Guard against dummy objects in 5.3+

Some time in the 5.3 dev stream (docs updated at
217f3a42), object refs started being invalidated
immediately on calling obj:remove(), such that
obj:get_pos() starts to return nil instead of the object's
last known position.

This can cause some crashes in NodeCore, where we
assume that our object is still valid (or usable as if it
were still valid) even though we're looping through
handlers and any one of them may have remove()d the
object before other handlers get a chance to fire.

Instead, just watch for unexpected nil returns from
functions we expect would never return nil (e.g.
get_pos or get_properties) and return if we hit one.
We can assume all other calls will be non-nil after that
one, as long as we stay in the same function flow.

mods/nc_api_active/aism.lua

@@ -116,15 +116,21 @@ nodecore.register_item_entity_step(function(self, dtime)
 		local t = (self.aismtimer or 0) + dtime
 		while t >= 1 do
 			t = t - 1
+			local pos = self.object:get_pos()
+			if not pos then return end
+			local setstack
 			checkstack(ItemStack(self.itemstring), {
-					pos = self.object:get_pos(),
+					pos = pos,
 					obj = self.object,
 					ent = self,
-					set = function(s)
-						if s:is_empty() then return self.object:remove() end
-						self.itemstring = s:to_string()
-					end
+					set = function(s) setstack = s end
 				})
+			if setstack then
+				if setstack:is_empty() then
+					return self.object:remove()
+				end
+				self.itemstring = setstack:to_string()
+			end
 		end
 		self.aismtimer = t
 	end)

mods/nc_api_active/dynalight.lua

@@ -73,6 +73,7 @@ nodecore.register_limited_abm({
 	})
 
 local function dynamic_light_add(pos, level, ttl)
+	if not pos then return end
 	local name = minetest.get_node(pos).name
 	if not canreplace[name] then return end
 	if level < 1 then return end

mods/nc_api_ents/api.lua

@@ -96,6 +96,7 @@ end
 function nodecore.entity_settle_check(on_settle, isnode)
 	return function(self)
 		local pos = self.object:get_pos()
+		if not pos then return end
 		if pos.y < nodecore.map_limit_min then
 			pos.y = nodecore.map_limit_min
 			self.object:set_pos(pos)

mods/nc_api_ents/item_merge.lua

@@ -8,6 +8,7 @@ local math_floor, math_random
 local cache = {}
 
 function nodecore.item_ent_merge(pos)
+	if not pos then return end
 	pos = vector.round(pos)
 	local hash = minetest.hash_node_position(pos)
 

mods/nc_api_ents/util.lua

@@ -21,6 +21,7 @@ end
 
 function nodecore.ent_prop_set(obj, def)
 	local old = obj:get_properties()
+	if not old then return end
 	if type(def) == "function" then
 		def = def(old, obj)
 	end

mods/nc_api_visinv/init.lua

@@ -20,16 +20,17 @@ minetest.register_entity(modname .. ":stackent", {
 		initial_properties = nodecore.stackentprops(),
 		is_stack = true,
 		itemcheck = function(self)
-			local pos = self.object:get_pos()
+			local obj = self.object
+			local pos = obj:get_pos()
+			if not pos then return end
 			local stack = nodecore.stack_get(pos)
-			if not stack or stack:is_empty() then return self.object:remove() end
+			if not stack or stack:is_empty() then return obj:remove() end
 
 			local rp = vector.round(pos)
 			local props, scale, yaw = nodecore.stackentprops(stack,
 				rp.x * 3 + rp.y * 5 + rp.z * 7)
 			rp.y = rp.y + scale - 31/64
 
-			local obj = self.object
 			nodecore.ent_prop_set(obj, props)
 			if obj:get_yaw() ~= yaw then
 				obj:set_yaw(yaw)

mods/nc_items/ent_falling.lua

@@ -10,7 +10,9 @@ nodecore.register_falling_node_on_setnode(function(self, node, meta)
 		and meta and meta.inventory and meta.inventory.solo then
 			local stack = ItemStack(meta.inventory.solo[1] or "")
 			if not stack:is_empty() then
-				local ent = minetest.add_item(self.object:get_pos(), stack)
+				local pos = self.object:get_pos()
+				if not pos then return end
+				local ent = minetest.add_item(pos, stack)
 				if ent then ent:set_velocity(self.object:get_velocity()) end
 				self.object:remove()
 				return true

mods/nc_nodefall/damage.lua

@@ -19,6 +19,7 @@ local function maketick(mult, getname, oldtick)
 		end
 
 		local pos = self.object:get_pos()
+		if not pos then return end
 		pos.y = pos.y - 1
 		local vel = self.object:get_velocity()
 		local v = vel and -vel.y or 0