From f641e7af14638105b5ec4c0177b1a803e8d61def Mon Sep 17 00:00:00 2001
From: Kegan Dougal <kegan@matrix.org>
Date: Wed, 17 May 2017 13:44:17 +0100
Subject: [PATCH] Use a single lock for protecting currPos and userStreams

---
 .../dendrite/syncapi/sync/notifier.go         | 39 ++++++++-----------
 1 file changed, 17 insertions(+), 22 deletions(-)

diff --git a/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go b/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
index b3753b7d7..2e54c2bde 100644
--- a/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
+++ b/src/github.com/matrix-org/dendrite/syncapi/sync/notifier.go
@@ -31,16 +31,15 @@ import (
 // the event, but the token has already advanced by the time they fetch it, resulting
 // in missed events.
 type Notifier struct {
+	// 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
-	currPosMutex *sync.RWMutex
-	// A map of RoomID => Set<UserID>
-	roomIDToJoinedUsers map[string]set
+	currPos types.StreamPosition
 	// A map of user_id => UserStream which can be used to wake a given user's /sync request.
-	// Map access is guarded by userStreamsMutex.
-	userStreams      map[string]*UserStream
-	userStreamsMutex *sync.Mutex
+	userStreams map[string]*UserStream
 }
 
 // NewNotifier creates a new notifier set to the given stream position.
@@ -49,10 +48,9 @@ type Notifier struct {
 func NewNotifier(pos types.StreamPosition) *Notifier {
 	return &Notifier{
 		currPos:             pos,
-		currPosMutex:        &sync.RWMutex{},
 		roomIDToJoinedUsers: make(map[string]set),
 		userStreams:         make(map[string]*UserStream),
-		userStreamsMutex:    &sync.Mutex{},
+		streamLock:          &sync.Mutex{},
 	}
 }
 
@@ -60,11 +58,11 @@ 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 relevant /sync streams.
+	// 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.currPosMutex.Lock()
+	n.streamLock.Lock()
+	defer n.streamLock.Unlock()
 	n.currPos = pos
-	n.currPosMutex.Unlock()
 
 	// Map this event's room_id to a list of joined users, and wake them up.
 	userIDs := n.joinedUsers(ev.RoomID())
@@ -107,20 +105,24 @@ func (n *Notifier) WaitForEvents(req syncRequest) types.StreamPosition {
 	//       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.currPosMutex.RLock()
+	n.streamLock.Lock()
 	currentPos := n.currPos
-	n.currPosMutex.RUnlock()
 
 	// 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")
-	return n.blockUser(req.userID)
+
+	// give up the stream lock prior to waiting on the user lock
+	stream := n.fetchUserStream(req.userID, true)
+	n.streamLock.Unlock()
+	return stream.Wait()
 }
 
 // Load the membership states required to notify users correctly.
@@ -156,17 +158,10 @@ func (n *Notifier) wakeupUser(userID string, newPos types.StreamPosition) {
 	stream.Broadcast(newPos) // wakeup all goroutines Wait()ing on this stream
 }
 
-func (n *Notifier) blockUser(userID string) types.StreamPosition {
-	stream := n.fetchUserStream(userID, true)
-	return stream.Wait()
-}
-
 // 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 {
-	n.userStreamsMutex.Lock()
-	defer n.userStreamsMutex.Unlock()
 	stream, ok := n.userStreams[userID]
 	if !ok {
 		// TODO: Unbounded growth of streams (1 per user)