Add short circuit if sub-node space check isn't possible:

mods/ENTITIES/mcl_mobs/spawning.lua

@@ -737,7 +737,7 @@ local function get_water_spawn(p)
 end
 
 local function has_room(self,pos)
-	local cb = self.collisionbox
+	local cb = self.spawnbox or self.collisionbox
 	local nodes = {}
 	if self.fly_in then
 		local t = type(self.fly_in)
@@ -748,18 +748,73 @@ local function has_room(self,pos)
 		end
 	end
 	table.insert(nodes,"air")
-	local x = cb[4] - cb[1]
-	local y = cb[5] - cb[2]
-	local z = cb[6] - cb[3]
-	local r = math.ceil(x * y * z)
-	local p1 = vector.offset(pos,cb[1],cb[2],cb[3])
-	local p2 = vector.offset(pos,cb[4],cb[5],cb[6])
-	local n = #minetest.find_nodes_in_area(p1,p2,nodes) or 0
-	if r > n then
-		minetest.log("warning","[mcl_mobs] No room for mob "..self.name.." at "..minetest.pos_to_string(vector.round(pos)))
-		return false
+
+	local p1 = vector.offset(pos,cb[1],cb[2] + 1,cb[3])
+	p1.x = math.floor(p1.x)
+	p1.y = math.floor(p1.y)
+	p1.z = math.floor(p1.z)
+
+	local cb_height = cb[5] - cb[2]
+	local p2 = vector.offset(p1,cb[4] - cb[1], cb_height, cb[6] - cb[3])
+	p2.x = math.ceil(p2.x) - 1
+	p2.y = math.ceil(p2.y) - 1
+	p2.z = math.ceil(p2.z) - 1
+
+	-- Check if the entire spawn volume is free
+	local dx = p2.x - p1.x + 1
+	local dy = p2.y - p1.y + 1
+	local dz = p2.z - p1.z + 1
+	local found_nodes = minetest.find_nodes_in_area(p1,p2,nodes) or 0
+	local n = #found_nodes
+	if n == ( dx * dy * dz ) then return true end
+
+	-- If we don't have an implementation of get_node_boxes, we can't check for sub-node space
+	if not minetest.get_node_boxes then return false end
+
+	-- Check if it's possible for a sub-node space check to succeed
+	local needed_in_bottom_section = (dx * dz * ( dy - 1))
+	if n < needed_in_bottom_section then return false end
+
+	-- Make sure the entire volume except for the top level is free before checking the top layer
+	if dy > 1 then
+		-- Remove nodes in the top layer from the count
+		for i = 1,#found_nodes do
+			if found_nodes[i].y == p2.y then
+				n = n - 1
+			end
+		end
+
+		-- If the entire volume except the top layer isn't air (or nodes) then we can't spawn this mob here
+		if n < needed_in_bottom_section then return false end
 	end
-	return true
+
+	-- Check the top layer to see if we have enough space to spawn in
+	local top_layer_height = 1
+	local processed = {}
+	for x = p1.x,p2.x do
+		for z = p1.z,p2.z do
+			local test_pos = vector.new(x,p2.y,z)
+			local node = minetest.get_node(test_pos) or { name = "ignore" }
+			local cache_name = string.format("%s-%d", node.name, node.param2)
+			if not processed[cache_name] then
+				-- Calculate node bounding box and select the lowest y value
+				local boxes = minetest.get_node_boxes("collision_box", test_pos, node)
+				for i = 1,#boxes do
+					local box = boxes[i]
+					local y_test = box[2] + 0.5
+					if y_test < top_layer_height then top_layer_height = y_test end
+
+					local y_test = box[5] + 0.5
+					if y_test < top_layer_height then top_layer_height = y_test end
+				end
+			end
+		end
+	end
+	if top_layer_height + dy - 1 >= cb_height then return true end
+
+	-- We don't have room
+	mcl_log("No room for mob "..self.name.." at "..minetest.pos_to_string(vector.round(pos)))
+	return false
 end
 
 mcl_mobs.custom_biomecheck = nil

mods/ENTITIES/mobs_mc/spider.lua

@@ -67,6 +67,7 @@ local spider = {
 	curiosity = 10,
 	head_yaw="z",
 	collisionbox = {-0.7, -0.01, -0.7, 0.7, 0.89, 0.7},
+	spawnbox = {-1.2, -0.01, -1.2, 1.2, 0.89, 1.2},
 	visual = "mesh",
 	mesh = "mobs_mc_spider.b3d",
 	textures = {