lib/events: Introduce per-subscription event IDs (fixes #3335)

Events API consumers rely on being able to detect that events were skipped
by the fact that the event ID has increased by more than 1. This is
documented, and is absolutely necessary when trying to maintain a local
model of Syncthing's state.

With the introduction of LocalChangeDetected, which is not exposed to the
Events API, this contract was broken.

This commit introduces separate concepts of a "Global ID" and a
"Subscription ID". The Global ID of an event is unique across all
subscriptions. The Subscription ID is local to a particular subscription,
and always increments by 1. They are both exposed over the Events API, but
the Subscription ID uses the key "id" for backwards compatibility, and
the "?since=xx" parameter refers to the Subscription ID (making the Global
ID for information only).

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3351
LGTM: calmh

cmd/syncthing/main.go

@@ -539,7 +539,7 @@ func syncthingMain(runtimeOptions RuntimeOptions) {
 	errors := logger.NewRecorder(l, logger.LevelWarn, maxSystemErrors, 0)
 	systemLog := logger.NewRecorder(l, logger.LevelDebug, maxSystemLog, initialSystemLog)
 
-	// Event subscription for the API; must start early to catch the early events.  The LocalDiskUpdated
+	// Event subscription for the API; must start early to catch the early events.  The LocalChangeDetected
 	// event might overwhelm the event reciever in some situations so we will not subscribe to it here.
 	apiSub := events.NewBufferedSubscription(events.Default.Subscribe(events.AllEvents&^events.LocalChangeDetected), 1000)
 

lib/events/events.go

@@ -112,12 +112,16 @@ const BufferSize = 64
 
 type Logger struct {
 	subs                []*Subscription
-	nextID int
+	nextSubscriptionIDs []int
+	nextGlobalID        int
 	mutex               sync.Mutex
 }
 
 type Event struct {
-	ID   int         `json:"id"`
+	// Per-subscription sequential event ID. Named "id" for backwards compatibility with the REST API
+	SubscriptionID int `json:"id"`
+	// Global ID of the event across all subscriptions
+	GlobalID int         `json:"globalID"`
 	Time     time.Time   `json:"time"`
 	Type     EventType   `json:"type"`
 	Data     interface{} `json:"data"`
@@ -144,16 +148,21 @@ func NewLogger() *Logger {
 
 func (l *Logger) Log(t EventType, data interface{}) {
 	l.mutex.Lock()
-	dl.Debugln("log", l.nextID, t, data)
-	l.nextID++
+	dl.Debugln("log", l.nextGlobalID, t, data)
+	l.nextGlobalID++
+
 	e := Event{
-		ID:   l.nextID,
+		GlobalID: l.nextGlobalID,
 		Time:     time.Now(),
 		Type:     t,
 		Data:     data,
 	}
-	for _, s := range l.subs {
+
+	for i, s := range l.subs {
 		if s.mask&t != 0 {
+			e.SubscriptionID = l.nextSubscriptionIDs[i]
+			l.nextSubscriptionIDs[i]++
+
 			select {
 			case s.events <- e:
 			default:
@@ -182,6 +191,7 @@ func (l *Logger) Subscribe(mask EventType) *Subscription {
 	}
 
 	l.subs = append(l.subs, s)
+	l.nextSubscriptionIDs = append(l.nextSubscriptionIDs, 1)
 	l.mutex.Unlock()
 	return s
 }
@@ -192,9 +202,15 @@ func (l *Logger) Unsubscribe(s *Subscription) {
 	for i, ss := range l.subs {
 		if s == ss {
 			last := len(l.subs) - 1
+
 			l.subs[i] = l.subs[last]
 			l.subs[last] = nil
 			l.subs = l.subs[:last]
+
+			l.nextSubscriptionIDs[i] = l.nextSubscriptionIDs[last]
+			l.nextSubscriptionIDs[last] = 0
+			l.nextSubscriptionIDs = l.nextSubscriptionIDs[:last]
+
 			break
 		}
 	}
@@ -234,7 +250,7 @@ type bufferedSubscription struct {
 	sub  *Subscription
 	buf  []Event
 	next int
-	cur  int
+	cur  int // Current SubscriptionID
 	mut  sync.Mutex
 	cond *stdsync.Cond
 }
@@ -270,7 +286,7 @@ func (s *bufferedSubscription) pollingLoop() {
 		s.mut.Lock()
 		s.buf[s.next] = ev
 		s.next = (s.next + 1) % len(s.buf)
-		s.cur = ev.ID
+		s.cur = ev.SubscriptionID
 		s.cond.Broadcast()
 		s.mut.Unlock()
 	}
@@ -285,12 +301,12 @@ func (s *bufferedSubscription) Since(id int, into []Event) []Event {
 	}
 
 	for i := s.next; i < len(s.buf); i++ {
-		if s.buf[i].ID > id {
+		if s.buf[i].SubscriptionID > id {
 			into = append(into, s.buf[i])
 		}
 	}
 	for i := 0; i < s.next; i++ {
-		if s.buf[i].ID > id {
+		if s.buf[i].SubscriptionID > id {
 			into = append(into, s.buf[i])
 		}
 	}

lib/events/events_test.go

@@ -128,7 +128,7 @@ func TestUnsubscribe(t *testing.T) {
 	}
 }
 
-func TestIDs(t *testing.T) {
+func TestGlobalIDs(t *testing.T) {
 	l := events.NewLogger()
 
 	s := l.Subscribe(events.AllEvents)
@@ -144,7 +144,7 @@ func TestIDs(t *testing.T) {
 	if ev.Data.(string) != "foo" {
 		t.Fatal("Incorrect event:", ev)
 	}
-	id := ev.ID
+	id := ev.GlobalID
 
 	ev, err = s.Poll(timeout)
 	if err != nil {
@@ -153,8 +153,48 @@ func TestIDs(t *testing.T) {
 	if ev.Data.(string) != "bar" {
 		t.Fatal("Incorrect event:", ev)
 	}
-	if ev.ID != id+1 {
-		t.Fatalf("ID not incremented (%d != %d)", ev.ID, id+1)
+	if ev.GlobalID != id+1 {
+		t.Fatalf("ID not incremented (%d != %d)", ev.GlobalID, id+1)
+	}
+}
+
+func TestSubscriptionIDs(t *testing.T) {
+	l := events.NewLogger()
+
+	s := l.Subscribe(events.DeviceConnected)
+	defer l.Unsubscribe(s)
+
+	l.Log(events.DeviceDisconnected, "a")
+	l.Log(events.DeviceConnected, "b")
+	l.Log(events.DeviceConnected, "c")
+	l.Log(events.DeviceDisconnected, "d")
+
+	ev, err := s.Poll(timeout)
+	if err != nil {
+		t.Fatal("Unexpected error:", err)
+	}
+
+	if ev.GlobalID != 2 {
+		t.Fatal("Incorrect GlobalID:", ev.GlobalID)
+	}
+	if ev.SubscriptionID != 1 {
+		t.Fatal("Incorrect SubscriptionID:", ev.SubscriptionID)
+	}
+
+	ev, err = s.Poll(timeout)
+	if err != nil {
+		t.Fatal("Unexpected error:", err)
+	}
+	if ev.GlobalID != 3 {
+		t.Fatal("Incorrect GlobalID:", ev.GlobalID)
+	}
+	if ev.SubscriptionID != 2 {
+		t.Fatal("Incorrect SubscriptionID:", ev.SubscriptionID)
+	}
+
+	ev, err = s.Poll(timeout)
+	if err != events.ErrTimeout {
+		t.Fatal("Unexpected error:", err)
 	}
 }
 
@@ -179,10 +219,10 @@ func TestBufferedSub(t *testing.T) {
 	for recv < 10*events.BufferSize {
 		evs := bs.Since(recv, nil)
 		for _, ev := range evs {
-			if ev.ID != recv+1 {
-				t.Fatalf("Incorrect ID; %d != %d", ev.ID, recv+1)
+			if ev.GlobalID != recv+1 {
+				t.Fatalf("Incorrect ID; %d != %d", ev.GlobalID, recv+1)
 			}
-			recv = ev.ID
+			recv = ev.GlobalID
 		}
 	}
 }
@@ -213,10 +253,10 @@ func BenchmarkBufferedSub(b *testing.B) {
 		for i := 0; i < b.N; {
 			evs = bs.Since(recv, evs[:0])
 			for _, ev := range evs {
-				if ev.ID != recv+1 {
-					b.Fatal("skipped event", ev.ID, recv)
+				if ev.GlobalID != recv+1 {
+					b.Fatal("skipped event", ev.GlobalID, recv)
 				}
-				recv = ev.ID
+				recv = ev.GlobalID
 				coord <- struct{}{}
 			}
 			i += len(evs)
@@ -237,3 +277,26 @@ func BenchmarkBufferedSub(b *testing.B) {
 	<-done
 	b.ReportAllocs()
 }
+
+func TestSinceUsesSubscriptionId(t *testing.T) {
+	l := events.NewLogger()
+
+	s := l.Subscribe(events.DeviceConnected)
+	defer l.Unsubscribe(s)
+	bs := events.NewBufferedSubscription(s, 10*events.BufferSize)
+
+	l.Log(events.DeviceConnected, "a") // SubscriptionID = 1
+	l.Log(events.DeviceDisconnected, "b")
+	l.Log(events.DeviceDisconnected, "c")
+	l.Log(events.DeviceConnected, "d") // SubscriptionID = 2
+
+	events := bs.Since(0, nil)
+	if len(events) != 2 {
+		t.Fatal("Incorrect number of events:", len(events))
+	}
+
+	events = bs.Since(1, nil)
+	if len(events) != 1 {
+		t.Fatal("Incorrect number of events:", len(events))
+	}
+}