feat(manager): add prestart hook support

When implementing a controller that uses leader election, there maybe be
work that needs to be done after winning the election but before
processing enqueued requests. For example, a controller may need to
build up an internal mapping of the current state of the cluster before
it can begin reconciling.

This changeset adds support for adding prestart hooks to
controller-runtime's manager implementation. This hook runs after the
manager has been elected leader, immediately before the leader election
controllers are started.

Related #607

pkg/manager/internal.go

@@ -173,6 +173,10 @@ type controllerManager struct {
 	// internalProceduresStop channel is used internally to the manager when coordinating
 	// the proper shutdown of servers. This channel is also used for dependency injection.
 	internalProceduresStop chan struct{}
+
+	// prestartHooks are functions that are run immediately before calling the Start functions
+	// of the leader election runnables.
+	prestartHooks []func(ctx context.Context) error
 }
 
 type hasCache interface {
@@ -269,6 +273,18 @@ func (cm *controllerManager) AddReadyzCheck(name string, check healthz.Checker)
 	return nil
 }
 
+func (cm *controllerManager) AddPrestartHook(hook func(ctx context.Context) error) error {
+	cm.Lock()
+	defer cm.Unlock()
+
+	if cm.started {
+		return fmt.Errorf("unable to add new prestart hook because the manager has already been started")
+	}
+
+	cm.prestartHooks = append(cm.prestartHooks, hook)
+	return nil
+}
+
 func (cm *controllerManager) GetConfig() *rest.Config {
 	return cm.cluster.GetConfig()
 }
@@ -606,6 +622,15 @@ func (cm *controllerManager) engageStopProcedure(stopComplete <-chan struct{}) e
 }
 
 func (cm *controllerManager) startLeaderElectionRunnables() error {
+	for _, hook := range cm.prestartHooks {
+		if err := hook(cm.internalCtx); err != nil {
+			return err
+		}
+	}
+
+	// All the prestart hooks have been run, clear the slice to free the underlying resources.
+	cm.prestartHooks = nil
+
 	return cm.runnables.LeaderElection.Start(cm.internalCtx)
 }
 

pkg/manager/manager.go

@@ -79,6 +79,12 @@ type Manager interface {
 	// AddReadyzCheck allows you to add Readyz checker
 	AddReadyzCheck(name string, check healthz.Checker) error
 
+	// AddPrestartHook allows you to add a hook that runs after leader election and immediately
+	// before starting controllers needing leader election. Prestart hooks block execution of
+	// leader election controllers until all return nil error. The manager is stopped on non-nil
+	// errors.
+	AddPrestartHook(func(ctx context.Context) error) error
+
 	// Start starts all registered Controllers and blocks until the context is cancelled.
 	// Returns an error if there is an error starting any controller.
 	//

pkg/manager/manager_test.go

@@ -1076,6 +1076,63 @@ var _ = Describe("manger.Manager", func() {
 				<-runnableStopped
 			})
 
+			It("should run prestart hooks before calling Start on leader election runnables", func() {
+				m, err := New(cfg, options)
+				Expect(err).NotTo(HaveOccurred())
+				for _, cb := range callbacks {
+					cb(m)
+				}
+
+				runnableRan := make(chan struct{})
+
+				Expect(m.Add(RunnableFunc(func(ctx context.Context) error {
+					close(runnableRan)
+					return nil
+				})))
+
+				Expect(m.AddPrestartHook(func(ctx context.Context) error {
+					Expect(m.Elected()).ShouldNot(BeClosed())
+					Consistently(runnableRan).ShouldNot(BeClosed())
+					return nil
+				}))
+
+				ctx, cancel := context.WithCancel(context.Background())
+				defer cancel()
+				go func() {
+					defer GinkgoRecover()
+					Expect(m.Elected()).ShouldNot(BeClosed())
+					Expect(m.Start(ctx)).NotTo(HaveOccurred())
+				}()
+
+				<-m.Elected()
+			})
+
+			It("should not run leader election runnables if prestart hooks fail", func() {
+				m, err := New(cfg, options)
+				Expect(err).NotTo(HaveOccurred())
+				for _, cb := range callbacks {
+					cb(m)
+				}
+
+				runnableRan := make(chan struct{})
+
+				Expect(m.Add(RunnableFunc(func(ctx context.Context) error {
+					close(runnableRan)
+					return nil
+				})))
+
+				Expect(m.AddPrestartHook(func(ctx context.Context) error {
+					Expect(m.Elected()).ShouldNot(BeClosed())
+					Consistently(runnableRan).ShouldNot(BeClosed())
+					return errors.New("prestart hook failed")
+				}))
+
+				ctx, cancel := context.WithCancel(context.Background())
+				defer cancel()
+
+				Expect(m.Elected()).ShouldNot(BeClosed())
+				Expect(m.Start(ctx)).Should(MatchError(ContainSubstring("prestart hook failed")))
+			})
 		}
 
 		Context("with defaults", func() {