Fix close deadlock (variant A)

Implement

.travis.yml

@@ -0,0 +1,30 @@
+os:
+  - linux
+
+language: go
+
+go:
+  - 1.12.x
+
+env:
+  global:
+    - GOTFLAGS="-race"
+  matrix:
+    - BUILD_DEPTYPE=gomod
+
+
+# disable travis install
+install:
+  - true
+
+script:
+  - bash <(curl -s https://raw.githubusercontent.com/ipfs/ci-helpers/master/travis-ci/run-standard-tests.sh)
+
+
+cache:
+  directories:
+    - $GOPATH/pkg/mod
+    - $HOME/.cache/go-build
+
+notifications:
+  email: false

basic.go

@@ -1,4 +1,4 @@
-package event
+package eventbus
 
 import (
 	"errors"
@@ -6,64 +6,104 @@ import (
 	"reflect"
 	"sync"
 	"sync/atomic"
+
+	"github.com/libp2p/go-libp2p-core/event"
 )
 
 ///////////////////////
 // BUS
 
-type bus struct {
-	lk sync.Mutex
-	nodes map[string]*node
+// basicBus is a type-based event delivery system
+type basicBus struct {
+	lk    sync.Mutex
+	nodes map[reflect.Type]*node
 }
 
-func NewBus() Bus {
-	return &bus{
-		nodes: map[string]*node{},
+var _ event.Bus = (*basicBus)(nil)
+
+type Emitter struct {
+	n       *node
+	typ     reflect.Type
+	closed  int32
+	dropper func(reflect.Type)
+}
+
+func (e *Emitter) Emit(evt interface{}) {
+	if atomic.LoadInt32(&e.closed) != 0 {
+		panic("emitter is closed")
+	}
+	e.n.emit(evt)
+}
+
+func (e *Emitter) Close() error {
+	if !atomic.CompareAndSwapInt32(&e.closed, 0, 1) {
+		panic("closed an emitter more than once")
+	}
+	if atomic.AddInt32(&e.n.nEmitters, -1) == 0 {
+		e.dropper(e.typ)
+	}
+	return nil
+}
+
+func NewBus() event.Bus {
+	return &basicBus{
+		nodes: map[reflect.Type]*node{},
 	}
 }
 
-func (b *bus) withNode(typ reflect.Type, cb func(*node)) error {
-	path := typePath(typ)
-
+func (b *basicBus) withNode(typ reflect.Type, cb func(*node), async func(*node)) error {
 	b.lk.Lock()
 
-	n, ok := b.nodes[path]
+	n, ok := b.nodes[typ]
 	if !ok {
 		n = newNode(typ)
-		b.nodes[path] = n
+		b.nodes[typ] = n
 	}
 
 	n.lk.Lock()
 	b.lk.Unlock()
-	defer n.lk.Unlock()
+
 	cb(n)
+
+	go func() {
+		defer n.lk.Unlock()
+		async(n)
+	}()
+
 	return nil
 }
 
-func (b *bus) tryDropNode(typ reflect.Type) {
-	path := typePath(typ)
-
+func (b *basicBus) tryDropNode(typ reflect.Type) {
 	b.lk.Lock()
-	n, ok := b.nodes[path]
+	n, ok := b.nodes[typ]
 	if !ok { // already dropped
 		b.lk.Unlock()
 		return
 	}
 
 	n.lk.Lock()
-	if n.nEmitters > 0 || len(n.sinks) > 0 {
+	if atomic.LoadInt32(&n.nEmitters) > 0 || len(n.sinks) > 0 {
 		n.lk.Unlock()
 		b.lk.Unlock()
 		return // still in use
 	}
 	n.lk.Unlock()
 
-	delete(b.nodes, path)
+	delete(b.nodes, typ)
 	b.lk.Unlock()
 }
 
-func (b *bus) Subscribe(typedChan interface{}, opts ...SubOption) (c CancelFunc, err error) {
-	var settings SubSettings
+// Subscribe creates new subscription. Failing to drain the channel will cause
+// publishers to get blocked. CancelFunc is guaranteed to return after last send
+// to the channel
+//
+// Example:
+// ch := make(chan EventT, 10)
+// defer close(ch)
+// cancel, err := eventbus.Subscribe(ch)
+// defer cancel()
+func (b *basicBus) Subscribe(typedChan interface{}, opts ...event.SubscriptionOpt) (c event.CancelFunc, err error) {
+	var settings subSettings
 	for _, opt := range opts {
 		if err := opt(&settings); err != nil {
 			return nil, err
@@ -75,7 +115,7 @@ func (b *bus) Subscribe(typedChan interface{}, opts ...SubOption) (c CancelFunc,
 	if typ.Kind() != reflect.Chan {
 		return nil, errors.New("expected a channel")
 	}
-	if typ.ChanDir() & reflect.SendDir == 0 {
+	if typ.ChanDir()&reflect.SendDir == 0 {
 		return nil, errors.New("channel doesn't allow send")
 	}
 
@@ -87,47 +127,64 @@ func (b *bus) Subscribe(typedChan interface{}, opts ...SubOption) (c CancelFunc,
 	}
 
 	err = b.withNode(typ.Elem(), func(n *node) {
-		// when all subs are waiting on this channel, setting this to 1 doesn't
-		// really affect benchmarks
-		i := n.sub(refCh)
+		n.sinks = append(n.sinks, refCh)
 		c = func() {
 			n.lk.Lock()
-			delete(n.sinks, i)
-			tryDrop := len(n.sinks) == 0 && n.nEmitters == 0
+			for i := 0; i < len(n.sinks); i++ {
+				if n.sinks[i] == refCh {
+					n.sinks[i], n.sinks[len(n.sinks)-1] = n.sinks[len(n.sinks)-1], reflect.Value{}
+					n.sinks = n.sinks[:len(n.sinks)-1]
+					break
+				}
+			}
+			tryDrop := len(n.sinks) == 0 && atomic.LoadInt32(&n.nEmitters) == 0
 			n.lk.Unlock()
 			if tryDrop {
 				b.tryDropNode(typ.Elem())
 			}
 		}
+	}, func(n *node) {
+		if n.keepLast {
+			lastVal, ok := n.last.Load().(reflect.Value)
+			if !ok {
+				return
+			}
+
+			refCh.Send(lastVal)
+		}
 	})
 	return
 }
 
-func (b *bus) Emitter(evtType interface{}, _ ...EmitterOption) (e EmitFunc, c CancelFunc, err error) {
+// Emitter creates new emitter
+//
+// eventType accepts typed nil pointers, and uses the type information to
+// select output type
+//
+// Example:
+// emit, err := eventbus.Emitter(new(EventT))
+// defer emit.Close() // MUST call this after being done with the emitter
+//
+// emit(EventT{})
+func (b *basicBus) Emitter(evtType interface{}, opts ...event.EmitterOpt) (e event.Emitter, err error) {
+	var settings emitterSettings
+	for _, opt := range opts {
+		if err := opt(&settings); err != nil {
+			return nil, err
+		}
+	}
+
 	typ := reflect.TypeOf(evtType)
 	if typ.Kind() != reflect.Ptr {
-		return nil, nil, errors.New("emitter called with non-pointer type")
+		return nil, errors.New("emitter called with non-pointer type")
 	}
 	typ = typ.Elem()
 
 	err = b.withNode(typ, func(n *node) {
 		atomic.AddInt32(&n.nEmitters, 1)
-		closed := false
-
-		e = func(event interface{}) {
-			if closed {
-				panic("emitter is closed")
-			}
-			n.emit(event)
-		}
-
-		c = func() {
-			closed = true
-			if atomic.AddInt32(&n.nEmitters, -1) == 0 {
-				b.tryDropNode(typ)
-			}
-		}
-	})
+		n.keepLast = n.keepLast || settings.makeStateful
+		e = &Emitter{n: n, typ: typ, dropper: b.tryDropNode}
+	}, func(_ *node) {})
 	return
 }
 
@@ -135,7 +192,7 @@ func (b *bus) Emitter(evtType interface{}, _ ...EmitterOption) (e EmitFunc, c Ca
 // NODE
 
 type node struct {
-	// Note: make sure to NEVER lock bus.lk when this lock is held
+	// Note: make sure to NEVER lock basicBus.lk when this lock is held
 	lk sync.RWMutex
 
 	typ reflect.Type
@@ -143,29 +200,18 @@ type node struct {
 	// emitter ref count
 	nEmitters int32
 
-	// sink index counter
-	sinkC     int
+	keepLast bool
+	last     atomic.Value
 
-	// TODO: we could make emit a bit faster by making this into an array, but
-	//  it doesn't seem needed for now
-	sinks  map[int]reflect.Value
+	sinks []reflect.Value
 }
 
 func newNode(typ reflect.Type) *node {
 	return &node{
 		typ: typ,
-
-		sinks: map[int]reflect.Value{},
 	}
 }
 
-func (n *node) sub(outChan reflect.Value) int {
-	i := n.sinkC
-	n.sinkC++
-	n.sinks[i] = outChan
-	return i
-}
-
 func (n *node) emit(event interface{}) {
 	eval := reflect.ValueOf(event)
 	if eval.Type() != n.typ {
@@ -173,18 +219,16 @@ func (n *node) emit(event interface{}) {
 	}
 
 	n.lk.RLock()
-	// TODO: try using reflect.Select
-	for _, ch := range n.sinks {
+	if n.keepLast {
+		n.last.Store(eval)
+	}
+
+	sinks := make([]reflect.Value, len(n.sinks))
+	copy(sinks, n.sinks)
+
+	n.lk.RUnlock()
+
+	for _, ch := range sinks {
 		ch.Send(eval)
 	}
-	n.lk.RUnlock()
 }
-
-///////////////////////
-// UTILS
-
-func typePath(t reflect.Type) string {
-	return t.PkgPath() + "/" + t.String()
-}
-
-var _ Bus = &bus{}

basic_test.go

@@ -1,4 +1,4 @@
-package event
+package eventbus
 
 import (
 	"fmt"
@@ -6,11 +6,21 @@ import (
 	"sync/atomic"
 	"testing"
 	"time"
+
+	"github.com/jbenet/go-detect-race"
 )
 
 type EventA struct{}
 type EventB int
 
+func getN() int {
+	n := 50000
+	if detectrace.WithRace() {
+		n = 1000
+	}
+	return n
+}
+
 func (EventA) String() string {
 	return "Oh, Hello"
 }
@@ -28,13 +38,13 @@ func TestEmit(t *testing.T) {
 		<-events
 	}()
 
-	emit, cancel, err := bus.Emitter(new(EventA))
+	em, err := bus.Emitter(new(EventA))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cancel()
+	defer em.Close()
 
-	emit(EventA{})
+	em.Emit(EventA{})
 }
 
 func TestSub(t *testing.T) {
@@ -56,13 +66,13 @@ func TestSub(t *testing.T) {
 		wait.Done()
 	}()
 
-	emit, cancel, err := bus.Emitter(new(EventB))
+	em, err := bus.Emitter(new(EventB))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cancel()
+	defer em.Close()
 
-	emit(EventB(7))
+	em.Emit(EventB(7))
 	wait.Wait()
 
 	if event != 7 {
@@ -73,23 +83,23 @@ func TestSub(t *testing.T) {
 func TestEmitNoSubNoBlock(t *testing.T) {
 	bus := NewBus()
 
-	emit, cancel, err := bus.Emitter(new(EventA))
+	em, err := bus.Emitter(new(EventA))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cancel()
+	defer em.Close()
 
-	emit(EventA{})
+	em.Emit(EventA{})
 }
 
 func TestEmitOnClosed(t *testing.T) {
 	bus := NewBus()
 
-	emit, cancel, err := bus.Emitter(new(EventA))
+	em, err := bus.Emitter(new(EventA))
 	if err != nil {
 		t.Fatal(err)
 	}
-	cancel()
+	em.Close()
 
 	defer func() {
 		r := recover()
@@ -101,12 +111,12 @@ func TestEmitOnClosed(t *testing.T) {
 		}
 	}()
 
-	emit(EventA{})
+	em.Emit(EventA{})
 }
 
 func TestClosingRaces(t *testing.T) {
-	subs := 50000
-	emits := 50000
+	subs := getN()
+	emits := getN()
 
 	var wg sync.WaitGroup
 	var lk sync.RWMutex
@@ -133,9 +143,9 @@ func TestClosingRaces(t *testing.T) {
 			lk.RLock()
 			defer lk.RUnlock()
 
-			_, cancel, _ := b.Emitter(new(EventA))
+			emit, _ := b.Emitter(new(EventA))
 			time.Sleep(10 * time.Millisecond)
-			cancel()
+			emit.Close()
 
 			wg.Done()
 		}()
@@ -146,7 +156,7 @@ func TestClosingRaces(t *testing.T) {
 
 	wg.Wait()
 
-	if len(b.(*bus).nodes) != 0 {
+	if len(b.(*basicBus).nodes) != 0 {
 		t.Error("expected no nodes")
 	}
 }
@@ -156,7 +166,7 @@ func TestSubMany(t *testing.T) {
 
 	var r int32
 
-	n := 50000
+	n := getN()
 	var wait sync.WaitGroup
 	var ready sync.WaitGroup
 	wait.Add(n)
@@ -177,18 +187,18 @@ func TestSubMany(t *testing.T) {
 		}()
 	}
 
-	emit, cancel, err := bus.Emitter(new(EventB))
+	em, err := bus.Emitter(new(EventB))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cancel()
+	defer em.Close()
 
 	ready.Wait()
 
-	emit(EventB(7))
+	em.Emit(EventB(7))
 	wait.Wait()
 
-	if int(r) != 7 * n {
+	if int(r) != 7*n {
 		t.Error("got wrong result")
 	}
 }
@@ -212,13 +222,13 @@ func TestSubType(t *testing.T) {
 		wait.Done()
 	}()
 
-	emit, cancel, err := bus.Emitter(new(EventA))
+	em, err := bus.Emitter(new(EventA))
 	if err != nil {
 		t.Fatal(err)
 	}
-	defer cancel()
+	defer em.Close()
 
-	emit(EventA{})
+	em.Emit(EventA{})
 	wait.Wait()
 
 	if event.String() != "Oh, Hello" {
@@ -226,7 +236,101 @@ func TestSubType(t *testing.T) {
 	}
 }
 
-func testMany(t testing.TB, subs, emits, msgs int) {
+func TestNonStateful(t *testing.T) {
+	bus := NewBus()
+	em, err := bus.Emitter(new(EventB))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer em.Close()
+
+	eventsA := make(chan EventB, 1)
+	cancelS, err := bus.Subscribe(eventsA)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cancelS()
+
+	select {
+	case <-eventsA:
+		t.Fatal("didn't expect to get an event")
+	default:
+	}
+
+	em.Emit(EventB(1))
+
+	select {
+	case e := <-eventsA:
+		if e != 1 {
+			t.Fatal("got wrong event")
+		}
+	default:
+		t.Fatal("expected to get an event")
+	}
+
+	eventsB := make(chan EventB, 1)
+	cancelS2, err := bus.Subscribe(eventsB)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cancelS2()
+
+	select {
+	case <-eventsA:
+		t.Fatal("didn't expect to get an event")
+	default:
+	}
+}
+
+func TestStateful(t *testing.T) {
+	bus := NewBus()
+	em, err := bus.Emitter(new(EventB), Stateful)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer em.Close()
+
+	em.Emit(EventB(2))
+
+	eventsA := make(chan EventB, 1)
+	cancelS, err := bus.Subscribe(eventsA)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer cancelS()
+
+	if <-eventsA != 2 {
+		t.Fatal("got wrong event")
+	}
+}
+
+func TestCloseBlocking(t *testing.T) {
+	bus := NewBus()
+	em, err := bus.Emitter(new(EventB))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	events := make(chan EventB)
+	cancel, err := bus.Subscribe(events)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	go func() {
+		em.Emit(EventB(159))
+	}()
+
+	time.Sleep(10 * time.Millisecond) // make sure that emit is blocked
+
+	cancel() // cancel sub
+}
+
+func testMany(t testing.TB, subs, emits, msgs int, stateful bool) {
+	if detectrace.WithRace() && subs+emits > 5000 {
+		t.SkipNow()
+	}
+
 	bus := NewBus()
 
 	var r int64
@@ -246,7 +350,7 @@ func testMany(t testing.TB, subs, emits, msgs int) {
 			defer cancel()
 
 			ready.Done()
-			for i := 0; i < emits * msgs; i++ {
+			for i := 0; i < emits*msgs; i++ {
 				atomic.AddInt64(&r, int64(<-events))
 			}
 			wait.Done()
@@ -255,16 +359,19 @@ func testMany(t testing.TB, subs, emits, msgs int) {
 
 	for i := 0; i < emits; i++ {
 		go func() {
-			emit, cancel, err := bus.Emitter(new(EventB))
+			em, err := bus.Emitter(new(EventB), func(settings interface{}) error {
+				settings.(*emitterSettings).makeStateful = stateful
+				return nil
+			})
 			if err != nil {
 				panic(err)
 			}
-			defer cancel()
+			defer em.Close()
 
 			ready.Wait()
 
 			for i := 0; i < msgs; i++ {
-				emit(EventB(97))
+				em.Emit(EventB(97))
 			}
 
 			wait.Done()
@@ -273,66 +380,78 @@ func testMany(t testing.TB, subs, emits, msgs int) {
 
 	wait.Wait()
 
-	if int(r) != 97 * subs * emits * msgs {
+	if int(r) != 97*subs*emits*msgs {
 		t.Fatal("got wrong result")
 	}
 }
 
 func TestBothMany(t *testing.T) {
-	testMany(t, 10000, 100, 10)
+	testMany(t, 10000, 100, 10, false)
 }
 
 func BenchmarkSubs(b *testing.B) {
 	b.ReportAllocs()
-	testMany(b, b.N, 100, 100)
+	testMany(b, b.N, 100, 100, false)
 }
 
 func BenchmarkEmits(b *testing.B) {
 	b.ReportAllocs()
-	testMany(b, 100, b.N, 100)
+	testMany(b, 100, b.N, 100, false)
 }
 
 func BenchmarkMsgs(b *testing.B) {
 	b.ReportAllocs()
-	testMany(b, 100, 100, b.N)
+	testMany(b, 100, 100, b.N, false)
 }
 
 func BenchmarkOneToMany(b *testing.B) {
 	b.ReportAllocs()
-	testMany(b, b.N, 1, 100)
+	testMany(b, b.N, 1, 100, false)
 }
 
 func BenchmarkManyToOne(b *testing.B) {
 	b.ReportAllocs()
-	testMany(b, 1, b.N, 100)
+	testMany(b, 1, b.N, 100, false)
 }
 
 func BenchmarkMs1e2m4(b *testing.B) {
 	b.N = 1000000
 	b.ReportAllocs()
-	testMany(b, 10, 100, 10000)
+	testMany(b, 10, 100, 10000, false)
 }
 
 func BenchmarkMs1e0m6(b *testing.B) {
 	b.N = 10000000
 	b.ReportAllocs()
-	testMany(b, 10, 1, 1000000)
+	testMany(b, 10, 1, 1000000, false)
 }
 
 func BenchmarkMs0e0m6(b *testing.B) {
 	b.N = 1000000
 	b.ReportAllocs()
-	testMany(b, 1, 1, 1000000)
+	testMany(b, 1, 1, 1000000, false)
+}
+
+func BenchmarkStatefulMs1e0m6(b *testing.B) {
+	b.N = 10000000
+	b.ReportAllocs()
+	testMany(b, 10, 1, 1000000, true)
+}
+
+func BenchmarkStatefulMs0e0m6(b *testing.B) {
+	b.N = 1000000
+	b.ReportAllocs()
+	testMany(b, 1, 1, 1000000, true)
 }
 
 func BenchmarkMs0e6m0(b *testing.B) {
 	b.N = 1000000
 	b.ReportAllocs()
-	testMany(b, 1, 1000000, 1)
+	testMany(b, 1, 1000000, 1, false)
 }
 
 func BenchmarkMs6e0m0(b *testing.B) {
 	b.N = 1000000
 	b.ReportAllocs()
-	testMany(b, 1000000, 1, 1)
+	testMany(b, 1000000, 1, 1, false)
 }

codecov.yml

@@ -0,0 +1,3 @@
+coverage:
+  range: "50...100"
+comment: off

go.mod

@@ -1,3 +1,8 @@
 module github.com/libp2p/go-eventbus
 
 go 1.12
+
+require (
+	github.com/jbenet/go-detect-race v0.0.0-20150302022421-3463798d9574
+	github.com/libp2p/go-libp2p-core v0.0.4-0.20190619224540-208efcb902d4
+)

go.sum

@@ -0,0 +1,79 @@
+github.com/aead/siphash v1.0.1/go.mod h1:Nywa3cDsYNNK3gaciGTWPwHt0wlpNV15vwmswBAUSII=
+github.com/btcsuite/btcd v0.0.0-20190523000118-16327141da8c h1:aEbSeNALREWXk0G7UdNhR3ayBV7tZ4M2PNmnrCAph6Q=
+github.com/btcsuite/btcd v0.0.0-20190523000118-16327141da8c/go.mod h1:3J08xEfcugPacsc34/LKRU2yO7YmuT8yt28J8k2+rrI=
+github.com/btcsuite/btclog v0.0.0-20170628155309-84c8d2346e9f/go.mod h1:TdznJufoqS23FtqVCzL0ZqgP5MqXbb4fg/WgDys70nA=
+github.com/btcsuite/btcutil v0.0.0-20190425235716-9e5f4b9a998d/go.mod h1:+5NJ2+qvTyV9exUAL/rxXi3DcLg2Ts+ymUAY5y4NvMg=
+github.com/btcsuite/go-socks v0.0.0-20170105172521-4720035b7bfd/go.mod h1:HHNXQzUsZCxOoE+CPiyCTO6x34Zs86zZUiwtpXoGdtg=
+github.com/btcsuite/goleveldb v0.0.0-20160330041536-7834afc9e8cd/go.mod h1:F+uVaaLLH7j4eDXPRvw78tMflu7Ie2bzYOH4Y8rRKBY=
+github.com/btcsuite/snappy-go v0.0.0-20151229074030-0bdef8d06723/go.mod h1:8woku9dyThutzjeg+3xrA5iCpBRH8XEEg3lh6TiUghc=
+github.com/btcsuite/websocket v0.0.0-20150119174127-31079b680792/go.mod h1:ghJtEyQwv5/p4Mg4C0fgbePVuGr935/5ddU9Z3TmDRY=
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interface.go

@@ -1,50 +0,0 @@
-package event
-
-import (
-	"errors"
-	"reflect"
-)
-
-type SubSettings struct {
-	forcedType reflect.Type
-}
-type SubOption func(*SubSettings) error
-
-func ForceSubType(evtType interface{}) SubOption {
-	return func(s *SubSettings) error {
-		typ := reflect.TypeOf(evtType)
-		if typ.Kind() != reflect.Ptr {
-			return errors.New("ForceSubType called with non-pointer type")
-		}
-		s.forcedType = typ
-		return nil
-	}
-}
-
-type EmitterSettings struct {}
-type EmitterOption func(*EmitterSettings)
-
-type Bus interface {
-	// Subscribe creates new subscription. Failing to drain the incoming channel
-	// will cause publishers to get blocked
-	//
-	// evtTypes only accepts typed nil pointers, and uses the type information to
-	// select output type
-	//
-	// Example:
-	// sub, cancel, err := eventbus.Subscribe(new(os.Signal))
-	// defer cancel()
-	//
-	// evt := (<-sub).(os.Signal) // guaranteed to be safe
-	Subscribe(typedChan interface{}, opts ...SubOption) (CancelFunc, error)
-
-	Emitter(eventType interface{}, opts ...EmitterOption) (EmitFunc, CancelFunc, error)
-}
-
-// EmitFunc emits events. If any channel subscribed to the topic is blocked,
-// calls to EmitFunc will block
-//
-// Calling this function with wrong event type will cause a panic
-type EmitFunc func(event interface{})
-
-type CancelFunc func()

opts.go

@@ -0,0 +1,56 @@
+package eventbus
+
+import (
+	"errors"
+	"reflect"
+
+	"github.com/libp2p/go-libp2p-core/event"
+)
+
+type subSettings struct {
+	forcedType reflect.Type
+}
+
+// ForceSubType is a Subscribe option which overrides the type to which
+// the subscription will be done. Note that the evtType must be assignable
+// to channel type.
+//
+// This also allows for subscribing to multiple eventbus channels with one
+// Go channel to get better ordering guarantees.
+//
+// Example:
+// type Event struct{}
+// func (Event) String() string {
+//    return "event"
+// }
+//
+// eventCh := make(chan fmt.Stringer) // interface { String() string }
+// cancel, err := eventbus.Subscribe(eventCh, event.ForceSubType(new(Event)))
+// [...]
+func ForceSubType(evtType interface{}) event.SubscriptionOpt {
+	return func(settings interface{}) error {
+		s := settings.(*subSettings)
+		typ := reflect.TypeOf(evtType)
+		if typ.Kind() != reflect.Ptr {
+			return errors.New("ForceSubType called with non-pointer type")
+		}
+		s.forcedType = typ
+		return nil
+	}
+}
+
+type emitterSettings struct {
+	makeStateful bool
+}
+
+// Stateful is an Emitter option which makes makes the eventbus channel
+// 'remember' last event sent, and when a new subscriber joins the
+// bus, the remembered event is immediately sent to the subscription
+// channel.
+//
+// This allows to provide state tracking for dynamic systems, and/or
+// allows new subscribers to verify that there are Emitters on the channel
+func Stateful(s interface{}) error {
+	s.(*emitterSettings).makeStateful = true
+	return nil
+}