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-- WaitQueue isa Queue with the added feature that when the queue is empty,
-- the Dequeue() operation blocks the calling coroutine until some other
-- coroutine Enqueue()s a new value.
local fiber = require('fiber')
local Queue = require('Queue')
local function dbg(...) end
-- local dbg = require('printf')
local WaitQueue = Queue:new()
function WaitQueue:new()
local obj = Queue:new()
setmetatable(obj, self)
self.__index = self
obj._waiters = {}
obj._closed = false
return obj
end
function WaitQueue:Enqueue(value)
if self._closed then
error("can't Enqueue() on closed Queue")
end
-- can't simply call Queue:Enqueue(value)! That calls the method on the
-- Queue class definition, instead of calling Queue:Enqueue() on self.
-- Hand-expand the Queue:Enqueue() syntactic sugar.
Queue.Enqueue(self, value)
self:_wake_waiters()
end
function WaitQueue:_wake_waiters()
-- WaitQueue is designed to support multi-producer, multi-consumer use
-- cases. With multiple consumers, if more than one is trying to
-- Dequeue() from an empty WaitQueue, we'll have multiple waiters.
-- Unlike OS threads, with cooperative concurrency it doesn't make sense
-- to "notify all": we need wake only one of the waiting Dequeue()
-- callers.
if ((not self:IsEmpty()) or self._closed) and next(self._waiters) then
-- Pop the oldest waiting coroutine instead of the most recent, for
-- more-or-less round robin fairness. But skip any coroutines that
-- have gone dead in the meantime.
local waiter = table.remove(self._waiters, 1)
while waiter and fiber.status(waiter) == "dead" do
waiter = table.remove(self._waiters, 1)
end
-- do we still have at least one waiting coroutine?
if waiter then
-- don't pass the head item: let the resumed coroutine retrieve it
fiber.wake(waiter)
end
end
end
function WaitQueue:Dequeue()
while self:IsEmpty() do
-- Don't check for closed until the queue is empty: producer can close
-- the queue while there are still items left, and we want the
-- consumer(s) to retrieve those last few items.
if self._closed then
dbg('WaitQueue:Dequeue(): closed')
return nil
end
dbg('WaitQueue:Dequeue(): waiting')
-- add the running coroutine to the list of waiters
dbg('WaitQueue:Dequeue() running %s', tostring(coroutine.running() or 'main'))
table.insert(self._waiters, fiber.running())
-- then let somebody else run
fiber.wait()
end
-- here we're sure this queue isn't empty
dbg('WaitQueue:Dequeue() calling Queue.Dequeue()')
return Queue.Dequeue(self)
end
function WaitQueue:close()
self._closed = true
-- close() is like Enqueueing an end marker. If there are waiting
-- consumers, give them a chance to see we're closed.
self:_wake_waiters()
end
return WaitQueue
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