Avoid duplication of mod metadata in memory (#12562)

Co-authored-by: sfan5 <sfan5@live.de>

games/devtest/mods/unittests/init.lua

@@ -179,6 +179,7 @@ dofile(modpath .. "/itemdescription.lua")
 dofile(modpath .. "/async_env.lua")
 dofile(modpath .. "/entity.lua")
 dofile(modpath .. "/content_ids.lua")
+dofile(modpath .. "/metadata.lua")
 
 --------------
 

games/devtest/mods/unittests/metadata.lua

@@ -0,0 +1,79 @@
+-- Tests of generic and specific metadata functionality
+
+local compare_meta = ItemStack("unittests:iron_lump"):get_meta()
+compare_meta:from_table({
+	fields = {
+		a = "1",
+		b = "2",
+		c = "3",
+		d = "4",
+		e = "e",
+	},
+})
+
+local function test_metadata(meta)
+	meta:from_table({fields = {a = 1, b = "2"}})
+	meta:set_string("c", "3")
+	meta:set_int("d", 4)
+	meta:set_string("e", "e")
+
+	meta:set_string("", "!")
+	meta:set_string("", "")
+
+	assert(meta:equals(compare_meta))
+
+	local tab = meta:to_table()
+	assert(tab.fields.a == "1")
+	assert(tab.fields.b == "2")
+	assert(tab.fields.c == "3")
+	assert(tab.fields.d == "4")
+	assert(tab.fields.e == "e")
+
+	assert(not meta:contains(""))
+	assert(meta:contains("a"))
+	assert(meta:contains("b"))
+	assert(meta:contains("c"))
+	assert(meta:contains("d"))
+	assert(meta:contains("e"))
+
+	assert(meta:get("") == nil)
+	assert(meta:get_string("") == "")
+	assert(meta:get_int("") == 0)
+	assert(meta:get_float("") == 0.0)
+	assert(meta:get("a") == "1")
+	assert(meta:get_string("a") == "1")
+	assert(meta:get_int("a") == 1)
+	assert(meta:get_float("a") == 1.0)
+	assert(meta:get_int("e") == 0)
+	assert(meta:get_float("e") == 0.0)
+
+	meta:set_float("f", 1.1)
+	meta:set_string("g", "${f}")
+	meta:set_string("h", "${g}")
+	meta:set_string("i", "${h}")
+	assert(meta:get_float("h") > 1)
+	assert(meta:get_string("i") == "${f}")
+
+	meta:from_table()
+	assert(next(meta:to_table().fields) == nil)
+
+	assert(not meta:equals(compare_meta))
+end
+
+local storage_a = core.get_mod_storage()
+local storage_b = core.get_mod_storage()
+local function test_mod_storage()
+	assert(rawequal(storage_a, storage_b))
+	test_metadata(storage_a)
+end
+unittests.register("test_mod_storage", test_mod_storage)
+
+local function test_item_metadata()
+	test_metadata(ItemStack("unittest:coal_lump"):get_meta())
+end
+unittests.register("test_item_metadata", test_item_metadata)
+
+local function test_node_metadata(player, pos)
+	test_metadata(minetest.get_meta(pos))
+end
+unittests.register("test_node_metadata", test_node_metadata, {map=true})