Make stack amount var private

code/game/objects/items/stacks/stack.dm

@@ -17,7 +17,7 @@
 	center_of_mass = null
 	var/list/datum/stack_recipe/recipes
 	var/singular_name
-	var/amount = 1
+	VAR_PROTECTED/amount = 1
 	var/max_amount //also see stack recipes initialisation, param "max_res_amount" must be equal to this max_amount
 	var/stacktype //determines whether different stack types can merge
 	var/build_type = null //used when directly applied to a turf
@@ -33,8 +33,10 @@
 	. = ..()
 	if(!stacktype)
 		stacktype = type
-	if(amount)
-		src.amount = amount
+	if(!isnull(amount)) // Could be 0
+		if(amount < 0)
+			amount = max_amount
+		set_amount(amount, TRUE)
 	update_icon()
 
 /obj/item/stack/Destroy()
@@ -229,12 +231,15 @@
 //Return 1 if an immediate subsequent call to use() would succeed.
 //Ensures that code dealing with stacks uses the same logic
 /obj/item/stack/proc/can_use(var/used)
-	if (get_amount() < used)
+	if(used < 0 || used % 1)
+		stack_trace("Tried to use a bad stack amount: [used]")
+		return 0
+	if(get_amount() < used)
 		return 0
 	return 1
 
 /obj/item/stack/proc/use(var/used)
-	if (!can_use(used))
+	if(!can_use(used))
 		return 0
 	if(!uses_charge)
 		amount -= used
@@ -251,6 +256,9 @@
 		return 1
 
 /obj/item/stack/proc/add(var/extra)
+	if(extra < 0 || extra % 1)
+		stack_trace("Tried to add a bad stack amount: [extra]")
+		return 0
 	if(!uses_charge)
 		if(amount + extra > get_max_amount())
 			return 0
@@ -265,6 +273,27 @@
 			var/datum/matter_synth/S = synths[i]
 			S.add_charge(charge_costs[i] * extra)
 
+/obj/item/stack/proc/set_amount(var/new_amount, var/no_limits = FALSE)
+	if(new_amount < 0 || new_amount % 1)
+		stack_trace("Tried to set a bad stack amount: [new_amount]")
+		return 0
+	
+	// Clean up the new amount
+	new_amount = max(round(new_amount), 0)
+	
+	// Can exceed max if you really want
+	if(new_amount > max_amount && !no_limits)
+		new_amount = max_amount
+	
+	amount = new_amount
+	
+	// Can set it to 0 without qdel if you really want
+	if(amount == 0 && !no_limits)
+		qdel(src)
+		return FALSE
+
+	return TRUE
+
 /*
 	The transfer and split procs work differently than use() and add().
 	Whereas those procs take no action if the desired amount cannot be added or removed these procs will try to transfer whatever they can.
@@ -282,6 +311,10 @@
 
 	if (isnull(tamount))
 		tamount = src.get_amount()
+	
+	if(tamount < 0 || tamount % 1)
+		stack_trace("Tried to transfer a bad stack amount: [tamount]")
+		return 0
 
 	var/transfer = max(min(tamount, src.get_amount(), (S.get_max_amount() - S.get_amount())), 0)
 
@@ -302,7 +335,10 @@
 	if(uses_charge)
 		return null
 
-	tamount = round(tamount)
+	if(tamount < 0 || tamount % 1)
+		stack_trace("Tried to split a bad stack amount: [tamount]")
+		return null
+	
 	var/transfer = max(min(tamount, src.amount, initial(max_amount)), 0)
 
 	var/orig_amount = src.amount