add OnNewLeader callback to leaderelection client

pkg/client/leaderelection/leaderelection.go

@@ -121,12 +121,15 @@ type LeaderElectionConfig struct {
 //
 // possible future callbacks:
 //  * OnChallenge()
-//  * OnNewLeader()
 type LeaderCallbacks struct {
 	// OnStartedLeading is called when a LeaderElector client starts leading
 	OnStartedLeading func(stop <-chan struct{})
 	// OnStoppedLeading is called when a LeaderElector client stops leading
 	OnStoppedLeading func()
+	// OnNewLeader is called when the client observes a leader that is
+	// not the previously observed leader. This includes the first observed
+	// leader when the client starts.
+	OnNewLeader func(identity string)
 }
 
 // LeaderElector is a leader election client.
@@ -139,6 +142,10 @@ type LeaderElector struct {
 	// internal bookkeeping
 	observedRecord LeaderElectionRecord
 	observedTime   time.Time
+	// used to implement OnNewLeader(), may lag slightly from the
+	// value observedRecord.HolderIdentity if the transistion has
+	// not yet been reported.
+	reportedLeader string
 }
 
 // LeaderElectionRecord is the record that is stored in the leader election annotation.
@@ -182,6 +189,7 @@ func (le *LeaderElector) acquire() {
 	stop := make(chan struct{})
 	util.Until(func() {
 		succeeded := le.tryAcquireOrRenew()
+		le.maybeReportTransition()
 		if !succeeded {
 			glog.V(4).Infof("failed to renew lease %v/%v", le.config.EndpointsMeta.Namespace, le.config.EndpointsMeta.Name)
 			time.Sleep(wait.Jitter(le.config.RetryPeriod, JitterFactor))
@@ -200,6 +208,7 @@ func (le *LeaderElector) renew() {
 		err := wait.Poll(le.config.RetryPeriod, le.config.RenewDeadline, func() (bool, error) {
 			return le.tryAcquireOrRenew(), nil
 		})
+		le.maybeReportTransition()
 		if err == nil {
 			glog.V(4).Infof("succesfully renewed lease %v/%v", le.config.EndpointsMeta.Namespace, le.config.EndpointsMeta.Name)
 			return
@@ -296,3 +305,13 @@ func (le *LeaderElector) tryAcquireOrRenew() bool {
 	le.observedTime = time.Now()
 	return true
 }
+
+func (l *LeaderElector) maybeReportTransition() {
+	if l.observedRecord.HolderIdentity == l.reportedLeader {
+		return
+	}
+	l.reportedLeader = l.observedRecord.HolderIdentity
+	if l.config.Callbacks.OnNewLeader != nil {
+		go l.config.Callbacks.OnNewLeader(l.reportedLeader)
+	}
+}

pkg/client/leaderelection/leaderelection_test.go

@@ -22,6 +22,7 @@ package leaderelection
 
 import (
 	"fmt"
+	"sync"
 	"testing"
 	"time"
 
@@ -195,14 +196,24 @@ func TestTryAcquireOrRenew(t *testing.T) {
 		},
 	}
 
-	lec := LeaderElectionConfig{
-		EndpointsMeta: api.ObjectMeta{Namespace: "foo", Name: "bar"},
-		Identity:      "baz",
-		EventRecorder: &record.FakeRecorder{},
-		LeaseDuration: 10 * time.Second,
-	}
-
 	for i, test := range tests {
+		// OnNewLeader is called async so we have to wait for it.
+		var wg sync.WaitGroup
+		wg.Add(1)
+		var reportedLeader string
+
+		lec := LeaderElectionConfig{
+			EndpointsMeta: api.ObjectMeta{Namespace: "foo", Name: "bar"},
+			Identity:      "baz",
+			EventRecorder: &record.FakeRecorder{},
+			LeaseDuration: 10 * time.Second,
+			Callbacks: LeaderCallbacks{
+				OnNewLeader: func(l string) {
+					defer wg.Done()
+					reportedLeader = l
+				},
+			},
+		}
 		c := &testclient.Fake{}
 		for _, reactor := range test.reactors {
 			c.AddReactor(reactor.verb, "endpoints", reactor.reaction)
@@ -237,5 +248,11 @@ func TestTryAcquireOrRenew(t *testing.T) {
 		if !test.transitionLeader && le.observedRecord.LeaderTransitions != 0 {
 			t.Errorf("[%v]leader should not have transitioned but did", i)
 		}
+
+		le.maybeReportTransition()
+		wg.Wait()
+		if reportedLeader != test.outHolder {
+			t.Errorf("[%v]reported leader was not the new leader. expected %q, got %q", i, test.outHolder, reportedLeader)
+		}
 	}
 }