Move areas:save() into async

On newer Minetest servers, handles saving jobs in async environment. To prevent conflicts, the save file is locked whie saving, and if a code requests saving while the file is locked, data is saved again immediately after finishing the current save.

async.lua

@@ -0,0 +1,26 @@
+local areas = {}
+local core = core
+
+local safe_file_write = core.safe_file_write
+if safe_file_write == nil then
+    function safe_file_write(path, content)
+        local file, err = io.open(path, "w")
+        if err then
+            return err
+        end
+        file:write(content)
+        file:close()
+    end
+end
+
+-- Save the areas table to a file
+function areas:do_save(areas_tb, filename)
+    local datastr = core.write_json(areas_tb)
+    if not datastr then
+        core.log("error", "[areas] Failed to serialize area data!")
+        return
+    end
+    return safe_file_write(filename, datastr)
+end
+
+_G.areas = areas

init.lua

@@ -19,6 +19,10 @@ dofile(areas.modpath.."/interact.lua")
 dofile(areas.modpath.."/legacy.lua")
 dofile(areas.modpath.."/hud.lua")
 
+if core.register_async_dofile then
+	core.register_async_dofile(areas.modpath.."/async.lua")
+end
+
 areas:load()
 
 local S = minetest.get_translator("areas")

internal.lua

@@ -4,26 +4,54 @@ function areas:player_exists(name)
 	return minetest.get_auth_handler().get_auth(name) ~= nil
 end
 
-local safe_file_write = minetest.safe_file_write
-if safe_file_write == nil then
-	function safe_file_write(path, content)
-		local file, err = io.open(path, "w")
-		if err then
-			return err
+if core.register_async_dofile then
+	-- nil: not locked nor requested to save again
+	-- false: locked and no request for saving again
+	-- true: locked and will save again on releasing
+	local file_lock = nil
+
+	local function async_func(areas_tb, filepath)
+		return areas:do_save(areas_tb, filepath)
+	end
+
+	local function done_callback()
+		local old_lock = file_lock
+		file_lock = nil
+		if old_lock == true then
+			return areas:save()
 		end
-		file:write(content)
-		file:close()
 	end
-end
 
--- Save the areas table to a file
-function areas:save()
-	local datastr = minetest.write_json(self.areas)
-	if not datastr then
-		minetest.log("error", "[areas] Failed to serialize area data!")
-		return
+	function areas:save()
+		if file_lock == false then
+			file_lock = true
+		elseif file_lock == nil then
+			file_lock = false
+			return core.handle_async(async_func, done_callback, self.areas, self.config.filename)
+		end
+	end
+else
+	local safe_file_write = minetest.safe_file_write
+	if safe_file_write == nil then
+		function safe_file_write(path, content)
+			local file, err = io.open(path, "w")
+			if err then
+				return err
+			end
+			file:write(content)
+			file:close()
+		end
+	end
+
+	-- Save the areas table to a file
+	function areas:save()
+		local datastr = minetest.write_json(self.areas)
+		if not datastr then
+			minetest.log("error", "[areas] Failed to serialize area data!")
+			return
+		end
+		return safe_file_write(self.config.filename, datastr)
 	end
-	return safe_file_write(self.config.filename, datastr)
 end
 
 -- Load the areas table from the save file