Nim线程间消息传递:如何避免全局TChannel？

Question

我有一个线程间通信问题的简单示例:我想在后台线程中运行任意"随时"算法.随时算法T逐步地执行结果类型的一些计算,即,它偶尔产生更新,更精确的结果.用Nim的说法,它们最好用迭代器代表.在主线程中,我现在想要将这些迭代器包装在它自己的线程中,并且有可能在线程中查询"是否有新值可用"或"当前计算结果是什么".

由于我不熟悉Nim的并发概念,因此无法实现所需的线程间通信.我的想法是使用a TChannel进行通信.根据这个论坛帖子,一个TChannel不能与之结合使用spawn但需要使用createThread.我设法得到以下内容来编译和运行:

import os, threadpool

proc spawnBackgroundJob[T](f: iterator (): T): TChannel[T] =

  type Args = tuple[iter: iterator (): T, channel: ptr TChannel[T]]

  # I think I have to wrap the iterator to pass it to createThread
  proc threadFunc(args: Args) {.thread.} =
    echo "Thread is starting"
    let iter = args.iter
    var channel = args.channel[]

    for i in iter():
      echo "Sending ", i
      channel.send(i)

  var thread: TThread[Args]
  var channel: TChannel[T]
  channel.open()

  let args = (f, channel.addr)
  createThread(thread, threadFunc, args)

  result = channel


# example use in some main thread:
iterator test(): int {.closure.} =
  sleep(500)
  yield 1
  sleep(500)
  yield 2

var channel = spawnBackgroundJob[int](test)

for i in 0 .. 10:
  sleep(200)
  echo channel.peek()

echo "Finished"

不幸的是,这没有预期的行为,即我从未在主线程中收到任何内容.IRC告诉我,问题是我不使用全局变量.但即使经过很长一段时间的思考,我也没有确切地看到为什么会失败,也没有办法解决它.问题是我不能简单地使变量thread和channel全局变量,因为它们取决于类型T.我还想避免将此限制为仅运行单个随时算法(或其他一些固定数N).我还被告知这种方法总体上没有意义,所以也许我只是错过了这个问题有一个完全不同的解决方案？

Answer 1

原因：

您在 send 和 receive 中使用两个不同的通道。

Nim 中的对象分配是深复制，它们是不同的对象。

var channel = args.channel[]

和

result = channel

为了解释它，请参见下面的代码片段：

type
  A = object
    x: int
    y: int

var a,b: A

var c = cast[ptr A](allocShared0(sizeof(A))) # shared memory allocation

a = c[]
b = c[]

echo a.x, a.y, b.x, b.y, c.x, c.y # output: 000000

a.x = 1
a.y = 2

echo a.x, a.y, b.x, b.y, c.x, c.y # output: 120000

b.x = 3
b.y = 4

echo a.x, a.y, b.x, b.y, c.x, c.y # output: 123400

传递通道进出过程的解决方案：

要将 Channel 作为参数和返回值传递，请参考Nim 论坛Jehan 的答案。

将 Jehan 的答案粘贴到此处以供快速参考，并使其在 Nim 0.11.2 中编译通过

type SharedChannel[T] = ptr TChannel[T]

proc newSharedChannel[T](): SharedChannel[T] =
  result = cast[SharedChannel[T]](allocShared0(sizeof(TChannel[T])))
  open(result[])

proc close[T](ch: var SharedChannel[T]) =
  close(ch[])
  deallocShared(ch)
  ch = nil

proc send[T](ch: SharedChannel[T], content: T) =
  ch[].send(content)


proc recv[T](ch: SharedChannel[T]): T =
  result = ch[].recv

proc someThread(ch: (SharedChannel[string], SharedChannel[bool])) {.thread.} =
  let (mainChannel, responseChannel) = ch
  while true:
    let s = mainChannel.recv
    if s == nil:
      break
    echo s
    responseChannel.send(true)
  responseChannel.send(false)

proc main() =
  var
    mainChannel = newSharedChannel[string]()
    responseChannel = newSharedChannel[bool]()
    th: TThread[(SharedChannel[string], SharedChannel[bool])]
  createThread(th, someThread, (mainChannel, responseChannel))
  for i in 0..2:
    echo("main thread send: " & $i)
    mainChannel.send($i)
    if not responseChannel.recv:
      break
  mainChannel.send(nil)
  joinThread(th)
  close(mainChannel)
  close(responseChannel)

main()

输出：

main thread send: 0
0
main thread send: 1
1
main thread send: 2
2

再一步，解决这个问题：

import os, threadpool, macros

template spawnBackgroundJob(t: typedesc, chan:ptr TChannel[t], iter: expr): stmt {.immediate.}=
  block:
    proc threadFunc(channel: ptr TChannel[t]) {.thread.} =
      echo "Thread is starting"

      for i in iter:
        echo "Sending ", i
        channel[].send(i)

    channel[].open()

    var thread: TThread[ptr TChannel[t]]
    createThread(thread, threadFunc, chan)
    #joinThread(thread)


# example use in some main thread:
iterator testJob(): int =
  yield 0
  sleep(500)
  yield 1
  sleep(500)
  yield 2

var channel: ptr TChannel[int]
channel = cast[ptr TChannel[int]](allocShared0(sizeof(TChannel[int])))
spawnBackgroundJob(type(int), channel, testJob())

for i in 1 .. 10:
  sleep(200)
  echo channel[].peek()

channel[].close()