Avoid deadlock in gc resync if available resources change during sync

pkg/controller/garbagecollector/garbagecollector.go

@@ -32,6 +32,7 @@ import (
 	"k8s.io/apimachinery/pkg/types"
 	utilerrors "k8s.io/apimachinery/pkg/util/errors"
 	utilruntime "k8s.io/apimachinery/pkg/util/runtime"
+	"k8s.io/apimachinery/pkg/util/sets"
 	"k8s.io/apimachinery/pkg/util/wait"
 	"k8s.io/client-go/discovery"
 	"k8s.io/client-go/dynamic"
@@ -170,10 +171,8 @@ func (gc *GarbageCollector) Sync(discoveryClient discovery.ServerResourcesInterf
 		newResources := GetDeletableResources(discoveryClient)
 
 		// This can occur if there is an internal error in GetDeletableResources.
-		// If the gc attempts to sync with 0 resources it will block forever.
-		// TODO: Implement a more complete solution for the garbage collector hanging.
 		if len(newResources) == 0 {
-			glog.V(5).Infof("no resources reported by discovery, skipping garbage collector sync")
+			glog.V(2).Infof("no resources reported by discovery, skipping garbage collector sync")
 			return
 		}
 
@@ -183,39 +182,61 @@ func (gc *GarbageCollector) Sync(discoveryClient discovery.ServerResourcesInterf
 			return
 		}
 
-		// Something has changed, time to sync.
-		glog.V(2).Infof("syncing garbage collector with updated resources from discovery: %v", newResources)
-
 		// Ensure workers are paused to avoid processing events before informers
 		// have resynced.
 		gc.workerLock.Lock()
 		defer gc.workerLock.Unlock()
 
-		// Resetting the REST mapper will also invalidate the underlying discovery
-		// client. This is a leaky abstraction and assumes behavior about the REST
-		// mapper, but we'll deal with it for now.
-		gc.restMapper.Reset()
+		// Once we get here, we should not unpause workers until we've successfully synced
+		attempt := 0
+		wait.PollImmediateUntil(100*time.Millisecond, func() (bool, error) {
+			attempt++
 
-		// Perform the monitor resync and wait for controllers to report cache sync.
-		//
-		// NOTE: It's possible that newResources will diverge from the resources
-		// discovered by restMapper during the call to Reset, since they are
-		// distinct discovery clients invalidated at different times. For example,
-		// newResources may contain resources not returned in the restMapper's
-		// discovery call if the resources appeared in-between the calls. In that
-		// case, the restMapper will fail to map some of newResources until the next
-		// sync period.
-		if err := gc.resyncMonitors(newResources); err != nil {
-			utilruntime.HandleError(fmt.Errorf("failed to sync resource monitors: %v", err))
-			return
-		}
-		// TODO: WaitForCacheSync can block forever during normal operation. Could
-		// pass a timeout channel, but we have to consider the implications of
-		// un-pausing the GC with a partially synced graph builder.
-		if !controller.WaitForCacheSync("garbage collector", stopCh, gc.dependencyGraphBuilder.IsSynced) {
-			utilruntime.HandleError(fmt.Errorf("timed out waiting for dependency graph builder sync during GC sync"))
-			return
-		}
+			// On a reattempt, check if available resources have changed
+			if attempt > 1 {
+				newResources = GetDeletableResources(discoveryClient)
+				if len(newResources) == 0 {
+					glog.V(2).Infof("no resources reported by discovery (attempt %d)", attempt)
+					return false, nil
+				}
+			}
+
+			glog.V(2).Infof("syncing garbage collector with updated resources from discovery (attempt %d): %s", attempt, printDiff(oldResources, newResources))
+
+			// Resetting the REST mapper will also invalidate the underlying discovery
+			// client. This is a leaky abstraction and assumes behavior about the REST
+			// mapper, but we'll deal with it for now.
+			gc.restMapper.Reset()
+			glog.V(4).Infof("reset restmapper")
+
+			// Perform the monitor resync and wait for controllers to report cache sync.
+			//
+			// NOTE: It's possible that newResources will diverge from the resources
+			// discovered by restMapper during the call to Reset, since they are
+			// distinct discovery clients invalidated at different times. For example,
+			// newResources may contain resources not returned in the restMapper's
+			// discovery call if the resources appeared in-between the calls. In that
+			// case, the restMapper will fail to map some of newResources until the next
+			// attempt.
+			if err := gc.resyncMonitors(newResources); err != nil {
+				utilruntime.HandleError(fmt.Errorf("failed to sync resource monitors (attempt %d): %v", attempt, err))
+				return false, nil
+			}
+			glog.V(4).Infof("resynced monitors")
+
+			// wait for caches to fill for a while (our sync period) before attempting to rediscover resources and retry syncing.
+			// this protects us from deadlocks where available resources changed and one of our informer caches will never fill.
+			// informers keep attempting to sync in the background, so retrying doesn't interrupt them.
+			// the call to resyncMonitors on the reattempt will no-op for resources that still exist.
+			// note that workers stay paused until we successfully resync.
+			if !controller.WaitForCacheSync("garbage collector", waitForStopOrTimeout(stopCh, period), gc.dependencyGraphBuilder.IsSynced) {
+				utilruntime.HandleError(fmt.Errorf("timed out waiting for dependency graph builder sync during GC sync (attempt %d)", attempt))
+				return false, nil
+			}
+
+			// success, break out of the loop
+			return true, nil
+		}, stopCh)
 
 		// Finally, keep track of our new state. Do this after all preceding steps
 		// have succeeded to ensure we'll retry on subsequent syncs if an error
@@ -225,6 +246,36 @@ func (gc *GarbageCollector) Sync(discoveryClient discovery.ServerResourcesInterf
 	}, period, stopCh)
 }
 
+// printDiff returns a human-readable summary of what resources were added and removed
+func printDiff(oldResources, newResources map[schema.GroupVersionResource]struct{}) string {
+	removed := sets.NewString()
+	for oldResource := range oldResources {
+		if _, ok := newResources[oldResource]; !ok {
+			removed.Insert(fmt.Sprintf("%+v", oldResource))
+		}
+	}
+	added := sets.NewString()
+	for newResource := range newResources {
+		if _, ok := oldResources[newResource]; !ok {
+			added.Insert(fmt.Sprintf("%+v", newResource))
+		}
+	}
+	return fmt.Sprintf("added: %v, removed: %v", added.List(), removed.List())
+}
+
+// waitForStopOrTimeout returns a stop channel that closes when the provided stop channel closes or when the specified timeout is reached
+func waitForStopOrTimeout(stopCh <-chan struct{}, timeout time.Duration) <-chan struct{} {
+	stopChWithTimeout := make(chan struct{})
+	go func() {
+		select {
+		case <-stopCh:
+		case <-time.After(timeout):
+		}
+		close(stopChWithTimeout)
+	}()
+	return stopChWithTimeout
+}
+
 func (gc *GarbageCollector) IsSynced() bool {
 	return gc.dependencyGraphBuilder.IsSynced()
 }

pkg/controller/garbagecollector/garbagecollector_test.go

@@ -800,6 +800,15 @@ func TestGarbageCollectorSync(t *testing.T) {
 			},
 		},
 	}
+	unsyncableServerResources := []*metav1.APIResourceList{
+		{
+			GroupVersion: "v1",
+			APIResources: []metav1.APIResource{
+				{Name: "pods", Namespaced: true, Kind: "Pod", Verbs: metav1.Verbs{"delete", "list", "watch"}},
+				{Name: "secrets", Namespaced: true, Kind: "Secret", Verbs: metav1.Verbs{"delete", "list", "watch"}},
+			},
+		},
+	}
 	fakeDiscoveryClient := &fakeServerResources{
 		PreferredResources: serverResources,
 		Error:              nil,
@@ -813,6 +822,10 @@ func TestGarbageCollectorSync(t *testing.T) {
 				200,
 				[]byte("{}"),
 			},
+			"GET" + "/api/v1/secrets": {
+				404,
+				[]byte("{}"),
+			},
 		},
 	}
 	srv, clientConfig := testServerAndClientConfig(testHandler.ServeHTTP)
@@ -849,7 +862,7 @@ func TestGarbageCollectorSync(t *testing.T) {
 	fmt.Printf("Test output")
 	time.Sleep(1 * time.Second)
 
-	err = expectSyncNotBlocked(fakeDiscoveryClient)
+	err = expectSyncNotBlocked(fakeDiscoveryClient, &gc.workerLock)
 	if err != nil {
 		t.Fatalf("Expected garbagecollector.Sync to be running but it is blocked: %v", err)
 	}
@@ -865,13 +878,29 @@ func TestGarbageCollectorSync(t *testing.T) {
 	fakeDiscoveryClient.setPreferredResources(serverResources)
 	fakeDiscoveryClient.setError(nil)
 
-	err = expectSyncNotBlocked(fakeDiscoveryClient)
+	err = expectSyncNotBlocked(fakeDiscoveryClient, &gc.workerLock)
+	if err != nil {
+		t.Fatalf("Expected garbagecollector.Sync to still be running but it is blocked: %v", err)
+	}
+
+	// Simulate the discovery client returning a resource the restmapper can resolve, but will not sync caches
+	fakeDiscoveryClient.setPreferredResources(unsyncableServerResources)
+	fakeDiscoveryClient.setError(nil)
+
+	// Wait until sync discovers the change
+	time.Sleep(1 * time.Second)
+
+	// Put the resources back to normal and ensure garbage collector sync recovers
+	fakeDiscoveryClient.setPreferredResources(serverResources)
+	fakeDiscoveryClient.setError(nil)
+
+	err = expectSyncNotBlocked(fakeDiscoveryClient, &gc.workerLock)
 	if err != nil {
 		t.Fatalf("Expected garbagecollector.Sync to still be running but it is blocked: %v", err)
 	}
 }
 
-func expectSyncNotBlocked(fakeDiscoveryClient *fakeServerResources) error {
+func expectSyncNotBlocked(fakeDiscoveryClient *fakeServerResources, workerLock *sync.RWMutex) error {
 	before := fakeDiscoveryClient.getInterfaceUsedCount()
 	t := 1 * time.Second
 	time.Sleep(t)
@@ -879,7 +908,19 @@ func expectSyncNotBlocked(fakeDiscoveryClient *fakeServerResources) error {
 	if before == after {
 		return fmt.Errorf("discoveryClient.ServerPreferredResources() called %d times over %v", after-before, t)
 	}
-	return nil
+
+	workerLockAcquired := make(chan struct{})
+	go func() {
+		workerLock.Lock()
+		workerLock.Unlock()
+		close(workerLockAcquired)
+	}()
+	select {
+	case <-workerLockAcquired:
+		return nil
+	case <-time.After(t):
+		return fmt.Errorf("workerLock blocked for at least %v", t)
+	}
 }
 
 type fakeServerResources struct {

pkg/controller/garbagecollector/graph_builder.go

@@ -288,11 +288,13 @@ func (gb *GraphBuilder) IsSynced() bool {
 	defer gb.monitorLock.Unlock()
 
 	if len(gb.monitors) == 0 {
+		glog.V(4).Info("garbage controller monitor not synced: no monitors")
 		return false
 	}
 
-	for _, monitor := range gb.monitors {
+	for resource, monitor := range gb.monitors {
 		if !monitor.controller.HasSynced() {
+			glog.V(4).Infof("garbage controller monitor not yet synced: %+v", resource)
 			return false
 		}
 	}