用于在 3 个进程之间写入(+锁定)/读取文件的 Python 脚本

Question

我是 Python 编程的新手。我需要编写一个脚本，该脚本将由 3 个新创建的进程在不同时间但非常接近地执行。资源始终需要检查文件是否被锁定，因为执行 python 脚本的第一个进程将执行命令，并锁定文件以将命令的结果写入其中。而其他 2 个进程将等待文件解锁，以便读取结果 ID 并执行与第一个进程不同的命令。

我怎样才能在 python 中实现它？谢谢

Answer 1

如果我没记错的话，这个任务可以通过使用 Lock 来完成。为了获得写入权限，一个 Semaphore通知未能获得写锁的进程将要读取文件，最后是 Barrier重新调整流程。一个例子:

import multiprocessing as mp
import time
from random import randint


def fun(lock_w, sem_r, barrier, task, filename):
  me = mp.current_process()
  for i in range(3):
    time.sleep(randint(1, 4))
    if(lock_w.acquire(block=False)):
      print(me.pid, "write access")
      task()
      sem_r.release()
      sem_r.release()
    else:
      sem_r.acquire()
      print(me.pid, "read access")
      task()
    if barrier.wait() == 0:
      print(me.pid, "releasing Lock")
      lock_w.release()

def task1():
  print("\tPerform Task 1")

def task2():
  print("\tPerform Task 2")

def task3():
  print("\tPerform Task 3")

lock_w = mp.Lock()
sem_r = mp.Semaphore(0)
bar = mp.Barrier(3)
t1 = mp.Process(target=fun, args=(lock_w, sem_r, bar, task1, "foo.txt", ))
t2 = mp.Process(target=fun, args=(lock_w, sem_r, bar, task2, "foo.txt", ))
t3 = mp.Process(target=fun, args=(lock_w, sem_r, bar, task3, "foo.txt", ))
t1.start()
t2.start()
t3.start()
t1.join()
t2.join()
t3.join()

示例输入:

585 write access
    Perform Task 1
586 read access
    Perform Task 2
587 read access
    Perform Task 3
585 releasing Lock
587 write access
    Perform Task 3
586 read access
    Perform Task 2
585 read access
    Perform Task 1
587 releasing Lock
586 write access
    Perform Task 2
585 read access
    Perform Task 1
587 read access
    Perform Task 3
586 releasing Lock