Set up database version v3 and its migration from v2

+ Check versions to choose v1->v2 or v2->v3

init.lua

@@ -2,6 +2,9 @@ mail = {
 	-- api version
 	apiversion = 1.1,
 
+	-- database version
+	dbversion = 3.0,
+	
 	-- mail directory
 	maildir = minetest.get_worldpath().."/mails",
 	contactsdir = minetest.get_worldpath().."/mails/contacts"

migrate.lua

@@ -1,7 +1,18 @@
+function mail.migrate()
+	local gen_file_v1 = io.open(minetest.get_worldpath().."/mail.db", "r")
+	if file then
+		mail.migrate_v1_to_v2()
+	end
+	
+	local info_file_v3 = mail.read_json_file(mail.maildir .. "/mail.info.json")
+	if not info_file_v3.dbversion then
+		mail.migrate_v2_to_v3()
+	end
+end
+	
 
 -- migrate from mail.db to player-file-based mailbox
-
+function mail.migrate_v1_to_v2()
-function mail.migrate()
 	-- create directory, just in case
 	minetest.mkdir(mail.maildir)
 	minetest.mkdir(mail.contactsdir)
@@ -25,8 +36,63 @@ function mail.migrate()
 
 end
 
+-- migrate from v2 to v3 database
+function mail.migrate_v2_to_v3()
+	minetest.log("info", "[mail] Migration from v2 to v3 database")
+	minetest.after(0,function()
+		for playername, value in minetest.get_auth_handler().iterate() do
+			mail.migrate_contacts_v2_to_v3(playername)
+		end
+	end)
+	mail.migrate_messages_v2_to_v3()
+	local info_file = mail.read_json_file(mail.maildir .. "/mail.info.json")
+	mail.write_json_file(mail.maildir .. "/mail.info.json", { dbversion = 3.0 })
+end
+
+function mail.migrate_messages_v2_to_v3()
+	local already_processed = {} -- store messages that are already process to avoid duplicates
+	minetest.after(0,function()
+		-- check in every inbox to fetch messages
+		for playername, value in minetest.get_auth_handler().iterate() do
+			local saneplayername = string.gsub(playername, "[.|/]", "")
+			local player_inbox = mail.read_json_file(mail.maildir .. "/" .. saneplayername .. ".json")
+			for _, msg in ipairs(player_inbox) do
+				local msg_id = tostring(msg.time) .. msg.sender -- id like "123456789.0singleplayer" -- it presumes that a same sender cannot send two mails within a second
+				local new_msg = true -- check if that mail was already processed with another player
+				for _, cur_id in ipairs(already_processed) do
+					if cur_id == msg_id then
+						new_msg = false
+						break
+					end
+				end
+				if new_msg then
+					local msg_table = {
+						sender  = msg.sender,
+						to      = msg.to,
+						cc      = msg.cc,
+						subject = msg.subject,
+						body    = msg.body,
+						time    = msg.time,
+					}
+					mail.addMessage(msg_table)
+					table.insert(already_processed, msg_id)
+				end
+			end
+		end
+	end)
+end
 
 function mail.migrate_contacts(playername)
+	local gen_file_v1 = io.open(minetest.get_worldpath().."/mail.db", "r")
+	if file then
+		mail.migrate_contacts_v1_to_v2()
+	end
+	
+	-- v2 to v3 directly in general function
+end
+
+
+function mail.migrate_contacts_v1_to_v2(playername)
 	local file = io.open(mail.getContactsFile(playername), 'r')
 	if not file then
 		-- file doesn't exist! This is a case for Migrate Man!
@@ -48,3 +114,11 @@ function mail.migrate_contacts(playername)
 		file:close()	-- uh, um, nope, let's leave those alone, shall we?
 	end
 end
+
+function mail.migrate_contacts_v2_to_v3(playername)
+	local player_contacts = mail.read_json_file(mail.maildir .. "/contacts/" .. playername .. ".json")
+	
+	for _, c in pairs(player_contacts) do
+		mail.addContact(playername, { name = c.name, note = c.note })
+	end
+end