Support checking assertion of Leadership

api.go

@@ -188,6 +188,10 @@ type Raft struct {
 	// to verify we are still the leader
 	verifyCh chan *verifyFuture
 
+	// assertedCh is used to check if the leader has fully asserted leadership.
+	// A leader does this by committing an entry to the log in the current term.
+	assertedCh chan *assertedFuture
+
 	// configurationsCh is used to get the configuration data safely from
 	// outside of the main thread.
 	configurationsCh chan *configurationsFuture
@@ -558,6 +562,7 @@ func NewRaft(conf *Config, fsm FSM, logs LogStore, stable StableStore, snaps Sna
 		stable:                stable,
 		trans:                 trans,
 		verifyCh:              make(chan *verifyFuture, 64),
+		assertedCh:            make(chan *assertedFuture, 64),
 		configurationsCh:      make(chan *configurationsFuture, 8),
 		bootstrapCh:           make(chan *bootstrapFuture),
 		observers:             make(map[uint64]*Observer),
@@ -883,6 +888,25 @@ func (r *Raft) VerifyLeader() Future {
 	}
 }
 
+// VerifyAssertedLeadership is used to check if the node, if acting as a leader,
+// has fully asserted leadership by committing an entry in this term. The returned
+// future indicates the term for which leadership has been asserted.
+//
+// Since this call may be relatively slow, it is not suitable for code paths that
+// require low latency. One suggested way to use it is to call this once per term,
+// storing the returned state in a variable. Once leadership has been asserted for
+// a term, the state will not change, so there is no need to call this again for
+// the same term.
+//
+// Must be run on the leader, or it will fail.
+func (r *Raft) VerifyAssertedLeadership() AssertedFuture {
+	metrics.IncrCounter([]string{"raft", "verify_asserted_leadership"}, 1)
+	assertedFuture := &assertedFuture{}
+	assertedFuture.init()
+	r.assertedCh <- assertedFuture
+	return assertedFuture
+}
+
 // GetConfiguration returns the latest configuration. This may not yet be
 // committed. The main loop can access this directly.
 func (r *Raft) GetConfiguration() ConfigurationFuture {

future.go

@@ -44,6 +44,18 @@ type ApplyFuture interface {
 	Response() interface{}
 }
 
+// AssertedFuture is used to check if the current node has asserted leadership.
+// A Leader asserts leadership by committing at least one log in this term.
+type AssertedFuture interface {
+	Future
+
+	// Asserted returns true if the current node has asserted leadership.
+	Asserted() bool
+
+	// Term returns the term in which the leadership was asserted.
+	Term() uint64
+}
+
 // ConfigurationFuture is used for GetConfiguration and can return the
 // latest configuration in use by Raft.
 type ConfigurationFuture interface {
@@ -243,6 +255,20 @@ type verifyFuture struct {
 	voteLock   sync.Mutex
 }
 
+type assertedFuture struct {
+	deferError
+	asserted bool
+	term     uint64
+}
+
+func (a *assertedFuture) Asserted() bool {
+	return a.asserted
+}
+
+func (a *assertedFuture) Term() uint64 {
+	return a.term
+}
+
 // leadershipTransferFuture is used to track the progress of a leadership
 // transfer internally.
 type leadershipTransferFuture struct {

raft.go

@@ -185,6 +185,11 @@ func (r *Raft) runFollower() {
 			// Reject any operations since we are not the leader
 			v.respond(ErrNotLeader)
 
+		case v := <-r.assertedCh:
+			r.mainThreadSaturation.working()
+			// Reject any operations since we are not the leader
+			v.respond(ErrNotLeader)
+
 		case ur := <-r.userRestoreCh:
 			r.mainThreadSaturation.working()
 			// Reject any restores since we are not the leader
@@ -398,6 +403,11 @@ func (r *Raft) runCandidate() {
 			// Reject any operations since we are not the leader
 			v.respond(ErrNotLeader)
 
+		case v := <-r.assertedCh:
+			r.mainThreadSaturation.working()
+			// Reject any operations since we are not the leader
+			v.respond(ErrNotLeader)
+
 		case ur := <-r.userRestoreCh:
 			r.mainThreadSaturation.working()
 			// Reject any restores since we are not the leader
@@ -676,6 +686,10 @@ func (r *Raft) leaderLoop() {
 	// based on the current config value.
 	lease := time.After(r.config().LeaderLeaseTimeout)
 
+	// Track whether leadership has been asserted. This becomes true once
+	// the leader has committed at least one log in this term.
+	leadershipAsserted := false
+
 	for r.getState() == Leader {
 		r.mainThreadSaturation.sleeping()
 
@@ -788,6 +802,7 @@ func (r *Raft) leaderLoop() {
 			oldCommitIndex := r.getCommitIndex()
 			commitIndex := r.leaderState.commitment.getCommitIndex()
 			r.setCommitIndex(commitIndex)
+			leadershipAsserted = true
 
 			// New configuration has been committed, set it as the committed
 			// value.
@@ -869,6 +884,12 @@ func (r *Raft) leaderLoop() {
 				v.respond(nil)
 			}
 
+		case v := <-r.assertedCh:
+			r.mainThreadSaturation.working()
+			v.asserted = leadershipAsserted
+			v.term = r.getCurrentTerm()
+			v.respond(nil)
+
 		case future := <-r.userRestoreCh:
 			r.mainThreadSaturation.working()
 			if r.getLeadershipTransferInProgress() {

raft_test.go

@@ -1971,6 +1971,87 @@ func TestRaft_VerifyLeader_PartialConnect(t *testing.T) {
 	}
 }
 
+func TestRaft_VerifyAssertedLeadership_Single(t *testing.T) {
+	// Make the cluster
+	c := MakeCluster(1, t, nil)
+	defer c.Close()
+
+	// Get the leader
+	leader := c.Leader()
+
+	time.Sleep(c.propagateTimeout)
+
+	// Check that leadership has been asserted.
+	asserted := leader.VerifyAssertedLeadership()
+
+	// Wait for the asserted to return
+	if err := asserted.Error(); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Verify that leadership has been asserted for the
+	// correct term.
+	if asserted.Asserted() != true {
+		t.Fatalf("expected leadership to be asserted")
+	}
+	if asserted.Term() != leader.getCurrentTerm() {
+		t.Fatalf("expected term %d, got %d", leader.getCurrentTerm(), asserted.Term())
+	}
+}
+
+func TestRaft_VerifyAssertedLeadership(t *testing.T) {
+	// Make the cluster
+	c := MakeCluster(3, t, nil)
+	defer c.Close()
+
+	// Get the leader
+	leader := c.Leader()
+
+	time.Sleep(c.propagateTimeout)
+
+	// Check that leadership has been asserted.
+	asserted := leader.VerifyAssertedLeadership()
+
+	// Wait for the asserted to return
+	if err := asserted.Error(); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Verify that leadership has been asserted for the
+	// correct term.
+	if asserted.Asserted() != true {
+		t.Fatalf("expected leadership to be asserted")
+	}
+	if asserted.Term() != leader.getCurrentTerm() {
+		t.Fatalf("expected term %d, got %d", leader.getCurrentTerm(), asserted.Term())
+	}
+}
+
+func TestRaft_VerifyAssertedLeadership_Fail(t *testing.T) {
+	// Make the cluster
+	c := MakeCluster(3, t, nil)
+	defer c.Close()
+
+	// Get the leader
+	leader := c.Leader()
+
+	time.Sleep(c.propagateTimeout)
+
+	// Check that leadership has been asserted.
+	asserted := leader.VerifyAssertedLeadership()
+
+	// Wait for the asserted to return
+	if err := asserted.Error(); err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Confirm that an error is received when called on a Follower.
+	follower := c.Followers()[0]
+	if err := follower.VerifyAssertedLeadership().Error(); err == nil {
+		t.Fatal("expected error asserting leadership on a Follower")
+	}
+}
+
 func TestRaft_NotifyCh(t *testing.T) {
 	ch := make(chan bool, 1)
 	conf := inmemConfig(t)