Address wagon aliasing issues

As it turns out, not fully testing new features is not necessarily a
good idea ...

This patch follows up 1F616EMO's patch by
* Making get_wagon_prototype return the resolved alias,
* Handling recursive wagon alises (in particular, loops), and
* Adding (partial) unittest for the wagon aliasing system. [v2]: The
testcases are complemented a bit more to cover situations where the
alias resolution system should return nil.

[v2]: This patch should hopefully also warn about not spawning wagons.
Note that this only warns about the missing wagon entity and does _not_
actually fix the issue.

How to test:
* In a world with both advtrains_train_subway and advtrains_train_japan
  enabled, place a subway wagon, a Japanese engine, and a regular
  Japanese wagon.
* Add the test mod to the world; do NOT remove advtrains_train_japan.
* Restart the world. Notice that the Japanese wagons still appear as
  Japanese wagons despite being aliased to subway wagons.
* Restart the world without the advtrains_train_japan mod. Notice that
  the engine appears as the subway wagon while the regular Japanese
  wagon appears as the wagon placeholder. [v2]: Also note that the
  warning message about the missing wagon prototype still mentions the
  regular Japanese wagon.
* Restart the world again with the advtrains_train_japan mod. Notice
  that both type of Japanese wagons reappear as Japanese wagons.
* Observe that unittests work.

Test mod:
advtrains.register_wagon_alias("advtrains:engine_japan", "advtrains:subway_wagon")
advtrains.register_wagon_alias("advtrains:wagon_japan", "advtrains:wagon_japan")

3 files changed, 74 insertions, 14 deletions

diff --git a/advtrains/spec/wagons_spec.lua b/advtrains/spec/wagons_spec.lua
new file mode 100644
index 0000000..df0687b
--- /dev/null
+++ b/advtrains/spec/wagons_spec.lua
@@ -0,0 +1,40 @@
+require "mineunit"
+mineunit "core"
+
+_G.advtrains = {
+	wagon_load_range = 32
+}
+sourcefile "wagons"
+
+local myproto = {_test = true}
+advtrains.register_wagon(":mywagon", myproto, "My wagon", "", false)
+advtrains.register_wagon_alias(":myalias", ":mywagon")
+advtrains.register_wagon_alias(":myotheralias", ":myalias")
+
+local myotherproto = {_other = true}
+advtrains.register_wagon(":noalias", myotherproto, "Not aliased wagon", "", false)
+advtrains.register_wagon_alias(":noalias", ":mywagon")
+
+advtrains.register_wagon_alias(":nilalias", ":nil")
+
+advtrains.register_wagon_alias(":R1", ":R2")
+advtrains.register_wagon_alias(":R2", ":R3")
+advtrains.register_wagon_alias(":R3", ":R1")
+
+describe("wagon alias system", function()
+	it("should work", function()
+		assert.same({":mywagon", myproto}, {advtrains.resolve_wagon_alias(":myalias")})
+		assert.equal(myproto, advtrains.wagon_prototypes[":myalias"])
+		assert.same({":mywagon", myproto}, {advtrains.resolve_wagon_alias(":myotheralias")})
+	end)
+	it("should respect wagon registration", function()
+		assert.same({":noalias", myotherproto}, {advtrains.resolve_wagon_alias(":noalias")})
+	end)
+	it("should handle recursive loops", function()
+		assert.same({}, {advtrains.resolve_wagon_alias(":R1")})
+	end)
+	it("should return nil for missing entries", function()
+		assert.same({}, {advtrains.resolve_wagon_alias(":what")})
+		assert.same({}, {advtrains.resolve_wagon_alias(":nilalias")})
+	end)
+end)
diff --git a/advtrains/trainlogic.lua b/advtrains/trainlogic.lua
index 796aae9..3b006d2 100644
--- a/advtrains/trainlogic.lua
+++ b/advtrains/trainlogic.lua
@@ -1113,8 +1113,17 @@ function advtrains.spawn_wagons(train_id)
 				if advtrains.position_in_range(pos, ablkrng) then
 					--atdebug("wagon",w_id,"spawning")
 					local wt = advtrains.get_wagon_prototype(data)
-					local wagon = minetest.add_entity(pos, wt):get_luaentity()
-					wagon:set_id(w_id)
+					local wobj = minetest.add_entity(pos, wt)
+					if not wobj then
+						atwarn("Failed to spawn wagon", w_id, "of type", wt)
+					else
+						local wagon = wobj:get_luaentity()
+						if not wagon then
+							atwarn("Wagon", w_id, "of type", wt, "spawned with nil luaentity")
+						else
+							wagon:set_id(w_id)
+						end
+					end
 				end
 			end
 		else
diff --git a/advtrains/wagons.lua b/advtrains/wagons.lua
index 50ce4ab..8bb12ee 100644
--- a/advtrains/wagons.lua
+++ b/advtrains/wagons.lua
@@ -16,15 +16,8 @@ advtrains.wagons = {}
 advtrains.wagon_alias = {}
 advtrains.wagon_prototypes = setmetatable({}, {
 	__index = function(t, k)
-		local rtn_val = rawget(t, k)
-		if rtn_val ~= nil then
-			return rtn_val
-		end
-		local alias = advtrains.wagon_alias[k]
-		if alias then
-			return rawget(t, alias)
-		end
-		return nil
+		local _, proto = advtrains.resolve_wagon_alias(k)
+		return proto
 	end
 })
 advtrains.wagon_objects = {}
@@ -1338,17 +1331,35 @@ function advtrains.get_wagon_prototype(data)
 		data.type = data.entity_name
 		data.entity_name = nil
 	end
-	if not wt or not advtrains.wagon_prototypes[wt] then
+	local rt, proto = advtrains.resolve_wagon_alias(wt)
+	if not rt then
 		atwarn("Unable to load wagon type",wt,", using placeholder")
-		wt="advtrains:wagon_placeholder"
+		rt = "advtrains:wagon_placeholder"
+		proto = advtrains.wagon_prototypes[rt]
 	end
-	return wt, advtrains.wagon_prototypes[wt]
+	return rt, proto
 end
 
 function advtrains.register_wagon_alias(src, dst)
 	advtrains.wagon_alias[src] = dst
 end
 
+local function recursive_resolve_alias(name, seen)
+	local prototype = rawget(advtrains.wagon_prototypes, name)
+	if prototype then
+		return name, prototype
+	end
+	local resolved = advtrains.wagon_alias[name]
+	if resolved and not seen[resolved] then
+		seen[name] = true
+		return recursive_resolve_alias(resolved, seen)
+	end
+end
+
+function advtrains.resolve_wagon_alias(name)
+	return recursive_resolve_alias(name, {})
+end
+
 function advtrains.standard_inventory_formspec(self, pname, invname)
 	--[[minetest.chat_send_player(pname, string.format("self=%s, pname=%s, invname=%s", self, pname, invname))
 	for k,v in pairs(self) do