Fix race-condition in populateUser() (#767)

Fix the root-cause of #759 by introducing synchronisation points for
individual user fetches.

bridge/slack/users_channels.go

@@ -20,6 +20,7 @@ type users struct {
 
 	users           map[string]*slack.User
 	usersMutex      sync.RWMutex
+	usersSyncPoints map[string]chan struct{}
 
 	refreshInProgress bool
 	earliestRefresh   time.Time
@@ -31,6 +32,7 @@ func newUserManager(log *logrus.Entry, sc *slack.Client) *users {
 		log:             log,
 		sc:              sc,
 		users:           make(map[string]*slack.User),
+		usersSyncPoints: make(map[string]chan struct{}),
 		earliestRefresh: time.Now(),
 	}
 }
@@ -68,14 +70,32 @@ func (b *users) getAvatar(id string) string {
 }
 
 func (b *users) populateUser(userID string) {
-	b.usersMutex.RLock()
+	for {
+		b.usersMutex.Lock()
 		_, exists := b.users[userID]
-	b.usersMutex.RUnlock()
 		if exists {
 			// already in cache
+			b.usersMutex.Unlock()
 			return
 		}
 
+		if syncPoint, ok := b.usersSyncPoints[userID]; ok {
+			// Another goroutine is already populating this user for us so wait on it to finish.
+			b.usersMutex.Unlock()
+			<-syncPoint
+			// We do not return and iterate again to check that the entry does indeed exist
+			// in case the previous query failed for some reason.
+		} else {
+			b.usersSyncPoints[userID] = make(chan struct{})
+			b.usersMutex.Unlock()
+			break
+		}
+	}
+
+	// Do not hold the lock while fetching information from Slack
+	// as this might take an unbounded amount of time.
+	b.usersMutex.Unlock()
+
 	user, err := b.sc.GetUserInfo(userID)
 	if err != nil {
 		b.log.Debugf("GetUserInfo failed for %v: %v", userID, err)
@@ -83,15 +103,20 @@ func (b *users) populateUser(userID string) {
 	}
 
 	b.usersMutex.Lock()
+	defer b.usersMutex.Unlock()
+
+	// Register user information.
 	b.users[userID] = user
-	b.usersMutex.Unlock()
+
+	// Wake up any waiting goroutines and remove the synchronization point.
+	close(b.usersSyncPoints[userID])
+	delete(b.usersSyncPoints, userID)
 }
 
 func (b *users) populateUsers(wait bool) {
 	b.refreshMutex.Lock()
 	if !wait && (time.Now().Before(b.earliestRefresh) || b.refreshInProgress) {
-		b.log.Debugf("Not refreshing user list as it was done less than %v ago.",
-			minimumRefreshInterval)
+		b.log.Debugf("Not refreshing user list as it was done less than %v ago.", minimumRefreshInterval)
 		b.refreshMutex.Unlock()
 
 		return
@@ -230,9 +255,10 @@ func (b *channels) getChannelMembers(users *users) config.ChannelMembers {
 
 func (b *channels) registerChannel(channel slack.Channel) {
 	b.channelsMutex.Lock()
+	defer b.channelsMutex.Unlock()
+
 	b.channelsByID[channel.ID] = &channel
 	b.channelsByName[channel.Name] = &channel
-	b.channelsMutex.Unlock()
 }
 
 func (b *channels) populateChannels(wait bool) {