Only wake up /sync requests which the event is for (#101)

src/github.com/matrix-org/dendrite/cmd/dendrite-sync-api-server/main.go

@@ -78,6 +78,9 @@ func main() {
 	}
 
 	n := sync.NewNotifier(types.StreamPosition(pos))
+	if err := n.Load(db); err != nil {
+		log.Panicf("startup: failed to set up notifier: %s", err)
+	}
 	server, err := consumers.NewServer(cfg, n, db)
 	if err != nil {
 		log.Panicf("startup: failed to create sync server: %s", err)

src/github.com/matrix-org/dendrite/syncapi/storage/current_room_state_table.go

@@ -61,11 +61,15 @@ const selectRoomIDsWithMembershipSQL = "" +
 const selectCurrentStateSQL = "" +
 	"SELECT event_json FROM current_room_state WHERE room_id = $1"
 
+const selectJoinedUsersSQL = "" +
+	"SELECT room_id, state_key FROM current_room_state WHERE type = 'm.room.member' AND membership = 'join'"
+
 type currentRoomStateStatements struct {
 	upsertRoomStateStmt             *sql.Stmt
 	deleteRoomStateByEventIDStmt    *sql.Stmt
 	selectRoomIDsWithMembershipStmt *sql.Stmt
 	selectCurrentStateStmt          *sql.Stmt
+	selectJoinedUsersStmt           *sql.Stmt
 }
 
 func (s *currentRoomStateStatements) prepare(db *sql.DB) (err error) {
@@ -85,8 +89,33 @@ func (s *currentRoomStateStatements) prepare(db *sql.DB) (err error) {
 	if s.selectCurrentStateStmt, err = db.Prepare(selectCurrentStateSQL); err != nil {
 		return
 	}
+	if s.selectJoinedUsersStmt, err = db.Prepare(selectJoinedUsersSQL); err != nil {
 		return
 	}
+	return
+}
+
+// JoinedMemberLists returns a map of room ID to a list of joined user IDs.
+func (s *currentRoomStateStatements) JoinedMemberLists() (map[string][]string, error) {
+	rows, err := s.selectJoinedUsersStmt.Query()
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+
+	result := make(map[string][]string)
+	for rows.Next() {
+		var roomID string
+		var userID string
+		if err := rows.Scan(&roomID, &userID); err != nil {
+			return nil, err
+		}
+		users := result[roomID]
+		users = append(users, userID)
+		result[roomID] = users
+	}
+	return result, nil
+}
 
 // SelectRoomIDsWithMembership returns the list of room IDs which have the given user in the given membership state.
 func (s *currentRoomStateStatements) SelectRoomIDsWithMembership(txn *sql.Tx, userID, membership string) ([]string, error) {

src/github.com/matrix-org/dendrite/syncapi/storage/syncserver.go

@@ -61,6 +61,11 @@ func NewSyncServerDatabase(dataSourceName string) (*SyncServerDatabase, error) {
 	return &SyncServerDatabase{db, partitions, events, state}, nil
 }
 
+// AllJoinedUsersInRooms returns a map of room ID to a list of all joined user IDs.
+func (d *SyncServerDatabase) AllJoinedUsersInRooms() (map[string][]string, error) {
+	return d.roomstate.JoinedMemberLists()
+}
+
 // WriteEvent into the database. It is not safe to call this function from multiple goroutines, as it would create races
 // when generating the stream position for this event. Returns the sync stream position for the inserted event.
 // Returns an error if there was a problem inserting this event.

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

@@ -15,27 +15,42 @@
 package sync
 
 import (
+	"encoding/json"
 	"sync"
 
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/dendrite/clientapi/events"
+	"github.com/matrix-org/dendrite/syncapi/storage"
 	"github.com/matrix-org/dendrite/syncapi/types"
 	"github.com/matrix-org/gomatrixserverlib"
 )
 
-// Notifier will wake up sleeping requests in the request pool when there
-// is some new data. It does not tell requests what that data is, only the
-// stream position which they can use to get at it.
+// Notifier will wake up sleeping requests when there is some new data.
+// It does not tell requests what that data is, only the stream position which
+// they can use to get at it. This is done to prevent races whereby we tell the caller
+// the event, but the token has already advanced by the time they fetch it, resulting
+// in missed events.
 type Notifier struct {
-	// The latest sync stream position: guarded by 'cond'.
+	// A map of RoomID => Set<UserID> : Must only be accessed by the OnNewEvent goroutine
+	roomIDToJoinedUsers map[string]set
+	// Protects currPos and userStreams.
+	streamLock *sync.Mutex
+	// The latest sync stream position: guarded by 'currPosMutex' which is RW to allow
+	// for concurrent reads on /sync requests
 	currPos types.StreamPosition
-	// A condition variable to notify all waiting goroutines of a new sync stream position
-	cond *sync.Cond
+	// A map of user_id => UserStream which can be used to wake a given user's /sync request.
+	userStreams map[string]*UserStream
 }
 
 // NewNotifier creates a new notifier set to the given stream position.
+// In order for this to be of any use, the Notifier needs to be told all rooms and
+// the joined users within each of them by calling Notifier.LoadFromDatabase().
 func NewNotifier(pos types.StreamPosition) *Notifier {
 	return &Notifier{
-		pos,
-		sync.NewCond(&sync.Mutex{}),
+		currPos:             pos,
+		roomIDToJoinedUsers: make(map[string]set),
+		userStreams:         make(map[string]*UserStream),
+		streamLock:          &sync.Mutex{},
 	}
 }
 
@@ -43,25 +58,157 @@ func NewNotifier(pos types.StreamPosition) *Notifier {
 // called from a single goroutine, to avoid races between updates which could set the
 // current position in the stream incorrectly.
 func (n *Notifier) OnNewEvent(ev *gomatrixserverlib.Event, pos types.StreamPosition) {
-	// update the current position in a guard and then notify all /sync streams
-	n.cond.L.Lock()
+	// update the current position then notify relevant /sync streams.
+	// This needs to be done PRIOR to waking up users as they will read this value.
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
 	n.currPos = pos
-	n.cond.L.Unlock()
 
-	n.cond.Broadcast() // notify ALL waiting goroutines
+	// Map this event's room_id to a list of joined users, and wake them up.
+	userIDs := n.joinedUsers(ev.RoomID())
+	// If this is an invite, also add in the invitee to this list.
+	if ev.Type() == "m.room.member" && ev.StateKey() != nil {
+		userID := *ev.StateKey()
+		var memberContent events.MemberContent
+		if err := json.Unmarshal(ev.Content(), &memberContent); err != nil {
+			log.WithError(err).WithField("event_id", ev.EventID()).Errorf(
+				"Notifier.OnNewEvent: Failed to unmarshal member event",
+			)
+		} else {
+			// Keep the joined user map up-to-date
+			switch memberContent.Membership {
+			case "invite":
+				userIDs = append(userIDs, userID)
+			case "join":
+				n.addJoinedUser(ev.RoomID(), userID)
+			case "leave":
+				fallthrough
+			case "ban":
+				n.removeJoinedUser(ev.RoomID(), userID)
+			}
+		}
+	}
+
+	for _, userID := range userIDs {
+		n.wakeupUser(userID, pos)
+	}
 }
 
 // WaitForEvents blocks until there are new events for this request.
 func (n *Notifier) WaitForEvents(req syncRequest) types.StreamPosition {
-	// In a guard, check if the /sync request should block, and block it until we get a new position
-	n.cond.L.Lock()
+	// 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
+	// - Incoming events wake requests for a matching user ID (needed for invites)
+
+	// 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
-	for req.since == currentPos {
-		// we need to wait for a new event.
-		// TODO: This waits for ANY new event, we need to only wait for events which we care about.
-		n.cond.Wait() // atomically unlocks and blocks goroutine, then re-acquires lock on unblock
-		currentPos = n.currPos
-	}
-	n.cond.L.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 req.since != currentPos {
+		n.streamLock.Unlock()
 		return currentPos
 	}
+
+	// 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)
+}
+
+// Load the membership states required to notify users correctly.
+func (n *Notifier) Load(db *storage.SyncServerDatabase) error {
+	roomToUsers, err := db.AllJoinedUsersInRooms()
+	if err != nil {
+		return err
+	}
+	n.setUsersJoinedToRooms(roomToUsers)
+	return nil
+}
+
+// setUsersJoinedToRooms marks the given users as 'joined' to the given rooms, such that new events from
+// these rooms will wake the given users /sync requests. This should be called prior to ANY calls to
+// OnNewEvent (eg on startup) to prevent racing.
+func (n *Notifier) setUsersJoinedToRooms(roomIDToUserIDs map[string][]string) {
+	// This is just the bulk form of addJoinedUser
+	for roomID, userIDs := range roomIDToUserIDs {
+		if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
+			n.roomIDToJoinedUsers[roomID] = make(set)
+		}
+		for _, userID := range userIDs {
+			n.roomIDToJoinedUsers[roomID].add(userID)
+		}
+	}
+}
+
+func (n *Notifier) wakeupUser(userID string, newPos types.StreamPosition) {
+	stream := n.fetchUserStream(userID, false)
+	if stream == nil {
+		return
+	}
+	stream.Broadcast(newPos) // wakeup all goroutines Wait()ing on this stream
+}
+
+// fetchUserStream retrieves a stream unique to the given user. If makeIfNotExists is true,
+// a stream will be made for this user if one doesn't exist and it will be returned. This
+// function does not wait for data to be available on the stream.
+func (n *Notifier) fetchUserStream(userID string, makeIfNotExists bool) *UserStream {
+	stream, ok := n.userStreams[userID]
+	if !ok {
+		// TODO: Unbounded growth of streams (1 per user)
+		stream = NewUserStream(userID)
+		n.userStreams[userID] = stream
+	}
+	return stream
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) addJoinedUser(roomID, userID string) {
+	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
+		n.roomIDToJoinedUsers[roomID] = make(set)
+	}
+	n.roomIDToJoinedUsers[roomID].add(userID)
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) removeJoinedUser(roomID, userID string) {
+	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
+		n.roomIDToJoinedUsers[roomID] = make(set)
+	}
+	n.roomIDToJoinedUsers[roomID].remove(userID)
+}
+
+// Not thread-safe: must be called on the OnNewEvent goroutine only
+func (n *Notifier) joinedUsers(roomID string) (userIDs []string) {
+	if _, ok := n.roomIDToJoinedUsers[roomID]; !ok {
+		return
+	}
+	return n.roomIDToJoinedUsers[roomID].values()
+}
+
+// A string set, mainly existing for improving clarity of structs in this file.
+type set map[string]bool
+
+func (s set) add(str string) {
+	s[str] = true
+}
+
+func (s set) remove(str string) {
+	delete(s, str)
+}
+
+func (s set) values() (vals []string) {
+	for str := range s {
+		vals = append(vals, str)
+	}
+	return
+}

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

@@ -0,0 +1,292 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sync
+
+import (
+	"context"
+	"fmt"
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+)
+
+var (
+	randomMessageEvent  gomatrixserverlib.Event
+	aliceInviteBobEvent gomatrixserverlib.Event
+	bobLeaveEvent       gomatrixserverlib.Event
+)
+
+var (
+	streamPositionVeryOld = types.StreamPosition(5)
+	streamPositionBefore  = types.StreamPosition(11)
+	streamPositionAfter   = types.StreamPosition(12)
+	streamPositionAfter2  = types.StreamPosition(13)
+	roomID                = "!test:localhost"
+	alice                 = "@alice:localhost"
+	bob                   = "@bob:localhost"
+)
+
+func init() {
+	var err error
+	randomMessageEvent, err = gomatrixserverlib.NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.message",
+		"content": {
+			"body": "Hello World",
+			"msgtype": "m.text"
+		},
+		"sender": "@noone:localhost",
+		"room_id": "`+roomID+`",
+		"origin_server_ts": 12345,
+		"event_id": "$randomMessageEvent:localhost"
+	}`), false)
+	if err != nil {
+		panic(err)
+	}
+	aliceInviteBobEvent, err = gomatrixserverlib.NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.member",
+		"state_key": "`+bob+`",
+		"content": {
+			"membership": "invite"
+		},
+		"sender": "`+alice+`",
+		"room_id": "`+roomID+`",
+		"origin_server_ts": 12345,
+		"event_id": "$aliceInviteBobEvent:localhost"
+	}`), false)
+	if err != nil {
+		panic(err)
+	}
+	bobLeaveEvent, err = gomatrixserverlib.NewEventFromTrustedJSON([]byte(`{
+		"type": "m.room.member",
+		"state_key": "`+bob+`",
+		"content": {
+			"membership": "leave"
+		},
+		"sender": "`+bob+`",
+		"room_id": "`+roomID+`",
+		"origin_server_ts": 12345,
+		"event_id": "$bobLeaveEvent:localhost"
+	}`), false)
+	if err != nil {
+		panic(err)
+	}
+}
+
+// Test that the current position is returned if a request is already behind.
+func TestImmediateNotification(t *testing.T) {
+	n := NewNotifier(streamPositionBefore)
+	pos, err := waitForEvents(n, newTestSyncRequest(alice, streamPositionVeryOld))
+	if err != nil {
+		t.Fatalf("TestImmediateNotification error: %s", err)
+	}
+	if pos != streamPositionBefore {
+		t.Fatalf("TestImmediateNotification want %d, got %d", streamPositionBefore, pos)
+	}
+}
+
+// Test that new events to a joined room unblocks the request.
+func TestNewEventAndJoinedToRoom(t *testing.T) {
+	n := NewNotifier(streamPositionBefore)
+	n.setUsersJoinedToRooms(map[string][]string{
+		roomID: []string{alice, bob},
+	})
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		pos, err := waitForEvents(n, newTestSyncRequest(bob, streamPositionBefore))
+		if err != nil {
+			t.Errorf("TestNewEventAndJoinedToRoom error: %s", err)
+		}
+		if pos != streamPositionAfter {
+			t.Errorf("TestNewEventAndJoinedToRoom want %d, got %d", streamPositionAfter, pos)
+		}
+		wg.Done()
+	}()
+
+	stream := n.fetchUserStream(bob, true)
+	waitForBlocking(stream, 1)
+
+	n.OnNewEvent(&randomMessageEvent, streamPositionAfter)
+
+	wg.Wait()
+}
+
+// Test that an invite unblocks the request
+func TestNewInviteEventForUser(t *testing.T) {
+	n := NewNotifier(streamPositionBefore)
+	n.setUsersJoinedToRooms(map[string][]string{
+		roomID: []string{alice, bob},
+	})
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go func() {
+		pos, err := waitForEvents(n, newTestSyncRequest(bob, streamPositionBefore))
+		if err != nil {
+			t.Errorf("TestNewInviteEventForUser error: %s", err)
+		}
+		if pos != streamPositionAfter {
+			t.Errorf("TestNewInviteEventForUser want %d, got %d", streamPositionAfter, pos)
+		}
+		wg.Done()
+	}()
+
+	stream := n.fetchUserStream(bob, true)
+	waitForBlocking(stream, 1)
+
+	n.OnNewEvent(&aliceInviteBobEvent, streamPositionAfter)
+
+	wg.Wait()
+}
+
+// Test that all blocked requests get woken up on a new event.
+func TestMultipleRequestWakeup(t *testing.T) {
+	n := NewNotifier(streamPositionBefore)
+	n.setUsersJoinedToRooms(map[string][]string{
+		roomID: []string{alice, bob},
+	})
+
+	var wg sync.WaitGroup
+	wg.Add(3)
+	poll := func() {
+		pos, err := waitForEvents(n, newTestSyncRequest(bob, streamPositionBefore))
+		if err != nil {
+			t.Errorf("TestMultipleRequestWakeup error: %s", err)
+		}
+		if pos != streamPositionAfter {
+			t.Errorf("TestMultipleRequestWakeup want %d, got %d", streamPositionAfter, pos)
+		}
+		wg.Done()
+	}
+	go poll()
+	go poll()
+	go poll()
+
+	stream := n.fetchUserStream(bob, true)
+	waitForBlocking(stream, 3)
+
+	n.OnNewEvent(&randomMessageEvent, streamPositionAfter)
+
+	wg.Wait()
+
+	numWaiting := stream.NumWaiting()
+	if numWaiting != 0 {
+		t.Errorf("TestMultipleRequestWakeup NumWaiting() want 0, got %d", numWaiting)
+	}
+}
+
+// Test that you stop getting woken up when you leave a room.
+func TestNewEventAndWasPreviouslyJoinedToRoom(t *testing.T) {
+	// listen as bob. Make bob leave room. Make alice send event to room.
+	// Make sure alice gets woken up only and not bob as well.
+	n := NewNotifier(streamPositionBefore)
+	n.setUsersJoinedToRooms(map[string][]string{
+		roomID: []string{alice, bob},
+	})
+
+	var leaveWG sync.WaitGroup
+
+	// Make bob leave the room
+	leaveWG.Add(1)
+	go func() {
+		pos, err := waitForEvents(n, newTestSyncRequest(bob, streamPositionBefore))
+		if err != nil {
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom error: %s", err)
+		}
+		if pos != streamPositionAfter {
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %d, got %d", streamPositionAfter, pos)
+		}
+		leaveWG.Done()
+	}()
+	bobStream := n.fetchUserStream(bob, true)
+	waitForBlocking(bobStream, 1)
+	n.OnNewEvent(&bobLeaveEvent, streamPositionAfter)
+	leaveWG.Wait()
+
+	// send an event into the room. Make sure alice gets it. Bob should not.
+	var aliceWG sync.WaitGroup
+	aliceStream := n.fetchUserStream(alice, true)
+	aliceWG.Add(1)
+	go func() {
+		pos, err := waitForEvents(n, newTestSyncRequest(alice, streamPositionAfter))
+		if err != nil {
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom error: %s", err)
+		}
+		if pos != streamPositionAfter2 {
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom want %d, got %d", streamPositionAfter2, pos)
+		}
+		aliceWG.Done()
+	}()
+
+	go func() {
+		// this should timeout with an error (but the main goroutine won't wait for the timeout explicitly)
+		_, err := waitForEvents(n, newTestSyncRequest(bob, streamPositionAfter))
+		if err == nil {
+			t.Errorf("TestNewEventAndWasPreviouslyJoinedToRoom expect error but got nil")
+		}
+	}()
+
+	waitForBlocking(aliceStream, 1)
+	waitForBlocking(bobStream, 1)
+
+	n.OnNewEvent(&randomMessageEvent, streamPositionAfter2)
+	aliceWG.Wait()
+
+	// it's possible that at this point alice has been informed and bob is about to be informed, so wait
+	// for a fraction of a second to account for this race
+	time.Sleep(1 * time.Millisecond)
+}
+
+// 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)
+		done <- newPos
+		close(done)
+	}()
+	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:
+		return p, nil
+	}
+}
+
+// Wait until something is Wait()ing on the user stream.
+func waitForBlocking(s *UserStream, numBlocking int) {
+	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)
+	}
+}
+
+func newTestSyncRequest(userID string, since types.StreamPosition) syncRequest {
+	return syncRequest{
+		userID:        userID,
+		timeout:       1 * time.Minute,
+		since:         since,
+		wantFullState: false,
+		limit:         defaultTimelineLimit,
+		log:           util.GetLogger(context.TODO()),
+	}
+}

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

@@ -15,10 +15,13 @@
 package sync
 
 import (
-	"github.com/matrix-org/dendrite/syncapi/types"
 	"net/http"
 	"strconv"
 	"time"
+
+	log "github.com/Sirupsen/logrus"
+	"github.com/matrix-org/dendrite/syncapi/types"
+	"github.com/matrix-org/util"
 )
 
 const defaultSyncTimeout = time.Duration(30) * time.Second
@@ -31,6 +34,7 @@ type syncRequest struct {
 	timeout       time.Duration
 	since         types.StreamPosition
 	wantFullState bool
+	log           *log.Entry
 }
 
 func newSyncRequest(req *http.Request, userID string) (*syncRequest, error) {
@@ -48,6 +52,7 @@ func newSyncRequest(req *http.Request, userID string) (*syncRequest, error) {
 		since:         since,
 		wantFullState: wantFullState,
 		limit:         defaultTimelineLimit, // TODO: read from filter
+		log:           util.GetLogger(req.Context()),
 	}, nil
 }
 

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

@@ -0,0 +1,79 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package sync
+
+import (
+	"sync"
+
+	"github.com/matrix-org/dendrite/syncapi/types"
+)
+
+// 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.
+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().
+	pos types.StreamPosition
+	// The number of goroutines blocked on Wait() - used for testing and metrics
+	numWaiting int
+}
+
+// NewUserStream creates a new user stream
+func NewUserStream(userID string) *UserStream {
+	return &UserStream{
+		UserID: userID,
+		cond:   sync.NewCond(&sync.Mutex{}),
+	}
+}
+
+// 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
+	}
+	s.numWaiting++
+	s.cond.Wait()
+	pos = s.pos
+	s.numWaiting--
+	s.cond.L.Unlock()
+	return
+}
+
+// Broadcast a new stream position for this user.
+func (s *UserStream) Broadcast(pos types.StreamPosition) {
+	s.cond.L.Lock()
+	s.pos = pos
+	s.cond.L.Unlock()
+	s.cond.Broadcast()
+}
+
+// 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()
+	return s.numWaiting
+}