Refactor Notifier to return channel (#311)

This has two benefits:

1. Using channels makes it easier to time out while waiting
2. Allows us to clean up goroutines that were waiting if we timeout the
   request

src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go

@@ -17,11 +17,12 @@ package sync
 import (
 	"context"
 	"sync"
+	"time"
 
-	log "github.com/sirupsen/logrus"
 	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
+	log "github.com/sirupsen/logrus"
 )
 
 // Notifier will wake up sleeping requests when there is some new data.
@@ -38,6 +39,8 @@ type Notifier struct {
 	currPos types.StreamPosition
 	// A map of user_id => UserStream which can be used to wake a given user's /sync request.
 	userStreams map[string]*UserStream
+	// The last time we cleaned out stale entries from the userStreams map
+	lastCleanUpTime time.Time
 }
 
 // NewNotifier creates a new notifier set to the given stream position.
@@ -49,6 +52,7 @@ func NewNotifier(pos types.StreamPosition) *Notifier {
 		roomIDToJoinedUsers: make(map[string]userIDSet),
 		userStreams:         make(map[string]*UserStream),
 		streamLock:          &sync.Mutex{},
+		lastCleanUpTime:     time.Now(),
 	}
 }
 
@@ -63,6 +67,8 @@ func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, userID string, pos ty
 	defer n.streamLock.Unlock()
 	n.currPos = pos
 
+	n.removeEmptyUserStreams()
+
 	if ev != nil {
 		// Map this event's room_id to a list of joined users, and wake them up.
 		userIDs := n.joinedUsers(ev.RoomID())
@@ -100,10 +106,10 @@ func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, userID string, pos ty
 	}
 }
 
-// WaitForEvents blocks until there are events for this request after sincePos.
-// In particular, it will return immediately if there are already events after
-// sincePos for the request, but otherwise blocks waiting for new events.
-func (n *Notifier) WaitForEvents(req syncRequest, sincePos types.StreamPosition) types.StreamPosition {
+// GetListener returns a UserStreamListener that can be used to wait for
+// updates for a user. Must be closed.
+// notify for anything before sincePos
+func (n *Notifier) GetListener(req syncRequest) UserStreamListener {
 	// Do what synapse does: https://github.com/matrix-org/synapse/blob/v0.20.0/synapse/notifier.py#L298
 	// - Bucket request into a lookup map keyed off a list of joined room IDs and separately a user ID
 	// - Incoming events wake requests for a matching room ID
@@ -112,25 +118,12 @@ func (n *Notifier) WaitForEvents(req syncRequest, sincePos types.StreamPosition)
 	// TODO: v1 /events 'peeking' has an 'explicit room ID' which is also tracked,
 	//       but given we don't do /events, let's pretend it doesn't exist.
 
-	// In a guard, check if the /sync request should block, and block it until we get woken up
 	n.streamLock.Lock()
-	currentPos := n.currPos
+	defer n.streamLock.Unlock()
 
-	// TODO: We increment the stream position for any event, so it's possible that we return immediately
-	//       with a pos which contains no new events for this user. We should probably re-wait for events
-	//       automatically in this case.
-	if sincePos != currentPos {
-		n.streamLock.Unlock()
-		return currentPos
-	}
+	n.removeEmptyUserStreams()
 
-	// wait to be woken up, and then re-check the stream position
-	req.log.WithField("user_id", req.userID).Info("Waiting for event")
-
-	// give up the stream lock prior to waiting on the user lock
-	stream := n.fetchUserStream(req.userID, true)
-	n.streamLock.Unlock()
-	return stream.Wait(currentPos)
+	return n.fetchUserStream(req.userID, true).GetListener(req.ctx)
 }
 
 // Load the membership states required to notify users correctly.
@@ -178,7 +171,7 @@ func (n *Notifier) fetchUserStream(userID string, makeIfNotExists bool) *UserStr
 	stream, ok := n.userStreams[userID]
 	if !ok && makeIfNotExists {
 		// TODO: Unbounded growth of streams (1 per user)
-		stream = NewUserStream(userID)
+		stream = NewUserStream(userID, n.currPos)
 		n.userStreams[userID] = stream
 	}
 	return stream
@@ -208,6 +201,29 @@ func (n *Notifier) joinedUsers(roomID string) (userIDs []string) {
 	return n.roomIDToJoinedUsers[roomID].values()
 }
 
+// removeEmptyUserStreams iterates through the user stream map and removes any
+// that have been empty for a certain amount of time. This is a crude way of
+// ensuring that the userStreams map doesn't grow forver.
+// This should be called when the notifier gets called for whatever reason,
+// the function itself is responsible for ensuring it doesn't iterate too
+// often.
+// NB: Callers should have locked the mutex before calling this function.
+func (n *Notifier) removeEmptyUserStreams() {
+	// Only clean up  now and again
+	now := time.Now()
+	if n.lastCleanUpTime.Add(time.Minute).After(now) {
+		return
+	}
+	n.lastCleanUpTime = now
+
+	deleteBefore := now.Add(-5 * time.Minute)
+	for key, value := range n.userStreams {
+		if value.TimeOfLastNonEmpty().Before(deleteBefore) {
+			delete(n.userStreams, key)
+		}
+	}
+}
+
 // A string set, mainly existing for improving clarity of structs in this file.
 type userIDSet map[string]bool
 

src/github.com/matrix-org/dendrite/syncapi/sync/notifier_test.go

@@ -256,24 +256,22 @@ func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
 
 // same as Notifier.WaitForEvents but with a timeout.
 func waitForEvents(n *Notifier, req syncRequest) (types.StreamPosition, error) {
-	done := make(chan types.StreamPosition, 1)
-	go func() {
-		newPos := n.WaitForEvents(req, req.since)
-		done <- newPos
-		close(done)
-	}()
+	listener := n.GetListener(req)
+	defer listener.Close()
+
 	select {
 	case <-time.After(5 * time.Second):
 		return types.StreamPosition(0), fmt.Errorf(
 			"waitForEvents timed out waiting for %s (pos=%d)", req.userID, req.since,
 		)
-	case p := <-done:
+	case <-listener.GetNotifyChannel(req.since):
+		p := listener.GetStreamPosition()
 		return p, nil
 	}
 }
 
 // Wait until something is Wait()ing on the user stream.
-func waitForBlocking(s *UserStream, numBlocking int) {
+func waitForBlocking(s *UserStream, numBlocking uint) {
 	for numBlocking != s.NumWaiting() {
 		// This is horrible but I don't want to add a signalling mechanism JUST for testing.
 		time.Sleep(1 * time.Microsecond)
@@ -288,5 +286,6 @@ func newTestSyncRequest(userID string, since types.StreamPosition) syncRequest {
 		wantFullState: false,
 		limit:         defaultTimelineLimit,
 		log:           util.GetLogger(context.TODO()),
+		ctx:           context.TODO(),
 	}
 }

src/github.com/matrix-org/dendrite/syncapi/sync/requestpool.go

@@ -19,7 +19,6 @@ import (
 	"net/http"
 	"time"
 
-	log "github.com/sirupsen/logrus"
 	"github.com/matrix-org/dendrite/clientapi/auth/authtypes"
 	"github.com/matrix-org/dendrite/clientapi/auth/storage/accounts"
 	"github.com/matrix-org/dendrite/clientapi/httputil"
@@ -28,6 +27,7 @@ import (
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 	"github.com/matrix-org/util"
+	log "github.com/sirupsen/logrus"
 )
 
 // RequestPool manages HTTP long-poll connections for /sync
@@ -82,10 +82,14 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	timer := time.NewTimer(syncReq.timeout) // case of timeout=0 is handled above
 	defer timer.Stop()
 
+	userStreamListener := rp.notifier.GetListener(*syncReq)
+	defer userStreamListener.Close()
+
 	for {
 		select {
 		// Wait for notifier to wake us up
-		case currPos = <-rp.makeNotifyChannel(*syncReq, currPos):
+		case <-userStreamListener.GetNotifyChannel(currPos):
+			currPos = userStreamListener.GetStreamPosition()
 		// Or for timeout to expire
 		case <-timer.C:
 			return util.JSONResponse{
@@ -116,24 +120,6 @@ func (rp *RequestPool) OnIncomingSyncRequest(req *http.Request, device *authtype
 	}
 }
 
-// makeNotifyChannel returns a channel that produces the current stream position
-// when there *may* be something to return to the client. Only produces a single
-// value and then closes the channel.
-func (rp *RequestPool) makeNotifyChannel(syncReq syncRequest, sincePos types.StreamPosition) chan types.StreamPosition {
-	notified := make(chan types.StreamPosition)
-
-	// TODO(#303): We need to ensure that WaitForEvents gets properly cancelled
-	// when the request is finished, or use some other mechanism to ensure we
-	// don't leak goroutines here
-	go (func() {
-		currentPos := rp.notifier.WaitForEvents(syncReq, sincePos)
-		notified <- currentPos
-		close(notified)
-	})()
-
-	return notified
-}
-
 type stateEventInStateResp struct {
 	gomatrixserverlib.ClientEvent
 	PrevContent   json.RawMessage `json:"prev_content,omitempty"`

src/github.com/matrix-org/dendrite/syncapi/sync/userstream.go

@@ -15,65 +15,148 @@
 package sync
 
 import (
+	"context"
+	"runtime"
 	"sync"
+	"time"
 
 	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/util"
 )
 
 // UserStream represents a communication mechanism between the /sync request goroutine
-// and the underlying sync server goroutines. Goroutines can Wait() for a stream position and
-// goroutines can Broadcast(streamPosition) to other goroutines.
+// and the underlying sync server goroutines.
+// Goroutines can get a UserStreamListener to wait for updates, and can Broadcast()
+// updates.
 type UserStream struct {
 	UserID string
-	// Because this is a Cond, we can notify all waiting goroutines so this works
-	// across devices for the same user. Protects pos.
-	cond *sync.Cond
-	// The position to broadcast to callers of Wait().
+	// The lock that protects changes to this struct
+	lock sync.Mutex
+	// Closed when there is an update.
+	signalChannel chan struct{}
+	// The last stream position that there may have been an update for the suser
 	pos types.StreamPosition
-	// The number of goroutines blocked on Wait() - used for testing and metrics
-	numWaiting int
+	// The last time when we had some listeners waiting
+	timeOfLastChannel time.Time
+	// The number of listeners waiting
+	numWaiting uint
+}
+
+// UserStreamListener allows a sync request to wait for updates for a user.
+type UserStreamListener struct {
+	userStream *UserStream
+
+	// Whether the stream has been closed
+	hasClosed bool
 }
 
 // NewUserStream creates a new user stream
-func NewUserStream(userID string) *UserStream {
+func NewUserStream(userID string, currPos types.StreamPosition) *UserStream {
 	return &UserStream{
-		UserID: userID,
-		cond:   sync.NewCond(&sync.Mutex{}),
+		UserID:            userID,
+		timeOfLastChannel: time.Now(),
+		pos:               currPos,
+		signalChannel:     make(chan struct{}),
 	}
 }
 
-// Wait blocks until there is a new stream position for this user, which is then returned.
-// waitAtPos should be the position the stream thinks it should be waiting at.
-func (s *UserStream) Wait(waitAtPos types.StreamPosition) (pos types.StreamPosition) {
-	s.cond.L.Lock()
-	// Before we start blocking, we need to make sure that we didn't race with a call
-	// to Broadcast() between calling Wait() and actually sleeping. We check the last
-	// broadcast pos to see if it is newer than the pos we are meant to wait at. If it
-	// is newer, something has Broadcast to this stream more recently so return immediately.
-	if s.pos > waitAtPos {
-		pos = s.pos
-		s.cond.L.Unlock()
-		return
+// GetListener returns UserStreamListener that a sync request can use to wait
+// for new updates with.
+// UserStreamListener must be closed
+func (s *UserStream) GetListener(ctx context.Context) UserStreamListener {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	s.numWaiting++ // We decrement when UserStreamListener is closed
+
+	listener := UserStreamListener{
+		userStream: s,
 	}
-	s.numWaiting++
-	s.cond.Wait()
-	pos = s.pos
-	s.numWaiting--
-	s.cond.L.Unlock()
-	return
+
+	// Lets be a bit paranoid here and check that Close() is being called
+	runtime.SetFinalizer(&listener, func(l *UserStreamListener) {
+		if !l.hasClosed {
+			util.GetLogger(ctx).Warn("Didn't call Close on UserStreamListener")
+			l.Close()
+		}
+	})
+
+	return listener
 }
 
 // Broadcast a new stream position for this user.
 func (s *UserStream) Broadcast(pos types.StreamPosition) {
-	s.cond.L.Lock()
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
 	s.pos = pos
-	s.cond.L.Unlock()
-	s.cond.Broadcast()
+
+	close(s.signalChannel)
+
+	s.signalChannel = make(chan struct{})
 }
 
-// NumWaiting returns the number of goroutines waiting for Wait() to return. Used for metrics and testing.
-func (s *UserStream) NumWaiting() int {
-	s.cond.L.Lock()
-	defer s.cond.L.Unlock()
+// NumWaiting returns the number of goroutines waiting for waiting for updates.
+// Used for metrics and testing.
+func (s *UserStream) NumWaiting() uint {
+	s.lock.Lock()
+	defer s.lock.Unlock()
 	return s.numWaiting
 }
+
+// TimeOfLastNonEmpty returns the last time that the number of waiting listeners
+// was non-empty, may be time.Now() if number of waiting listeners is currently
+// non-empty.
+func (s *UserStream) TimeOfLastNonEmpty() time.Time {
+	s.lock.Lock()
+	defer s.lock.Unlock()
+
+	if s.numWaiting > 0 {
+		return time.Now()
+	}
+
+	return s.timeOfLastChannel
+}
+
+// GetStreamPosition returns last stream position which the UserStream was
+// notified about
+func (s *UserStreamListener) GetStreamPosition() types.StreamPosition {
+	s.userStream.lock.Lock()
+	defer s.userStream.lock.Unlock()
+
+	return s.userStream.pos
+}
+
+// GetNotifyChannel returns a channel that is closed when there may be an
+// update for the user.
+// sincePos specifies from which point we want to be notified about. If there
+// has already been an update after sincePos we'll return a closed channel
+// immediately.
+func (s *UserStreamListener) GetNotifyChannel(sincePos types.StreamPosition) <-chan struct{} {
+	s.userStream.lock.Lock()
+	defer s.userStream.lock.Unlock()
+
+	if sincePos < s.userStream.pos {
+		// If the listener is behind, i.e. missed a potential update, then we
+		// want them to wake up immediately. We do this by returning a new
+		// closed stream, which returns immediately when selected.
+		closedChannel := make(chan struct{})
+		close(closedChannel)
+		return closedChannel
+	}
+
+	return s.userStream.signalChannel
+}
+
+// Close cleans up resources used
+func (s *UserStreamListener) Close() {
+	s.userStream.lock.Lock()
+	defer s.userStream.lock.Unlock()
+
+	if !s.hasClosed {
+		s.userStream.numWaiting--
+		s.userStream.timeOfLastChannel = time.Now()
+	}
+
+	s.hasClosed = true
+}