rcu-based msgbox

Author: jperon

lib/mailbox.lua

@@ -1,14 +1,18 @@
 --
 -- SPDX-FileCopyrightText: (c) 2024 Ring Zero Desenvolvimento de Software LTDA
--- SPDX-License-Identifier: MIT OR GPL-2.0-only 
+-- SPDX-License-Identifier: MIT OR GPL-2.0-only
 --
 
 local fifo       = require("fifo")
 local completion = require("completion")
 
+local lunatik    = require("lunatik")
+local rcu_table  = require("rcu").table
+local new_data   = require("data").new
+
 -- Mailbox using fifo and completion
 
-local mailbox = {} 
+local mailbox = {}
 local MailBox = {}
 MailBox.__index = MailBox
 
@@ -61,5 +65,69 @@ function MailBox:send(message)
 	if self.event then self.event:complete() end
 end
 
+-- Msgbox using rcu
+
+local env = lunatik._ENV
+
+local MsgBox = {}
+MsgBox.__index = MsgBox
+
+local function new_rcu(name, hsize, allowed, forbidden)
+	hsize = hsize or 1024
+
+	local mbox = env[name]
+	if not mbox then
+		mbox = rcu_table(hsize)
+		mbox.last_write = new_data(8)
+		mbox.last_read = new_data(8)
+		env[name] = mbox
+	end
+	local self = {
+		name = name,
+	  [forbidden] = function () error(allowed .. "-only mailbox") end
+	}
+	return setmetatable(self, MsgBox)
+end
+
+function MsgBox:send(msg)
+	-- We don’t set mbox within the object, to avoid duplicates in race conditions
+	local mbox = env[self.name]
+	local last_write = mbox.last_write
+	local index = 1 + last_write:getnumber(0)
+
+	local msg_data = new_data(#msg)
+	msg_data:setstring(0, msg)
+	mbox[index] = msg_data
+
+	last_write:setnumber(0, index)
+end
+
+function MsgBox:receive()
+	local mbox = env[self.name]
+	local last_read = mbox.last_read
+	local last_write = mbox.last_write
+	local index = last_read:getnumber(0)
+
+	if index ~= last_write:getnumber(0) then
+		index = index + 1
+		local msg_data = mbox[index]
+
+		if msg_data then  -- This should be, but this condition avoids race conditions
+			last_read:setnumber(0, index)
+			local msg = msg_data:getstring(0)
+			mbox[index] = nil
+			return msg
+		end
+	end
+end
+
+function mailbox.rcu_inbox(name, hsize)
+	return new_rcu(name, hsize, 'receive', 'send')
+end
+
+function mailbox.rcu_outbox(name, hsize)
+	return new_rcu(name, hsize, 'send', 'receive')
+end
+
 return mailbox