openstack/deb-python-eventlet
Code Issues Proposed changes

Documented the hell out of semaphore, noticed that BoundedSemaphore had the wrong semantics so created a new BoundedSemaphore, renaming the old to CappedSemaphore, to correct for that.

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This commit is contained in:
Ryan Williams
2010-02-20 14:29:02 -05:00
parent 092b17ce9f
commit 4623461141
4 changed files with 136 additions and 15 deletions
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9
doc/modules/semaphore.rst
View File

@@ -1,6 +1,11 @@
:mod:`semaphore` -- Semaphore classes
==================================================
.. automodule:: eventlet.semaphore
.. autoclass:: eventlet.semaphore.Semaphore
:members:
:undoc-members:
.. autoclass:: eventlet.semaphore.BoundedSemaphore
:members:
.. autoclass:: eventlet.semaphore.CappedSemaphore
:members:

136
eventlet/semaphore.py
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@@ -5,12 +5,25 @@ class Semaphore(object):
"""An unbounded semaphore.
Optionally initialize with a resource *count*, then :meth:`acquire` and
:meth:`release` resources as needed. Attempting to :meth:`acquire` when
*count* is zero suspends the calling coroutine until *count* becomes
*count* is zero suspends the calling greenthread until *count* becomes
nonzero again.
This is API-compatible with :class:`threading.Semaphore`.
It is a context manager, and thus can be used in a with block::
sem = Semaphore(2)
with sem:
do_some_stuff()
If not specified, *value* defaults to 1.
"""
def __init__(self, count=0):
self.counter = count
def __init__(self, value=1):
self.counter = value
if value < 0:
raise ValueError("Semaphore must be initialized with a positive "
"number, got %s" % value)
self._waiters = set()
def __repr__(self):
@@ -22,13 +35,31 @@ class Semaphore(object):
return '<%s c=%s _w[%s]>' % params
def locked(self):
""" Returns true if a call to acquire would block."""
return self.counter <= 0
def bounded(self):
# for consistency with BoundedSemaphore
""" Returns False; for consistency with :class:`~eventlet.semaphore.CappedSemaphore`."""
return False
def acquire(self, blocking=True):
"""Acquire a semaphore.
When invoked without arguments: if the internal counter is larger than
zero on entry, decrement it by one and return immediately. If it is zero
on entry, block, waiting until some other thread has called release() to
make it larger than zero. This is done with proper interlocking so that
if multiple acquire() calls are blocked, release() will wake exactly one
of them up. The implementation may pick one at random, so the order in
which blocked threads are awakened should not be relied on. There is no
return value in this case.
When invoked with blocking set to true, do the same thing as when called
without arguments, and return true.
When invoked with blocking set to false, do not block. If a call without
an argument would block, return false immediately; otherwise, do the
same thing as when called without arguments, and return true."""
if not blocking and self.locked():
return False
if self.counter <= 0:
@@ -45,7 +76,12 @@ class Semaphore(object):
self.acquire()
def release(self, blocking=True):
# `blocking' parameter is for consistency with BoundedSemaphore and is ignored
"""Release a semaphore, incrementing the internal counter by one. When
it was zero on entry and another thread is waiting for it to become
larger than zero again, wake up that thread.
The *blocking* argument is for consistency with CappedSemaphore and is
ignored"""
self.counter += 1
if self._waiters:
hubs.get_hub().schedule_call_global(0, self._do_acquire)
@@ -61,22 +97,69 @@ class Semaphore(object):
@property
def balance(self):
"""An integer value that represents how many new calls to
:meth:`acquire` or :meth:`release` would be needed to get the counter to
0. If it is positive, then its value is the number of acquires that can
happen before the next acquire would block. If it is negative, it is
the negative of the number of releases that would be required in order
to make the counter 0 again (one more release would push the counter to
1 and unblock acquirers). It takes into account how many greenthreads
are currently blocking in :meth:`acquire`.
"""
# positive means there are free items
# zero means there are no free items but nobody has requested one
# negative means there are requests for items, but no items
return self.counter - len(self._waiters)
class BoundedSemaphore(object):
"""A bounded semaphore.
class BoundedSemaphore(Semaphore):
"""A bounded semaphore checks to make sure its current value doesn't exceed
its initial value. If it does, ValueError is raised. In most situations
semaphores are used to guard resources with limited capacity. If the
semaphore is released too many times it's a sign of a bug. If not given,
*value* defaults to 1."""
def __init__(self, value=1):
super(BoundedSemaphore, self).__init__(value)
self.original_counter = value
def release(self, blocking=True):
"""Release a semaphore, incrementing the internal counter by one. If
the counter would exceed the initial value, raises ValueError. When
it was zero on entry and another thread is waiting for it to become
larger than zero again, wake up that thread.
The *blocking* argument is for consistency with :class:`CappedSemaphore`
and is ignored"""
if self.counter >= self.original_counter:
raise ValueError, "Semaphore released too many times"
return super(BoundedSemaphore, self).release(blocking)
class CappedSemaphore(object):
"""A blockingly bounded semaphore.
Optionally initialize with a resource *count*, then :meth:`acquire` and
:meth:`release` resources as needed. Attempting to :meth:`acquire` when
*count* is zero suspends the calling coroutine until count becomes nonzero
*count* is zero suspends the calling greenthread until count becomes nonzero
again. Attempting to :meth:`release` after *count* has reached *limit*
suspends the calling coroutine until *count* becomes less than *limit*
suspends the calling greenthread until *count* becomes less than *limit*
again.
This has the same API as :class:`threading.Semaphore`, though its
semantics and behavior differ subtly due to the upper limit on calls
to :meth:`release`. It is **not** compatible with
:class:`threading.BoundedSemaphore` because it blocks when reaching *limit*
instead of raising a ValueError.
It is a context manager, and thus can be used in a with block::
sem = CappedSemaphore(2)
with sem:
do_some_stuff()
"""
def __init__(self, count, limit):
if count < 0:
raise ValueError("CappedSemaphore must be initialized with a "
"positive number, got %s" % count)
if count > limit:
# accidentally, this also catches the case when limit is None
raise ValueError("'count' cannot be more than 'limit'")
@@ -92,12 +175,31 @@ class BoundedSemaphore(object):
return '<%s b=%s l=%s u=%s>' % params
def locked(self):
"""Returns true if a call to acquire would block."""
return self.lower_bound.locked()
def bounded(self):
def bounded(self):
"""Returns true if a call to release would block."""
return self.upper_bound.locked()
def acquire(self, blocking=True):
"""Acquire a semaphore.
When invoked without arguments: if the internal counter is larger than
zero on entry, decrement it by one and return immediately. If it is zero
on entry, block, waiting until some other thread has called release() to
make it larger than zero. This is done with proper interlocking so that
if multiple acquire() calls are blocked, release() will wake exactly one
of them up. The implementation may pick one at random, so the order in
which blocked threads are awakened should not be relied on. There is no
return value in this case.
When invoked with blocking set to true, do the same thing as when called
without arguments, and return true.
When invoked with blocking set to false, do not block. If a call without
an argument would block, return false immediately; otherwise, do the
same thing as when called without arguments, and return true."""
if not blocking and self.locked():
return False
self.upper_bound.release()
@@ -114,6 +216,12 @@ class BoundedSemaphore(object):
self.acquire()
def release(self, blocking=True):
"""Release a semaphore. In this class, this behaves very much like
an :meth:`acquire` but in the opposite direction.
Imagine the docs of :meth:`acquire` here, but with every direction
reversed. When calling this method, it will block if the internal
counter is greater than or equal to *limit*."""
if not blocking and self.bounded():
return False
self.lower_bound.release()
@@ -128,4 +236,12 @@ class BoundedSemaphore(object):
@property
def balance(self):
"""An integer value that represents how many new calls to
:meth:`acquire` or :meth:`release` would be needed to get the counter to
0. If it is positive, then its value is the number of acquires that can
happen before the next acquire would block. If it is negative, it is
the negative of the number of releases that would be required in order
to make the counter 0 again (one more release would push the counter to
1 and unblock acquirers). It takes into account how many greenthreads
are currently blocking in :meth:`acquire` and :meth:`release`."""
return self.lower_bound.balance - self.upper_bound.balance

4
tests/semaphore_test.py
View File

@@ -5,7 +5,7 @@ from tests import LimitedTestCase
class TestSemaphore(LimitedTestCase):
def test_bounded(self):
sem = semaphore.BoundedSemaphore(2, limit=3)
sem = semaphore.CappedSemaphore(2, limit=3)
self.assertEqual(sem.acquire(), True)
self.assertEqual(sem.acquire(), True)
gt1 = eventlet.spawn(sem.release)
@@ -21,7 +21,7 @@ class TestSemaphore(LimitedTestCase):
gt2.wait()
def test_bounded_with_zero_limit(self):
sem = semaphore.BoundedSemaphore(0, 0)
sem = semaphore.CappedSemaphore(0, 0)
gt = eventlet.spawn(sem.acquire)
sem.release()
gt.wait()

2
tests/stdlib/test_thread__boundedsem.py
View File

@@ -8,7 +8,7 @@ def allocate_lock():
original_allocate_lock = thread.allocate_lock
thread.allocate_lock = allocate_lock
original_LockType = thread.LockType
thread.LockType = coros.BoundedSemaphore
thread.LockType = coros.CappedSemaphore
try:
import os.path