Fix Consul Lock TTL with store failure

If using the Lock TTL feature with Consul, the code
path in libkv is issuing a Put in the background through
the PeriodicRenewal call. The error is then eaten up and
ignored on the candidate loop. This would lead to the
candidate and followers being stuck in their candidate
loop. Consequence would be that they would not retry to
take the lock ending in a state with no Leader.

This patch restores an explicit error check instead of
wrongfully passing on the error to the channel before
giving it back to the caller.

Signed-off-by: Alexandre Beslic <abronan@docker.com>

cli/flags.go

@@ -138,7 +138,7 @@ var (
 	}
 	flLeaderTTL = cli.StringFlag{
 		Name:  "replication-ttl",
-		Value: "30s",
+		Value: "15s",
 		Usage: "Leader lock release time on failure",
 	}
 )

cli/manage.go

@@ -161,7 +161,10 @@ func setupReplication(c *cli.Context, cluster cluster.Cluster, server *api.Serve
 }
 
 func run(candidate *leadership.Candidate, server *api.Server, primary *mux.Router, replica *api.Replica) {
-	electedCh, errCh := candidate.RunForElection()
+	electedCh, errCh, err := candidate.RunForElection()
+	if err != nil {
+		return
+	}
 	for {
 		select {
 		case isElected := <-electedCh:
@@ -181,7 +184,10 @@ func run(candidate *leadership.Candidate, server *api.Server, primary *mux.Route
 }
 
 func follow(follower *leadership.Follower, replica *api.Replica, addr string) {
-	leaderCh, errCh := follower.FollowElection()
+	leaderCh, errCh, err := follower.FollowElection()
+	if err != nil {
+		return
+	}
 	for {
 		select {
 		case leader := <-leaderCh:

leadership/README.md

@@ -16,7 +16,10 @@ if err != nil {
 }
 
 underwood := leadership.NewCandidate(client, "service/swarm/leader", "underwood")
-electedCh, _ := underwood.RunForElection()
+electedCh, _, err := underwood.RunForElection()
+if err != nil {
+    log.Fatal("Cannot run for election, store is probably down")
+}
 
 for isElected := range electedCh {
 	// This loop will run every time there is a change in our leadership
@@ -46,7 +49,10 @@ It is possible to follow an election in real-time and get notified whenever
 there is a change in leadership:
 ```go
 follower := leadership.NewFollower(client, "service/swarm/leader")
-leaderCh, _ := follower.FollowElection()
+leaderCh, _, err := follower.FollowElection()
+if err != nil {
+    log.Fatal("Cannot follow the election, store is probably down")
+}
 for leader := <-leaderCh {
 	// Leader is a string containing the value passed to `NewCandidate`.
 	log.Printf("%s is now the leader", leader)
@@ -83,7 +89,10 @@ func participate() {
 }
 
 func run(candidate *leadership.Candidate) {
-    electedCh, errCh := candidate.RunForElection()
+    electedCh, errCh, err := candidate.RunForElection()
+    if err != nil {
+        return
+    }
     for {
         select {
             case elected := <-electedCh:

leadership/candidate.go

@@ -7,6 +7,10 @@ import (
 	"github.com/docker/libkv/store"
 )
 
+const (
+	defaultLockTTL = 15 * time.Second
+)
+
 // Candidate runs the leader election algorithm asynchronously
 type Candidate struct {
 	client store.Store
@@ -47,23 +51,28 @@ func (c *Candidate) IsLeader() bool {
 // ElectedCh is used to get a channel which delivers signals on
 // acquiring or losing leadership. It sends true if we become
 // the leader, and false if we lose it.
-func (c *Candidate) RunForElection() (<-chan bool, <-chan error) {
+func (c *Candidate) RunForElection() (<-chan bool, <-chan error, error) {
 	c.electedCh = make(chan bool)
 	c.errCh = make(chan error)
 
-	lock, err := c.client.NewLock(c.key, &store.LockOptions{
+	lockOpts := &store.LockOptions{
 		Value: []byte(c.node),
-		TTL:       c.lockTTL,
-		RenewLock: make(chan struct{}),
-	})
-
-	if err != nil {
-		c.errCh <- err
-	} else {
-		go c.campaign(lock)
 	}
 
-	return c.electedCh, c.errCh
+	if c.lockTTL != defaultLockTTL {
+		lockOpts.TTL = c.lockTTL
+		lockOpts.RenewLock = make(chan struct{})
+	}
+
+	lock, err := c.client.NewLock(c.key, lockOpts)
+
+	if err != nil {
+		return nil, nil, err
+	}
+
+	go c.campaign(lock)
+
+	return c.electedCh, c.errCh, nil
 }
 
 // Stop running for election.

leadership/candidate_test.go

@@ -25,7 +25,8 @@ func TestCandidate(t *testing.T) {
 	mockLock.On("Unlock").Return(nil)
 
 	candidate := NewCandidate(kv, "test_key", "test_node", 0)
-	electedCh, _ := candidate.RunForElection()
+	electedCh, _, err := candidate.RunForElection()
+	assert.Nil(t, err)
 
 	// Should issue a false upon start, no matter what.
 	assert.False(t, <-electedCh)

leadership/follower.go

@@ -33,18 +33,18 @@ func (f *Follower) Leader() string {
 }
 
 // FollowElection starts monitoring the election.
-func (f *Follower) FollowElection() (<-chan string, <-chan error) {
+func (f *Follower) FollowElection() (<-chan string, <-chan error, error) {
 	f.leaderCh = make(chan string)
 	f.errCh = make(chan error)
 
 	ch, err := f.client.Watch(f.key, f.stopCh)
 	if err != nil {
-		f.errCh <- err
-	} else {
-		go f.follow(ch)
+		return nil, nil, err
 	}
 
-	return f.leaderCh, f.errCh
+	go f.follow(ch)
+
+	return f.leaderCh, f.errCh, nil
 }
 
 // Stop stops monitoring an election.

leadership/follower_test.go

@@ -21,7 +21,8 @@ func TestFollower(t *testing.T) {
 	mockStore.On("Watch", "test_key", mock.Anything).Return(mockKVCh, nil)
 
 	follower := NewFollower(kv, "test_key")
-	leaderCh, errCh := follower.FollowElection()
+	leaderCh, errCh, err := follower.FollowElection()
+	assert.Nil(t, err)
 
 	// Simulate leader updates
 	go func() {