path: root/src/task.lua

----------------------------------------------------------------------------
-- Go style Channels for Lua
-- https://github.com/majek/lua-channels
--
-- This code is derived from libtask library by Russ Cox, mainly from
-- channel.c. Semantically channels as implemented here are quite
-- similar to channels from the Go language.
--
-- Usage (we're using unbuffered channel here):
--
-- local task = require('task')
--
-- local function counter(channel)
--    local i = 1
--    while true do
--        channel:send(i)
--        i = i + 1
--    end
-- end
--
-- local function main()
--     local channel = task.Channel:new()
--     task.spawn(counter, channel)
--     assert(channel:recv() == 1)
--     assert(channel:recv() == 2)
--     assert(channel:recv() == 3)
-- end
--
-- task.spawn(main)
-- task.scheduler()
--
--
-- This module exposes:
--
--  task.spawn(fun, [...]) - run fun as a coroutine with given
--                         parameters. You should use this instead of
--                         coroutine.create()
--
--  task.scheduler() - can be run only from the main thread, executes
--                     all the stuff, resumes the coroutines that are
--                     blocked on channels that became available. You
--                     can only do non-blocking sends / receives from
--                     the main thread.
--
--  task.Channel:new([buffer size]) - create a new channel with given size
--
--  task.chanalt(alts, can_block) - run alt / select / multiplex over
--                                  the alts structure. For example:
--
-- task.chanalt({{c = channel_1, op = task.RECV},
--               {c = channel_2, op = task.SEND, p = "hello"}}, true)
--
-- This will block current coroutine until it's possible to receive
-- from channel_1 or send to channel_2. chanalt returns a number of
-- statement from alts that succeeded (1 or 2 here) and a received
-- value if executed statement was RECV.
--
-- Finally, if two alt statements can be fulfilled at the same time,
-- we use math.random() to decide which one should go first. So it
-- makes sense to initialize seed with something random. If you don't
-- have access to an entropy source you can do:
--   math.randomseed(os.time())
-- but beware, the results of random() will predictable to a attacker.
----------------------------------------------------------------------------

local _M = {}

-- Constants
local RECV = 0x1
local SEND = 0x2
local NOP  = 0x3

-- Global objects for scheduler
local tasks_runnable = {}       -- list of coroutines ready to be resumed


----------------------------------------------------------------------------
--- Helpers

local function random_choice(arr)
   if #arr > 1 then
      return arr[math.random(#arr)]
   else
      return arr[1]
   end
end

-- Specialised Set data structure (with random element selection)
local Set = {
   new = function(self)
      local o = {a = {}, l = {}}; setmetatable(o, self); self.__index = self
      return o
   end,

   add = function(self, v)
      local a, l = self.a, self.l
      if a[v] == nil then
         table.insert(l, v)
         a[v] = #l
         return true
      end
   end,

   remove = function(self, v)
      local a, l = self.a, self.l
      local i = a[v]
      if i > 0 then
         local t = l[#l]
         a[t], l[i] = i, t
         a[i], l[#l] = nil, nil
         return true
      end
   end,

   random = function(self, v)
      return random_choice(self.l)
   end,

   len = function(self)
      return #self.l
   end,
}

-- Circular Buffer data structure
local CircularBuffer = {
   new = function(self, size)
      local o = {b = {}, slots = size + 1, size = size, l = 0, r = 0}
      setmetatable(o, self); self.__index = self
      return o
   end,

   len = function(self)
      return (self.r - self.l) % self.slots
   end,

   pop = function(self)
      assert(self.l ~= self.r)
      local v = self.b[self.l]
      self.l = (self.l + 1) % self.slots
      return v
   end,

   push = function(self, v)
      self.b[self.r] = v
      self.r = (self.r + 1) % self.slots
      assert(self.l ~= self.r)
   end,
}

----------------------------------------------------------------------------
-- Scheduling
--
-- Tasks ready to be run are placed on a stack and it's possible to
-- starve a coroutine.
local function scheduler()
   local self_coro, is_main = coroutine.running()

   -- We actually don't care if scheduler is run from the main
   -- coroutine. But we do need to make sure that user doesn't do
   -- blocking operation from it, as it can't yield.

   -- Be compatible with 5.1 and 5.2
   assert(not(self_coro ~= nil and is_main ~= true),
          "Scheduler must be run from the main coroutine.")

   local i = 0
   while #tasks_runnable > 0 do
      local co = table.remove(tasks_runnable)
      local okay, emsg = coroutine.resume(co)
      if not okay then
         error(emsg)
      end
      i = i + 1
   end
   return i
end

local function task_ready(co)
   table.insert(tasks_runnable, co)
end

local function spawn(fun, ...)
   local args = {...}

   local f = function()
      fun(unpack(args))
   end
   local co = coroutine.create(f)
   task_ready(co)
end

----------------------------------------------------------------------------
-- Channels - chanalt and helpers

-- Given two Alts from a single channel exchange data between
-- them. It's implied that one is RECV and another is SEND. Channel
-- may be buffered.
local function altcopy(a, b)
   local r, s, c = a, b, a.c
   if r.op == SEND then
      r, s = s, r
   end

   assert(s == nil or s.op == SEND)
   assert(r == nil or r.op == RECV)

   -- Channel is empty or unbuffered, copy directly
   if s ~= nil and r and c._buf:len() == 0 then
      r.alt_array.value = s.p
      return
   end

   -- Otherwise it's always okay to receive and then send.
   if r ~= nil then
      r.alt_array.value = c._buf:pop()
   end
   if s ~= nil then
      c._buf:push(s.p)
   end
end

-- Given enqueued alt_array from a chanalt statement remove all alts
-- from the associated channels.
local function altalldequeue(alt_array)
   for i = 1, #alt_array do
      local a = alt_array[i]
      if a.op == RECV or a.op == SEND then
         a.c:_get_alts(a.op):remove(a)
      end
   end
end

-- Can this Alt be execed without blocking?
local function altcanexec(a)
   local c, op = a.c, a.op
   if c._buf.size == 0 then
      if op ~= NOP then
         return c:_get_other_alts(op):len() > 0
      end
   else
      if op == SEND then
         return c._buf:len() < c._buf.size
      elseif op == RECV then
         return c._buf:len() > 0
      end
   end
end

-- Alt can be execed so find a counterpart Alt and exec it!
local function altexec(a)
   local c, op = a.c, a.op
   local other_alts = c:_get_other_alts(op)
   local other_a = other_alts:random()
   -- other_a may be nil
   altcopy(a, other_a)
   if other_a ~= nil then
      -- Disengage from channels used by the other Alt and make it ready.
      altalldequeue(other_a.alt_array)
      other_a.alt_array.resolved = other_a.alt_index
      task_ready(other_a.alt_array.task)
   end
end

function ischannel(x)
    return type(x) == "table" and x.__index == _M.Channel
end

-- The main entry point. Call it `alt` or `select` or just a
-- multiplexing statement. This is user facing function so make sure
-- the parameters passed are sane.
local function chanalt(alt_array, canblock)
   assert(#alt_array)

   local list_of_canexec_i = {}
   for i = 1, #alt_array do
      local a = alt_array[i]
      a.alt_array = alt_array
      a.alt_index = i
      assert(type(a.op) == "number" and
                (a.op == RECV or a.op == SEND or a.op == NOP),
             "op field must be RECV, SEND or NOP in alt")
      assert(ischannel(a.c),
             "pass valid channel to a c field of alt")
      if altcanexec(a) == true then
         table.insert(list_of_canexec_i, i)
      end
   end

   if #list_of_canexec_i > 0 then
      local i = random_choice(list_of_canexec_i)
      altexec(alt_array[i])
      return i, alt_array.value
   end

   if canblock ~= true then
      return nil
   end

   local self_coro, is_main = coroutine.running()
   alt_array.task = self_coro
   assert(self_coro ~= nil and is_main ~= true,
          "Unable to block from the main thread, run scheduler.")

   for i = 1, #alt_array do
      local a = alt_array[i]
      if a.op ~= NOP then
         a.c:_get_alts(a.op):add(a)
      end
   end

   -- Make sure we're not woken by someone who is not the scheduler.
   alt_array.resolved = nil
   coroutine.yield()
   assert(alt_array.resolved > 0)

   local r = alt_array.resolved
   return r, alt_array.value
end


----------------------------------------------------------------------------
-- Channel object

local Channel = {
   new = function(self, buf_size)
      local o = {}; setmetatable(o, self); self.__index = self
      o._buf = CircularBuffer:new(buf_size or 0)
      o._recv_alts, o._send_alts = Set:new(), Set:new()
      return o
   end,

   send = function(self, msg)
      assert(chanalt({{c = self, op = SEND, p = msg}}, true) == 1)
      return true
   end,

   recv = function(self)
      local alts = {{c = self, op = RECV}}
      local s, msg = chanalt(alts, true)
      assert(s == 1)
      return msg
   end,

   nbsend = function(self, msg)
      local s = chanalt({{c = self, op = SEND, p = msg}}, false)
      return s == 1
   end,

   nbrecv = function(self)
      local s, msg = chanalt({{c = self, op = RECV}}, false)
      return s == 1, msg
   end,

   _get_alts = function(self, op)
      return (op == RECV) and self._recv_alts or self._send_alts
   end,

   _get_other_alts = function(self, op)
      return (op == SEND) and self._recv_alts or self._send_alts
   end,

   __tostring = function(self)
      return string.format("<Channel size=%i/%i send_alt=%i recv_alt=%i>",
                           self._buf:len(), self._buf.size, self._send_alts:len(),
                           self._recv_alts:len())
   end,

   __call = function(self)
      local function f(s, v)
         return true, self:recv()
      end
      return f, nil, nil
   end,
}

----------------------------------------------------------------------------
-- Public interface

_M.scheduler = scheduler
_M.spawn     = spawn
_M.Channel   = Channel
_M.chanalt   = chanalt
_M.RECV      = RECV
_M.SEND      = SEND
_M.NOP       = NOP

----------------------------------------------------------------------------
----------------------------------------------------------------------------
-- Tests

local task = _M

function test_counter()
   local done
   local function counter(c)
      local i = 1
      while true do
         c:send(i)
         i = i + 1
      end
   end
   local function main()
      local c = task.Channel:new()
      task.spawn(counter, c)
      assert(c:recv() == 1)
      assert(c:recv() == 2)
      assert(c:recv() == 3)
      assert(c:recv() == 4)
      assert(c:recv() == 5)
      done = true
   end
   task.spawn(main)
   task.scheduler()
   assert(done)
end

function test_nonblocking_channel()
   local done
   local function main()
      local b = task.Channel:new()
      assert(b:nbsend(1) == false)
      assert(b:nbrecv() == false)

      local c = task.Channel:new(1)
      assert(c:nbrecv() == false)
      assert(c:nbsend(1) == true)
      assert(c:nbsend(1) == false)
      local r, v = c:nbrecv()
      assert(r == true)
      assert(v == 1)
      assert(c:nbrecv() == false)
      done = true
   end
   task.spawn(main)
   task.scheduler()
   assert(done)
end

function test_concurrent_send_and_recv()
   local l = {}
   local function a(c, name)
      -- Blocking send and recv from the same process
      local alt = {{c = c, op = task.SEND, p = 1},
                   {c = c, op = task.RECV}}
      local i, v = task.chanalt(alt, true)
      local k = string.format('%s %s', name, i == 1 and "send" or "recv")
      l[k] = (l[k] or 0) + 1
   end

   for i = 0, 1000 do
      -- On Mac OS X in lua 5.1 initializing seed with a
      -- predictable value makes no sense. For all seeds from 1 to
      -- 1000 the result of math.random(1,3) is _exactly_ the same!
      -- So beware, when seeding!
      -- math.randomseed(i)
      local c = task.Channel:new()
      task.spawn(a, c, "a")
      task.spawn(a, c, "b")
      task.scheduler()
   end

   -- Make sure we have randomness, that is: events occur in both
   -- orders in 1000 runs
   assert(l['a recv'] > 0)
   assert(l['a send'] > 0)
   assert(l['b recv'] > 0)
   assert(l['b send'] > 0)
end

function test_channels_from_a_coroutine()
   local done
   local c = task.Channel:new()
   local function a()
      for i = 1, 100 do
         c:send(i)
      end
   end
   local function b()
      assert(c:recv() == 1)
      assert(c:recv() == 2)
      assert(c:recv() == 3)
      assert(c:recv() == 4)
      assert(c:recv() == 5)
      done = true
   end
   local a_co = coroutine.create(a)
   local b_co = coroutine.create(b)
   coroutine.resume(a_co)
   coroutine.resume(b_co)
   task.scheduler()
   assert(done)
end

function test_fibonacci()
   local done
   local function fib(c)
      local x, y = 0, 1
      while true do
         c:send(x)
         x, y = y, x + y
      end
   end
   local function main(c)
      assert(c:recv() == 0)
      assert(c:recv() == 1)
      assert(c:recv() == 1)
      assert(c:recv() == 2)
      assert(c:recv() == 3)
      assert(c:recv() == 5)
      assert(c:recv() == 8)
      assert(c:recv() == 13)
      assert(c:recv() == 21)
      assert(c:recv() == 34)
      done = true
   end

   local c = task.Channel:new()
   task.spawn(fib, c)
   task.spawn(main, c)
   task.scheduler()
   assert(done)
end

function test_non_blocking_chanalt()
   local done
   local function main()
      local c = task.Channel:new()
      local alts = {{c = c, op = task.RECV},
                    {c = c, op = task.NOP},
                    {c = c, op = task.SEND, p = 1}}
      assert(task.chanalt(alts, false) == nil)

      local c = task.Channel:new(1)
      local alts = {{c = c, op = task.RECV},
                    {c = c, op = task.NOP},
                    {c = c, op = task.SEND, p = 1}}
      assert(task.chanalt(alts, false) == 3)
      assert(task.chanalt(alts, false) == 1)

      local alts = {{c = c, op = task.NOP}}
      assert(task.chanalt(alts, false) == nil)

      done = true
   end
   task.spawn(main)
   task.scheduler()
   assert(done)
end

-- Apparently it's not really a Sieve of Eratosthenes:
--   http://www.cs.hmc.edu/~oneill/papers/Sieve-JFP.pdf
function test_eratosthenes_sieve()
   local done
   local function counter(c)
      local i = 2
      while true do
         c:send(i)
         i = i + 1
      end
   end

   local function filter(p, recv_ch, send_ch)
      while true do
         local i = recv_ch:recv()
         if i % p ~= 0 then
            send_ch:send(i)
         end
      end
   end

   local function sieve(primes_ch)
      local c = task.Channel:new()
      task.spawn(counter, c)
      while true do
         local p, newc = c:recv(), task.Channel:new()
         primes_ch:send(p)
         task.spawn(filter, p, c, newc)
         c = newc
      end
   end

   local function main()
      local primes = task.Channel:new()
      task.spawn(sieve, primes)
      assert(primes:recv() == 2)
      assert(primes:recv() == 3)
      assert(primes:recv() == 5)
      assert(primes:recv() == 7)
      assert(primes:recv() == 11)
      assert(primes:recv() == 13)
      done = true
   end

   task.spawn(main)
   task.scheduler()
   assert(done)
end

function test_channel_as_iterator()
   local done
   local function counter(c)
      local i = 2
      while true do
         c:send(i)
         i = i + 1
      end
   end

   local function main()
      local numbers = task.Channel:new()
      task.spawn(counter, numbers)
      for _, j in numbers() do
         if j == 100 then
            break
         end
         done = true
      end
   end
   if _VERSION == "Lua 5.1" then
      -- sorry, this test doesn't work in 5.1
      print('S')
      done = true
   else
      task.spawn(main)
      task.scheduler()
   end
   assert(done)
end

return _M