Source code for rwlock.imp_0.imp

"""
Module **rwlock.imp_0.imp** has class ``Imp`` that implements
the read-write-lock service except that it allows writes to starve.
To instantiate and drive ``Imp`` from **rwlock/user.py**, in the parent
directory run "``python user.py <num_threads> <num_ops> imp_0.imp``".
Or instantiate ``Imp`` in your own program and drive it from there.
"""

from threading import Lock, Condition

class Imp:
  """
  An implementation of the read-write-lock service except that ``acqw()``
  can starve if there is a continuous stream of ``acqr(); relr()``s.
  It has four functions: ``acqr()``,  ``relr()``, ``acqw()`` and ``relw()``.
  It maintains the following variables:

  - ``nr`` (int): number of threads holding a read lock; initially 0.
  - ``nw`` (int): number of threads holding a write lock; initially 0.
  - ``lck`` (lock): protects ``nr`` and ``nw``.
  - ``cvr`` (condition): to wait for ``nw`` equal to 0.
  - ``cvw`` (condition): to wait for ``nr`` and ``nw`` equal to 0.

  When a thread executes ``acqw()``, it waits on ``cvw`` until ``nr``
  and ``nw`` are 0, then increments ``nw`` and returns.
  At this point, no thread can acquire a write lock or a read lock.
  When a thread executes ``relw()``, it decrements ``nw`` and returns.

  When a thread executes ``acqr()``, it waits on ``cvr`` until ``nw``
  is 0, then increments ``nr`` and returns.
  At this point a thread can get a read lock but not a write lock.
  When a thread executes ``relr()``, it decrements ``nr`` and returns.
  """

  def __init__(self):
    self.nr = 0
    self.nw = 0
    self.lck = Lock()
    self.cvr = Condition(self.lck)
    self.cvw = Condition(self.lck)

  def acqr(self):
    with self.cvr:
      while self.nw > 0:
        self.cvr.wait()
      self.nr += 1

  def relr(self):
    with self.lck:
      self.nr -= 1
      self.cvw.notify()

  def acqw(self):
    with self.cvw:
      while self.nr + self.nw > 0:
        self.cvw.wait()
      self.nw += 1

  def relw(self):
    with self.lck:
      self.nw -= 1
      self.cvr.notify_all()
      self.cvw.notify()

##### end class Imp #############################

if __name__ == '__main__':
  imp = Imp()