Fix data race in display configuration that can cause crashes.

field/arena_notifiers.go

@@ -105,11 +105,16 @@ func (arena *Arena) generateAudienceDisplayModeMessage() interface{} {
 }
 
 func (arena *Arena) generateDisplayConfigurationMessage() interface{} {
+	// Make a copy of the map to avoid potential data races; otherwise the same map would get iterated through as it is
+	// serialized to JSON.
+	displaysCopy := make(map[string]*Display)
 	displayUrls := make(map[string]string)
 	for displayId, display := range arena.Displays {
+		displaysCopy[displayId] = display
 		displayUrls[displayId] = display.ToUrl()
 	}
-	return &DisplayConfigurationMessage{arena.Displays, displayUrls}
+
+	return &DisplayConfigurationMessage{displaysCopy, displayUrls}
 }
 
 func (arena *Arena) generateLedModeMessage() interface{} {

field/display.go

@@ -131,6 +131,9 @@ func (display *Display) ToUrl() string {
 
 // Returns an unused ID that can be used for a new display.
 func (arena *Arena) NextDisplayId() string {
+	displayRegistryMutex.Lock()
+	defer displayRegistryMutex.Unlock()
+
 	// Loop until we get an ID that isn't already used. This is inefficient if there is a large number of displays, but
 	// that should never be the case.
 	for {