Python：构建可重入信号量（结合 RLock 和信号量）

Question

您将如何将threading.RLock 与threading.Semaphore 结合起来？还是这样的结构已经存在？

在 Python 中，有一个可重入锁的原语threading.RLock(N)，它允许同一个线程多次获取锁，但其他线程不能。还有threading.Semaphore(N)，它允许在阻塞之前获取锁N次。如何将这两种结构结合起来？我希望最多 N 单独的线程能够获取锁，但我希望线程上的每个单独的锁都是可重入的。

Answer 1

所以我猜不存在可重入信号量。这是我想出的实现，很高兴招待 cmets。

import threading
import datetime
class ReentrantSemaphore(object):
  '''A counting Semaphore which allows threads to reenter.'''
  def __init__(self, value = 1):
    self.local = threading.local()
    self.sem = threading.Semaphore(value)

  def acquire(self):
    if not getattr(self.local, 'lock_level', 0):
      # We do not yet have the lock, acquire it.
      start = datetime.datetime.utcnow()
      self.sem.acquire()
      end = datetime.datetime.utcnow()
      if end - start > datetime.timedelta(seconds = 3):
        logging.info("Took %d Sec to lock."%((end - start).total_seconds()))
      self.local.lock_time = end
      self.local.lock_level = 1
    else:
      # We already have the lock, just increment it due to the recursive call.
      self.local.lock_level += 1

  def release(self):
    if getattr(self.local, 'lock_level', 0) < 1:
      raise Exception("Trying to release a released lock.")

    self.local.lock_level -= 1
    if self.local.lock_level == 0:
      self.sem.release()

  __enter__ = acquire
  def __exit__(self, t, v, tb):
    self.release()