Merge pull request #3867 from Citadel-Station-13/upstream-merge-32504

[MIRROR] Weakrefs

code/datums/datum.dm

@@ -4,6 +4,7 @@
     var/list/datum_components //for /datum/components
     var/ui_screen = "home"  //for tgui
     var/use_tag = FALSE
+    var/datum/weakref/weak_reference
 
 #ifdef TESTING
     var/running_find_references
@@ -15,6 +16,7 @@
 // Return the appropriate QDEL_HINT; in most cases this is QDEL_HINT_QUEUE.
 /datum/proc/Destroy(force=FALSE, ...)
 	tag = null
+	weak_reference = null	//ensure prompt GCing of weakref.
 	var/list/timers = active_timers
 	active_timers = null
 	for(var/thing in timers)

code/datums/weakrefs.dm

@@ -0,0 +1,19 @@
+
+/proc/WEAKREF(datum/input)
+	if(istype(input) && !QDELETED(input))
+		if(!input.weak_reference)
+			input.weak_reference = new /datum/weakref(input)
+		return input.weak_reference
+
+/datum/weakref
+	var/reference
+
+/datum/weakref/New(datum/thing)
+	reference = REF(thing)
+
+/datum/weakref/Destroy()
+	return QDEL_HINT_LETMELIVE	//Let BYOND autoGC thiswhen nothing is using it anymore.
+
+/datum/weakref/proc/resolve()
+	var/datum/D = locate(reference)
+	return D.weak_reference == src? D : null