raft: refactor network simulator

Signed-off-by: Blake Mizerany <blake.mizerany@gmail.com>

raft/node.go

@@ -2,6 +2,7 @@ package raft
 
 type Interface interface {
 	Step(m Message)
+	Msgs() []Message
 }
 
 type tick int
@@ -14,18 +15,15 @@ type Node struct {
 	// elapsed ticks after the last reset
 	elapsed tick
 	sm      *stateMachine
-
-	next Interface
 }
 
-func New(k, addr int, heartbeat, election tick, next Interface) *Node {
+func New(k, addr int, heartbeat, election tick) *Node {
 	if election < heartbeat*3 {
 		panic("election is least three times as heartbeat [election: %d, heartbeat: %d]")
 	}
 
 	n := &Node{
 		sm:        newStateMachine(k, addr),
-		next:      next,
 		heartbeat: heartbeat,
 		election:  election,
 	}
@@ -39,10 +37,14 @@ func (n *Node) Propose(data []byte) {
 	n.Step(m)
 }
 
+func (n *Node) Msgs() []Message {
+	return n.sm.Msgs()
+}
+
 func (n *Node) Step(m Message) {
+	l := len(n.sm.msgs)
 	n.sm.Step(m)
-	ms := n.sm.Msgs()
-	for _, m := range ms {
+	for _, m := range n.sm.msgs[l:] {
 		// reset elapsed in two cases:
 		// msgAppResp -> heard from the leader of the same term
 		// msgVoteResp with grant -> heard from the candidate the node voted for
@@ -54,7 +56,6 @@ func (n *Node) Step(m Message) {
 				n.elapsed = 0
 			}
 		}
-		n.next.Step(m)
 	}
 }
 

raft/node_test.go

@@ -1,6 +1,8 @@
 package raft
 
-import "testing"
+import (
+	"testing"
+)
 
 const (
 	defaultHeartbeat = 1
@@ -8,17 +10,17 @@ const (
 )
 
 func TestTickMsgHub(t *testing.T) {
-	n := New(3, 0, defaultHeartbeat, defaultElection, nil)
+	n := New(3, 0, defaultHeartbeat, defaultElection)
+
+	for i := 0; i < defaultElection+1; i++ {
+		n.Tick()
+	}
 
 	called := false
-	n.next = stepperFunc(func(m Message) {
+	for _, m := range n.Msgs() {
 		if m.Type == msgVote {
 			called = true
 		}
-	})
-
-	for i := 0; i < defaultElection+1; i++ {
-		n.Tick()
 	}
 
 	if !called {
@@ -28,24 +30,26 @@ func TestTickMsgHub(t *testing.T) {
 
 func TestTickMsgBeat(t *testing.T) {
 	k := 3
-	n := New(k, 0, defaultHeartbeat, defaultElection, nil)
+	n := New(k, 0, defaultHeartbeat, defaultElection)
 
-	called := 0
-	n.next = stepperFunc(func(m Message) {
-		if m.Type == msgApp {
-			called++
-		}
+	n.Step(Message{Type: msgHup}) // become leader please
+	for _, m := range n.Msgs() {
 		if m.Type == msgVote {
 			n.Step(Message{From: 1, Type: msgVoteResp, Index: 1, Term: 1})
 		}
-	})
-
-	n.Step(Message{Type: msgHup}) // become leader please
+	}
 
 	for i := 0; i < defaultHeartbeat+1; i++ {
 		n.Tick()
 	}
 
+	called := 0
+	for _, m := range n.Msgs() {
+		if m.Type == msgApp {
+			called++
+		}
+	}
+
 	// becomeLeader -> k-1 append
 	// msgBeat -> k-1 append
 	w := (k - 1) * 2

raft/raft_test.go

@@ -2,8 +2,7 @@ package raft
 
 import (
 	"bytes"
-	"fmt"
-	"reflect"
+	"math/rand"
 	"testing"
 )
 
@@ -18,34 +17,16 @@ func TestLeaderElection(t *testing.T) {
 		{newNetwork(nil, nopStepper, nopStepper, nil), stateCandidate},
 		{newNetwork(nil, nopStepper, nopStepper, nil, nil), stateLeader},
 
-		// three nodes are have logs further along than 0
-		{
-			newNetwork(
-				nil,
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 1}}}}, nil},
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 2}}}}, nil},
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 1}, {Term: 3}}}}, nil},
-				nil,
-			),
-			stateFollower,
-		},
+		// three logs further along than 0
+		{newNetwork(nil, ents(1), ents(2), ents(1, 3), nil), stateFollower},
 
 		// logs converge
-		{
-			newNetwork(
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 1}}}}, nil},
-				nil,
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 2}}}}, nil},
-				&nsm{stateMachine{log: &log{ents: []Entry{{}, {Term: 1}}}}, nil},
-				nil,
-			),
-			stateLeader,
-		},
+		{newNetwork(ents(1), nil, ents(2), ents(1), nil), stateLeader},
 	}
 
 	for i, tt := range tests {
-		tt.Step(Message{To: 0, Type: msgHup})
-		sm := tt.network.ss[0].(*nsm)
+		tt.send(Message{To: 0, Type: msgHup})
+		sm := tt.network.peers[0].(*stateMachine)
 		if sm.state != tt.state {
 			t.Errorf("#%d: state = %s, want %s", i, sm.state, tt.state)
 		}
@@ -64,33 +45,30 @@ func TestLogReplication(t *testing.T) {
 		{
 			newNetwork(nil, nil, nil),
 			[]Message{
-				Message{To: 0, Type: msgProp, Data: []byte("somedata")},
+				{To: 0, Type: msgProp, Data: []byte("somedata")},
 			},
 			1,
 		},
 		{
 			newNetwork(nil, nil, nil),
 			[]Message{
-				Message{To: 0, Type: msgProp, Data: []byte("somedata")},
-				Message{To: 1, Type: msgHup},
-				Message{To: 1, Type: msgProp, Data: []byte("somedata")},
+				{To: 0, Type: msgProp, Data: []byte("somedata")},
+				{To: 1, Type: msgHup},
+				{To: 1, Type: msgProp, Data: []byte("somedata")},
 			},
 			2,
 		},
 	}
 
 	for i, tt := range tests {
-		tt.tee = stepperFunc(func(m Message) {
-			t.Logf("#%d: m = %+v", i, m)
-		})
-		tt.Step(Message{To: 0, Type: msgHup})
+		tt.send(Message{To: 0, Type: msgHup})
 
 		for _, m := range tt.msgs {
-			tt.Step(m)
+			tt.send(m)
 		}
 
-		for j, ism := range tt.ss {
-			sm := ism.(*nsm)
+		for j, x := range tt.network.peers {
+			sm := x.(*stateMachine)
 
 			if sm.log.committed != tt.wcommitted {
 				t.Errorf("#%d.%d: committed = %d, want %d", i, j, sm.log.committed, tt.wcommitted)
@@ -114,43 +92,33 @@ func TestLogReplication(t *testing.T) {
 
 func TestSingleNodeCommit(t *testing.T) {
 	tt := newNetwork(nil)
-	tt.Step(Message{To: 0, Type: msgHup})
-	tt.Step(Message{To: 0, Type: msgProp, Data: []byte("some data")})
-	tt.Step(Message{To: 0, Type: msgProp, Data: []byte("some data")})
+	tt.send(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgProp, Data: []byte("some data")})
+	tt.send(Message{To: 0, Type: msgProp, Data: []byte("some data")})
 
-	sm := tt.ss[0].(*nsm)
+	sm := tt.peers[0].(*stateMachine)
 	if sm.log.committed != 2 {
 		t.Errorf("committed = %d, want %d", sm.log.committed, 2)
 	}
 }
 
-func TestDualingCandidates(t *testing.T) {
-	a := &nsm{stateMachine{log: defaultLog()}, nil}
-	c := &nsm{stateMachine{log: defaultLog()}, nil}
+func TestDuelingCandidates(t *testing.T) {
+	a := newStateMachine(0, 0) // k, addr are set later
+	c := newStateMachine(0, 0)
 
 	tt := newNetwork(a, nil, c)
+	tt.drop(0, 2, 1.0)
+	tt.drop(2, 0, 1.0)
 
-	heal := false
-	next := stepperFunc(func(m Message) {
-		if heal {
-			tt.Step(m)
-		}
-	})
-	a.next = next
-	c.next = next
-
-	tt.tee = stepperFunc(func(m Message) {
-		t.Logf("m = %+v", m)
-	})
-	tt.Step(Message{To: 0, Type: msgHup})
-	tt.Step(Message{To: 2, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 2, Type: msgHup})
 
-	t.Log("healing")
-	heal = true
-	tt.Step(Message{To: 2, Type: msgHup})
+	tt.drop(0, 2, 0)
+	tt.drop(2, 0, 0)
+	tt.send(Message{To: 2, Type: msgHup})
 
 	tests := []struct {
-		sm    *nsm
+		sm    *stateMachine
 		state stateType
 		term  int
 	}{
@@ -166,52 +134,59 @@ func TestDualingCandidates(t *testing.T) {
 			t.Errorf("#%d: term = %d, want %d", i, g, tt.term)
 		}
 	}
-	if g := diffLogs(defaultLog().ents, tt.logs()); g != nil {
-		for _, diff := range g {
-			t.Errorf("bag log:\n%s", diff)
+
+	base := ltoa(newLog())
+	for i, p := range tt.peers {
+		if sm, ok := p.(*stateMachine); ok {
+			l := ltoa(sm.log)
+			if g := diffu(base, l); g != "" {
+				t.Errorf("#%d: diff:\n%s", i, g)
+			}
+		} else {
+			t.Logf("#%d: empty log", i)
 		}
 	}
 }
 
 func TestCandidateConcede(t *testing.T) {
-	a := &nsm{stateMachine{log: defaultLog()}, nil}
-
-	tt := newNetwork(a, nil, nil)
-	tt.tee = stepperFunc(func(m Message) {
-		t.Logf("m = %+v", m)
-	})
-
-	a.next = nopStepper
+	tt := newNetwork(nil, nil, nil)
+	tt.isolate(0)
 
-	tt.Step(Message{To: 0, Type: msgHup})
-	tt.Step(Message{To: 2, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 2, Type: msgHup})
 
 	// heal the partition
-	a.next = tt
+	tt.recover()
 
 	data := []byte("force follower")
 	// send a proposal to 2 to flush out a msgApp to 0
-	tt.Step(Message{To: 2, Type: msgProp, Data: data})
+	tt.send(Message{To: 2, Type: msgProp, Data: data})
 
+	a := tt.peers[0].(*stateMachine)
 	if g := a.state; g != stateFollower {
 		t.Errorf("state = %s, want %s", g, stateFollower)
 	}
 	if g := a.term; g != 1 {
 		t.Errorf("term = %d, want %d", g, 1)
 	}
-	wantLog := []Entry{{}, {Term: 1, Data: data}}
-	if g := diffLogs(wantLog, tt.logs()); g != nil {
-		for _, diff := range g {
-			t.Errorf("bag log:\n%s", diff)
+	wantLog := ltoa(&log{ents: []Entry{{}, {Term: 1, Data: data}}, committed: 1})
+	for i, p := range tt.peers {
+		if sm, ok := p.(*stateMachine); ok {
+			l := ltoa(sm.log)
+			if g := diffu(wantLog, l); g != "" {
+				t.Errorf("#%d: diff:\n%s", i, g)
+			}
+		} else {
+			t.Logf("#%d: empty log", i)
 		}
 	}
 }
 
 func TestSingleNodeCandidate(t *testing.T) {
 	tt := newNetwork(nil)
-	tt.Step(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgHup})
 
-	sm := tt.ss[0].(*nsm)
+	sm := tt.peers[0].(*stateMachine)
 	if sm.state != stateLeader {
 		t.Errorf("state = %d, want %d", sm.state, stateLeader)
 	}
@@ -220,14 +195,21 @@ func TestSingleNodeCandidate(t *testing.T) {
 func TestOldMessages(t *testing.T) {
 	tt := newNetwork(nil, nil, nil)
 	// make 0 leader @ term 3
-	tt.Step(Message{To: 0, Type: msgHup})
-	tt.Step(Message{To: 1, Type: msgHup})
-	tt.Step(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgHup})
+	tt.send(Message{To: 1, Type: msgHup})
+	tt.send(Message{To: 0, Type: msgHup})
 	// pretend we're an old leader trying to make progress
-	tt.Step(Message{To: 0, Type: msgApp, Term: 1, Entries: []Entry{{Term: 1}}})
-	if g := diffLogs(defaultLog().ents, tt.logs()); g != nil {
-		for _, diff := range g {
-			t.Errorf("bag log:\n%s", diff)
+	tt.send(Message{To: 0, Type: msgApp, Term: 1, Entries: []Entry{{Term: 1}}})
+
+	base := ltoa(newLog())
+	for i, p := range tt.peers {
+		if sm, ok := p.(*stateMachine); ok {
+			l := ltoa(sm.log)
+			if g := diffu(base, l); g != "" {
+				t.Errorf("#%d: diff:\n%s", i, g)
+			}
+		} else {
+			t.Logf("#%d: empty log", i)
 		}
 	}
 }
@@ -247,11 +229,7 @@ func TestProposal(t *testing.T) {
 	}
 
 	for i, tt := range tests {
-		tt.tee = stepperFunc(func(m Message) {
-			t.Logf("#%d: m = %+v", i, m)
-		})
-
-		step := stepperFunc(func(m Message) {
+		send := func(m Message) {
 			defer func() {
 				// only recover is we expect it to panic so
 				// panics we don't expect go up.
@@ -262,27 +240,31 @@ func TestProposal(t *testing.T) {
 					}
 				}
 			}()
-			tt.Step(m)
-		})
+			tt.send(m)
+		}
 
 		data := []byte("somedata")
 
 		// promote 0 the leader
-		step(Message{To: 0, Type: msgHup})
-		step(Message{To: 0, Type: msgProp, Data: data})
+		send(Message{To: 0, Type: msgHup})
+		send(Message{To: 0, Type: msgProp, Data: data})
 
-		var wantLog []Entry
+		wantLog := newLog()
 		if tt.success {
-			wantLog = []Entry{{}, {Term: 1, Data: data}}
-		} else {
-			wantLog = defaultLog().ents
+			wantLog = &log{ents: []Entry{{}, {Term: 1, Data: data}}, committed: 1}
 		}
-		if g := diffLogs(wantLog, tt.logs()); g != nil {
-			for _, diff := range g {
-				t.Errorf("#%d: diff:%s", i, diff)
+		base := ltoa(wantLog)
+		for i, p := range tt.peers {
+			if sm, ok := p.(*stateMachine); ok {
+				l := ltoa(sm.log)
+				if g := diffu(base, l); g != "" {
+					t.Errorf("#%d: diff:\n%s", i, g)
+				}
+			} else {
+				t.Logf("#%d: empty log", i)
 			}
 		}
-		sm := tt.network.ss[0].(*nsm)
+		sm := tt.network.peers[0].(*stateMachine)
 		if g := sm.term; g != 1 {
 			t.Errorf("#%d: term = %d, want %d", i, g, 1)
 		}
@@ -297,23 +279,25 @@ func TestProposalByProxy(t *testing.T) {
 	}
 
 	for i, tt := range tests {
-		tt.tee = stepperFunc(func(m Message) {
-			t.Logf("#%d: m = %+v", i, m)
-		})
-
 		// promote 0 the leader
-		tt.Step(Message{To: 0, Type: msgHup})
+		tt.send(Message{To: 0, Type: msgHup})
 
 		// propose via follower
-		tt.Step(Message{To: 1, Type: msgProp, Data: []byte("somedata")})
-
-		wantLog := []Entry{{}, {Term: 1, Data: data}}
-		if g := diffLogs(wantLog, tt.logs()); g != nil {
-			for _, diff := range g {
-				t.Errorf("#%d: bad entry: %s", i, diff)
+		tt.send(Message{To: 1, Type: msgProp, Data: []byte("somedata")})
+
+		wantLog := &log{ents: []Entry{{}, {Term: 1, Data: data}}, committed: 1}
+		base := ltoa(wantLog)
+		for i, p := range tt.peers {
+			if sm, ok := p.(*stateMachine); ok {
+				l := ltoa(sm.log)
+				if g := diffu(base, l); g != "" {
+					t.Errorf("#%d: diff:\n%s", i, g)
+				}
+			} else {
+				t.Logf("#%d: empty log", i)
 			}
 		}
-		sm := tt.ss[0].(*nsm)
+		sm := tt.peers[0].(*stateMachine)
 		if g := sm.term; g != 1 {
 			t.Errorf("#%d: term = %d, want %d", i, g, 1)
 		}
@@ -391,22 +375,20 @@ func TestVote(t *testing.T) {
 
 	for i, tt := range tests {
 		called := false
-		sm := &nsm{
-			stateMachine{
-				state: tt.state,
-				vote:  tt.voteFor,
-				log:   &log{ents: []Entry{{}, {Term: 2}, {Term: 2}}},
-			},
-			nil,
+		sm := &stateMachine{
+			state: tt.state,
+			vote:  tt.voteFor,
+			log:   &log{ents: []Entry{{}, {Term: 2}, {Term: 2}}},
 		}
 
-		sm.next = stepperFunc(func(m Message) {
+		sm.Step(Message{Type: msgVote, From: 1, Index: tt.i, LogTerm: tt.term})
+
+		for _, m := range sm.Msgs() {
 			called = true
 			if m.Index != tt.w {
 				t.Errorf("#%d, m.Index = %d, want %d", i, m.Index, tt.w)
 			}
-		})
-		sm.Step(Message{Type: msgVote, From: 1, Index: tt.i, LogTerm: tt.term})
+		}
 		if !called {
 			t.Fatal("#%d: not called", i)
 		}
@@ -487,163 +469,89 @@ func TestLeaderAppResp(t *testing.T) {
 	}
 }
 
-func TestLogDiff(t *testing.T) {
-	a := []Entry{{}, {Term: 1}, {Term: 2}}
-	b := []Entry{{}, {Term: 1}, {Term: 2}}
-	c := []Entry{{}, {Term: 2}}
-	d := []Entry(nil)
-
-	w := []diff{
-		diff{1, []*Entry{{Term: 1}, {Term: 1}, {Term: 2}, nilLogEntry}},
-		diff{2, []*Entry{{Term: 2}, {Term: 2}, noEntry, nilLogEntry}},
+func ents(terms ...int) *stateMachine {
+	ents := []Entry{{}}
+	for _, term := range terms {
+		ents = append(ents, Entry{Term: term})
 	}
 
-	if g := diffLogs(a, [][]Entry{b, c, d}); !reflect.DeepEqual(w, g) {
-		t.Errorf("g = %s", g)
-		t.Errorf("want %s", w)
-	}
+	sm := &stateMachine{log: &log{ents: ents}}
+	sm.reset()
+	return sm
 }
 
 type network struct {
-	tee Interface
-	ss  []Interface
+	peers []Interface
+	dropm map[connem]float64
 }
 
-// newNetwork initializes a network from nodes. A nil node will be replaced
-// with a new *stateMachine. A *stateMachine will get its k, addr, and next
-// fields set.
-func newNetwork(nodes ...Interface) *network {
-	nt := &network{ss: nodes}
-	for i, n := range nodes {
-		switch v := n.(type) {
+// newNetwork initializes a network from peers. A nil node will be replaced
+// with a new *stateMachine. A *stateMachine will get its k, addr.
+func newNetwork(peers ...Interface) *network {
+	for addr, p := range peers {
+		switch v := p.(type) {
 		case nil:
-			nt.ss[i] = &nsm{*newStateMachine(len(nodes), i), nt}
-		case *nsm:
-			v.k = len(nodes)
-			v.addr = i
-			if v.next == nil {
-				v.next = nt
-			}
-		default:
-			nt.ss[i] = v
+			sm := newStateMachine(len(peers), addr)
+			peers[addr] = sm
+		case *stateMachine:
+			v.k = len(peers)
+			v.addr = addr
 		}
 	}
-	return nt
+	return &network{peers: peers, dropm: make(map[connem]float64)}
 }
 
-func (nt network) Step(m Message) {
-	if nt.tee != nil {
-		nt.tee.Step(m)
+func (nw *network) send(msgs ...Message) {
+	for len(msgs) > 0 {
+		m := msgs[0]
+		p := nw.peers[m.To]
+		p.Step(m)
+		msgs = append(msgs[1:], nw.filter(p.Msgs())...)
 	}
-	nt.ss[m.To].Step(m)
 }
 
-// logs returns all logs in nt prepended with want. If a node is not a
-// *stateMachine, its log will be nil.
-func (nt network) logs() [][]Entry {
-	ls := make([][]Entry, len(nt.ss))
-	for i, node := range nt.ss {
-		if sm, ok := node.(*nsm); ok {
-			ls[i] = sm.log.ents
+func (nw *network) drop(from, to int, perc float64) {
+	nw.dropm[connem{from, to}] = perc
+}
+
+func (nw *network) isolate(addr int) {
+	for i := 0; i < len(nw.peers); i++ {
+		if i != addr {
+			nw.drop(addr, i, 1.0)
+			nw.drop(i, addr, 1.0)
 		}
 	}
-	return ls
 }
 
-type diff struct {
-	i    int
-	ents []*Entry // pointers so they can be nil for N/A
+func (nw *network) recover() {
+	nw.dropm = make(map[connem]float64)
 }
 
-var noEntry = &Entry{}
-var nilLogEntry = &Entry{}
-
-func (d diff) String() string {
-	s := fmt.Sprintf("[%d] ", d.i)
-	for i, e := range d.ents {
-		switch e {
-		case nilLogEntry:
-			s += fmt.Sprintf("o")
-		case noEntry:
-			s += fmt.Sprintf("-")
-		case nil:
-			s += fmt.Sprintf("<nil>")
+func (nw *network) filter(msgs []Message) []Message {
+	mm := make([]Message, 0)
+	for _, m := range msgs {
+		switch m.Type {
+		case msgHup:
+			// hups never go over the network, so don't drop them but panic
+			panic("unexpected msgHup")
 		default:
-			s += fmt.Sprintf("<%d:%q>", e.Term, string(e.Data))
-		}
-		if i != len(d.ents)-1 {
-			s += "\t\t"
-		}
-	}
-	return s
-}
-
-func diffLogs(base []Entry, logs [][]Entry) []diff {
-	var (
-		d   []diff
-		max int
-	)
-	logs = append([][]Entry{base}, logs...)
-	for _, log := range logs {
-		if l := len(log); l > max {
-			max = l
-		}
-	}
-	ediff := func(i int) (result []*Entry) {
-		e := make([]*Entry, len(logs))
-		found := false
-		for j, log := range logs {
-			if log == nil {
-				e[j] = nilLogEntry
+			perc := nw.dropm[connem{m.From, m.To}]
+			if n := rand.Float64(); n < perc {
 				continue
 			}
-			if len(log) <= i {
-				e[j] = noEntry
-				found = true
-				continue
-			}
-			e[j] = &log[i]
-			if j > 0 {
-				switch prev := e[j-1]; {
-				case prev == nilLogEntry:
-				case prev == noEntry:
-				case !reflect.DeepEqual(prev, e[j]):
-					found = true
-				}
-			}
-		}
-		if found {
-			return e
-		}
-		return nil
-	}
-	for i := 0; i < max; i++ {
-		if e := ediff(i); e != nil {
-			d = append(d, diff{i, e})
 		}
+		mm = append(mm, m)
 	}
-	return d
+	return mm
 }
 
-type stepperFunc func(Message)
-
-func (f stepperFunc) Step(m Message) { f(m) }
-
-var nopStepper = stepperFunc(func(Message) {})
-
-type nsm struct {
-	stateMachine
-	next Interface
+type connem struct {
+	from, to int
 }
 
-func (n *nsm) Step(m Message) {
-	(&n.stateMachine).Step(m)
-	ms := n.Msgs()
-	for _, m := range ms {
-		n.next.Step(m)
-	}
-}
+type blackHole struct{}
 
-func defaultLog() *log {
-	return &log{ents: []Entry{{}}}
-}
+func (blackHole) Step(Message)    {}
+func (blackHole) Msgs() []Message { return nil }
+
+var nopStepper = &blackHole{}