Fix a number of crashes involving unknown nodes, also fix fishbuckets on_place (#3914)

Fixes: #3913 #3915

~~You can reproduce the crash by placing a fish bucket on top snow above an unknown node.

I also noticed that the code always uses pointed_thing.above so I fixed that and also added a function to mcl_utils to figure out where a node should be placed (either above or below). Looks like the rest of the code could also use improvement but at least it does not crash now.~~

Cora fixed a bunch of related crashes in Mineclona so I am replacing my commit and cherry picking all her commits here.
https://codeberg.org/mineclonia/mineclonia/pulls/549

Here is the list of fixes from that PR:
- Crash when placing snow layer on unknown nodes
- Crash when snow layers on unknown nodes are flooded
- Crash when placing fishbucket on snow on top of unknown nodes
- Crash when placing chorus flower and stem on unknown
- Crash when placing mob spawners on unknown
- The fishbucket on place to actually replace buildable_to

Co-authored-by: cora <coradelamouche@gmx.ch>
Reviewed-on: MineClone2/MineClone2#3914
Reviewed-by: ancientmarinerdev <ancientmariner_dev@proton.me>
Co-authored-by: 𝕵𝖔𝖍𝖆𝖓𝖓𝖊𝖘 𝕱𝖗𝖎𝖙𝖟 <mrrar@noreply.git.minetest.land>
Co-committed-by: 𝕵𝖔𝖍𝖆𝖓𝖓𝖊𝖘 𝕱𝖗𝖎𝖙𝖟 <mrrar@noreply.git.minetest.land>

mods/ITEMS/mcl_buckets/fishbuckets.lua

@@ -18,13 +18,21 @@ local function on_place_fish(itemstack, placer, pointed_thing)
 		return new_stack
 	end
 
-	local pos = pointed_thing.above or pointed_thing.under
-	if not pos then return end
-	local n = minetest.get_node_or_nil(pos)
-	if n.name and minetest.registered_nodes[n.name].buildable_to or n.name == "mcl_portals:portal" then
-		local fish = itemstack:get_name():gsub(fishbucket_prefix,"")
+	if pointed_thing.type ~= "node" then return end
+
+	local pos = pointed_thing.above
+	local n = minetest.get_node(pointed_thing.above)
+	local def = minetest.registered_nodes[minetest.get_node(pointed_thing.under).name]
+
+	if ( def and def.buildable_to ) or n.name == "mcl_portals:portal" then
+		pos = pointed_thing.under
+		n = minetest.get_node(pointed_thing.under)
+	end
+
+	local fish = itemstack:get_definition()._mcl_buckets_fish
 	if fish_names[fish] then
 		local o = minetest.add_entity(pos, "mobs_mc:" .. fish)
+		if o and o:get_pos() then
 			local props = itemstack:get_meta():get_string("properties")
 			if props ~= "" then
 				o:set_properties(minetest.deserialize(props))
@@ -60,6 +68,7 @@ for techname, fishname in pairs(fish_names) do
 		stack_max = 1,
 		groups = {bucket = 1, fish_bucket = 1},
 		liquids_pointable = false,
+		_mcl_buckets_fish = techname,
 		on_place = on_place_fish,
 		on_secondary_use = on_place_fish,
 		_on_dispense = function(stack, pos, droppos, dropnode, dropdir)

mods/ITEMS/mcl_core/functions.lua

@@ -1581,7 +1581,7 @@ end
 -- MUST NOT be called if there is a snow cover node above pos.
 function mcl_core.clear_snow_dirt(pos, node)
 	local def = minetest.registered_nodes[node.name]
-	if def._mcl_snowless then
+	if def and def._mcl_snowless then
 		minetest.swap_node(pos, {name = def._mcl_snowless, param2=node.param2})
 	end
 end
@@ -1602,7 +1602,7 @@ function mcl_core.on_snowable_construct(pos)
 	-- Make snowed if needed
 	if minetest.get_item_group(anode.name, "snow_cover") == 1 then
 		local def = minetest.registered_nodes[node.name]
-		if def._mcl_snowed then
+		if def and def._mcl_snowed then
 			minetest.swap_node(pos, {name = def._mcl_snowed, param2=node.param2})
 		end
 	end
@@ -1623,7 +1623,7 @@ function mcl_core.on_snow_construct(pos)
 	local npos = {x=pos.x, y=pos.y-1, z=pos.z}
 	local node = minetest.get_node(npos)
 	local def = minetest.registered_nodes[node.name]
-	if def._mcl_snowed then
+	if def and def._mcl_snowed then
 		minetest.swap_node(npos, {name = def._mcl_snowed, param2=node.param2})
 	end
 end

mods/ITEMS/mcl_core/nodes_base.lua

@@ -1038,7 +1038,7 @@ for i=1,8 do
 
 		-- Get position where snow would be placed
 		local target
-		if minetest.registered_nodes[unode.name].buildable_to then
+		if def and def.buildable_to then
 			target = under
 		else
 			target = above

mods/ITEMS/mcl_end/chorus_plant.lua

@@ -155,7 +155,8 @@ minetest.register_node("mcl_end:chorus_flower", {
 		1) On top of end stone or chorus plant
 		2) On top of air and horizontally adjacent to exactly 1 chorus plant ]]
 		local pos
-		if minetest.registered_nodes[node_under.name].buildable_to then
+		local def = minetest.registered_nodes[node_under.name]
+		if def and def.buildable_to then
 			pos = pointed_thing.under
 		else
 			pos = pointed_thing.above
@@ -283,7 +284,8 @@ minetest.register_node("mcl_end:chorus_plant", {
 		condition is met:
 		- placed on end stone or any chorus node ]]
 		local pos_place, node_check
-		if minetest.registered_nodes[node_under.name].buildable_to then
+		local def = minetest.registered_nodes[node_under.name]
+		if def and def.buildable_to then
 			pos_place = pointed_thing.under
 			node_check = node_above
 		else

mods/ITEMS/mcl_mobspawners/init.lua

@@ -301,7 +301,8 @@ minetest.register_node("mcl_mobspawners:spawner", {
 		local new_itemstack, success = minetest.item_place(itemstack, placer, pointed_thing)
 		if success then
 			local placepos
-			if minetest.registered_nodes[node_under.name].buildable_to then
+			local def = minetest.registered_nodes[node_under.name]
+			if def and def.buildable_to then
 				placepos = pointed_thing.under
 			else
 				placepos = pointed_thing.above