world/world.go: Protect viewers with mutex.

Resolves #967.

server/world/loader.go

@@ -53,7 +53,9 @@ func (l *Loader) ChangeWorld(tx *Tx, new *World) {
 		}
 	})
 	clear(l.loaded)
+	l.w.viewerMu.Lock()
 	delete(l.w.viewers, l)
+	l.w.viewerMu.Unlock()
 
 	l.world(new)
 }
@@ -130,7 +132,10 @@ func (l *Loader) Close(tx *Tx) {
 		tx.World().removeViewer(tx, pos, l)
 	}
 	l.loaded = map[ChunkPos]*Column{}
+
+	l.w.viewerMu.Lock()
 	delete(l.w.viewers, l)
+	l.w.viewerMu.Unlock()
 
 	l.closed = true
 	l.viewer = nil

server/world/world.go

@@ -66,7 +66,8 @@ type World struct {
 	scheduledUpdates *scheduledTickQueue
 	neighbourUpdates []neighbourUpdate
 
-	viewers map[*Loader]Viewer
+	viewerMu sync.Mutex
+	viewers  map[*Loader]Viewer
 }
 
 // transaction is a type that may be added to the transaction queue of a World.
@@ -1023,6 +1024,9 @@ func (w *World) close() {
 // allViewers returns all viewers and loaders, regardless of where in the world
 // they are viewing.
 func (w *World) allViewers() ([]Viewer, []*Loader) {
+	w.viewerMu.Lock()
+	defer w.viewerMu.Unlock()
+
 	viewers, loaders := make([]Viewer, 0, len(w.viewers)), make([]*Loader, 0, len(w.viewers))
 	for k, v := range w.viewers {
 		viewers = append(viewers, v)
@@ -1034,7 +1038,10 @@ func (w *World) allViewers() ([]Viewer, []*Loader) {
 // addWorldViewer adds a viewer to the world. Should only be used while the
 // viewer isn't viewing any chunks.
 func (w *World) addWorldViewer(l *Loader) {
+	w.viewerMu.Lock()
 	w.viewers[l] = l.viewer
+	w.viewerMu.Unlock()
+
 	l.viewer.ViewTime(w.Time())
 	w.set.Lock()
 	raining, thundering := w.set.Raining, w.set.Raining && w.set.Thundering