Fix PriorityQueue to no longer store function in global.

Changed the PriorityQueue to store the comparator function in a metatable to avoid serializing the comparator function to the global table.
This means that the PriorityQueue has to be restored on load.
Plus side is we can use comparators that are closures.

utils/priority_queue.lua

@@ -1,6 +1,6 @@
-local Debug = require 'utils.debug'
-local is_closure = Debug.is_closure
 local floor = math.floor
+local getmetatable = getmetatable
+local setmetatable = setmetatable
 
 local PriorityQueue = {}
 
@@ -26,15 +26,30 @@ end
 function PriorityQueue.new(comparator)
     if comparator == nil then
         comparator = default_comparator
-    elseif is_closure(comparator) then
-        error('comparator cannot be a closure.', 2)
     end
 
-    return {_comparator = comparator}
+    local mt = {comparator = comparator}
+
+    return setmetatable({}, mt)
+end
+
+function PriorityQueue.load(self, comparator)
+    if comparator == nil then
+        comparator = default_comparator
+    end
+
+    local mt = {comparator = comparator}
+
+    return setmetatable(self or {}, mt)
+end
+
+local function get_comparator(self)
+    local mt = getmetatable(self)
+    return mt.comparator
 end
 
 local function heapify_from_end_to_start(self)
-    local comparator = self._comparator
+    local comparator = get_comparator(self)
     local pos = #self
     while pos > 1 do
         local parent = floor(pos * 0.5)
@@ -49,7 +64,7 @@ local function heapify_from_end_to_start(self)
 end
 
 local function heapify_from_start_to_end(self)
-    local comparator = self._comparator
+    local comparator = get_comparator(self)
     local parent = 1
     local smallest = 1
     local count = #self

utils/task.lua

@@ -43,6 +43,8 @@ Global.register(
         callbacks = tbl.callbacks
         task_queue = tbl.task_queue
         primitives = tbl.primitives
+
+        PriorityQueue.load(callbacks, comparator)
     end
 )