	"k8s.io/apiserver/pkg/features"
 	"k8s.io/apiserver/pkg/storage"
 	utilfeature "k8s.io/apiserver/pkg/util/feature"
+	utilflowcontrol "k8s.io/apiserver/pkg/util/flowcontrol"
 	"k8s.io/client-go/tools/cache"
 	"k8s.io/klog/v2"
 	utiltrace "k8s.io/utils/trace"
@@ -1413,6 +1414,14 @@ func (c *cacheWatcher) process(ctx context.Context, initEvents []*watchCacheEven
 		klog.V(2).Infof("processing %d initEvents of %s (%s) took %v", len(initEvents), objType, c.identifier, processingTime)
 	}
 
+	// At this point we already start processing incoming watch events.
+	// However, the init event can still be processed because their serialization
+	// and sending to the client happens asynchrnously.
+	// TODO: As describe in the KEP, we would like to estimate that by delaying
+	//   the initialization signal proportionally to the number of events to
+	//   process, but we're leaving this to the tuning phase.
+	utilflowcontrol.WatchInitialized(ctx)
+
 	defer close(c.result)
 	defer c.Stop()
 	for {

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

@@ -42,6 +42,7 @@ import (
 	"k8s.io/apiserver/pkg/apis/example"
 	examplev1 "k8s.io/apiserver/pkg/apis/example/v1"
 	"k8s.io/apiserver/pkg/storage"
+	utilflowcontrol "k8s.io/apiserver/pkg/util/flowcontrol"
 )
 
 var (
@@ -701,6 +702,26 @@ func TestCacherNoLeakWithMultipleWatchers(t *testing.T) {
 	}
 }
 
+func TestWatchInitializationSignal(t *testing.T) {
+	backingStorage := &dummyStorage{}
+	cacher, _, err := newTestCacher(backingStorage)
+	if err != nil {
+		t.Fatalf("Couldn't create cacher: %v", err)
+	}
+	defer cacher.Stop()
+
+	ctx, _ := context.WithTimeout(context.Background(), 5*time.Second)
+	initSignal := utilflowcontrol.NewInitializationSignal()
+	ctx = utilflowcontrol.WithInitializationSignal(ctx, initSignal)
+
+	_, err = cacher.Watch(ctx, "pods/ns", storage.ListOptions{ResourceVersion: "0", Predicate: storage.Everything})
+	if err != nil {
+		t.Fatalf("Failed to create watch: %v", err)
+	}
+
+	initSignal.Wait()
+}
+
 func testCacherSendBookmarkEvents(t *testing.T, allowWatchBookmarks, expectedBookmarks bool) {
 	backingStorage := &dummyStorage{}
 	cacher, _, err := newTestCacher(backingStorage)

staging/src/k8s.io/apiserver/pkg/storage/etcd3/watcher.go

@@ -32,6 +32,7 @@ import (
 	"k8s.io/apiserver/pkg/storage"
 	"k8s.io/apiserver/pkg/storage/etcd3/metrics"
 	"k8s.io/apiserver/pkg/storage/value"
+	utilflowcontrol "k8s.io/apiserver/pkg/util/flowcontrol"
 
 	"go.etcd.io/etcd/clientv3"
 	"k8s.io/klog/v2"
@@ -120,6 +121,14 @@ func (w *watcher) Watch(ctx context.Context, key string, rev int64, recursive, p
 	}
 	wc := w.createWatchChan(ctx, key, rev, recursive, progressNotify, pred)
 	go wc.run()
+
+	// For etcd watch we don't have an easy way to answer whether the watch
+	// has already caught up. So in the initial version (given that watchcache
+	// is by default enabled for all resources but Events), we just deliver
+	// the initialization signal immediately. Improving this will be explored
+	// in the future.
+	utilflowcontrol.WatchInitialized(ctx)
+
 	return wc, nil
 }
 

staging/src/k8s.io/apiserver/pkg/storage/etcd3/watcher_test.go

@@ -38,6 +38,7 @@ import (
 	"k8s.io/apiserver/pkg/apis/example"
 	examplev1 "k8s.io/apiserver/pkg/apis/example/v1"
 	"k8s.io/apiserver/pkg/storage"
+	utilflowcontrol "k8s.io/apiserver/pkg/util/flowcontrol"
 )
 
 func TestWatch(t *testing.T) {
@@ -313,6 +314,23 @@ func TestWatchDeleteEventObjectHaveLatestRV(t *testing.T) {
 	}
 }
 
+func TestWatchInitializationSignal(t *testing.T) {
+	_, store, cluster := testSetup(t)
+	defer cluster.Terminate(t)
+
+	ctx, _ := context.WithTimeout(context.Background(), 5*time.Second)
+	initSignal := utilflowcontrol.NewInitializationSignal()
+	ctx = utilflowcontrol.WithInitializationSignal(ctx, initSignal)
+
+	key, storedObj := testPropogateStore(ctx, t, store, &example.Pod{ObjectMeta: metav1.ObjectMeta{Name: "foo"}})
+	_, err := store.Watch(ctx, key, storage.ListOptions{ResourceVersion: storedObj.ResourceVersion, Predicate: storage.Everything})
+	if err != nil {
+		t.Fatalf("Watch failed: %v", err)
+	}
+
+	initSignal.Wait()
+}
+
 func TestProgressNotify(t *testing.T) {
 	codec := apitesting.TestCodec(codecs, examplev1.SchemeGroupVersion)
 	clusterConfig := &integration.ClusterConfig{

staging/src/k8s.io/apiserver/pkg/util/flowcontrol/apf_context.go

@@ -0,0 +1,82 @@
+/*
+Copyright 2021 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package flowcontrol
+
+import (
+	"context"
+	"sync"
+)
+
+type priorityAndFairnessKeyType int
+
+const (
+	// priorityAndFairnessInitializationSignalKey is a key under which
+	// initialization signal function for watch requests is stored
+	// in the context.
+	priorityAndFairnessInitializationSignalKey priorityAndFairnessKeyType = iota
+)
+
+// WithInitializationSignal creates a copy of parent context with
+// priority and fairness initialization signal value.
+func WithInitializationSignal(ctx context.Context, signal InitializationSignal) context.Context {
+	return context.WithValue(ctx, priorityAndFairnessInitializationSignalKey, signal)
+}
+
+// initializationSignalFrom returns an initialization signal function
+// which when called signals that watch initialization has already finished
+// to priority and fairness dispatcher.
+func initializationSignalFrom(ctx context.Context) (InitializationSignal, bool) {
+	signal, ok := ctx.Value(priorityAndFairnessInitializationSignalKey).(InitializationSignal)
+	return signal, ok && signal != nil
+}
+
+// WatchInitialized sends a signal to priority and fairness dispatcher
+// that a given watch request has already been initialized.
+func WatchInitialized(ctx context.Context) {
+	if signal, ok := initializationSignalFrom(ctx); ok {
+		signal.Signal()
+	}
+}
+
+// InitializationSignal is an interface that allows sending and handling
+// initialization signals.
+type InitializationSignal interface {
+	// Signal notifies the dispatcher about finished initialization.
+	Signal()
+	// Wait waits for the initialization signal.
+	Wait()
+}
+
+type initializationSignal struct {
+	once sync.Once
+	done chan struct{}
+}
+
+func NewInitializationSignal() InitializationSignal {
+	return &initializationSignal{
+		once: sync.Once{},
+		done: make(chan struct{}),
+	}
+}
+
+func (i *initializationSignal) Signal() {
+	i.once.Do(func() { close(i.done) })
+}
+
+func (i *initializationSignal) Wait() {
+	<-i.done
+}