# cython: auto_pickle=False,embedsignature=True,always_allow_keywords=False
###
# This file is ``gevent._semaphore`` so that it can be compiled by Cython
# individually. However, this is not the place to import from. Everyone,
# gevent internal code included, must import from ``gevent.lock``.
# The only exception are .pxd files which need access to the
# C code; the PURE_PYTHON things that have to happen and which are
# handled in ``gevent.lock``, do not apply to them.
###
from __future__ import print_function, absolute_import, division
__all__ = [
'Semaphore',
'BoundedSemaphore',
]
from time import sleep as _native_sleep
from gevent._compat import monotonic
from gevent.exceptions import InvalidThreadUseError
from gevent.exceptions import LoopExit
from gevent.timeout import Timeout
def _get_linkable():
x = __import__('gevent._abstract_linkable')
return x._abstract_linkable.AbstractLinkable
locals()['AbstractLinkable'] = _get_linkable()
del _get_linkable
from gevent._hub_local import get_hub_if_exists
from gevent._hub_local import get_hub
from gevent.hub import spawn_raw
class _LockReleaseLink(object):
__slots__ = (
'lock',
)
def __init__(self, lock):
self.lock = lock
def __call__(self, _):
self.lock.release()
_UNSET = object()
_MULTI = object()
class Semaphore(AbstractLinkable): # pylint:disable=undefined-variable
"""
Semaphore(value=1) -> Semaphore
.. seealso:: :class:`BoundedSemaphore` for a safer version that prevents
some classes of bugs. If unsure, most users should opt for `BoundedSemaphore`.
A semaphore manages a counter representing the number of `release`
calls minus the number of `acquire` calls, plus an initial value.
The `acquire` method blocks if necessary until it can return
without making the counter negative. A semaphore does not track ownership
by greenlets; any greenlet can call `release`, whether or not it has previously
called `acquire`.
If not given, ``value`` defaults to 1.
The semaphore is a context manager and can be used in ``with`` statements.
This Semaphore's ``__exit__`` method does not call the trace function
on CPython, but does under PyPy.
.. versionchanged:: 1.4.0
Document that the order in which waiters are awakened is not specified. It was not
specified previously, but due to CPython implementation quirks usually went in FIFO order.
.. versionchanged:: 1.5a3
Waiting greenlets are now awakened in the order in which they waited.
.. versionchanged:: 1.5a3
The low-level ``rawlink`` method (most users won't use this) now automatically
unlinks waiters before calling them.
.. versionchanged:: 20.12.0
Improved support for multi-threaded usage. When multi-threaded usage is detected,
instances will no longer create the thread's hub if it's not present.
.. versionchanged:: 24.2.1
Uses Python 3 native lock timeouts for cross-thread operations instead
of spinning.
"""
__slots__ = (
'counter',
# long integer, signed (Py2) or unsigned (Py3); see comments
# in the .pxd file for why we store as Python object. Set to ``_UNSET``
# initially. Set to the ident of the first thread that
# acquires us. If we later see a different thread ident, set
# to ``_MULTI``.
'_multithreaded',
)
def __init__(self, value=1, hub=None):
self.counter = value
if self.counter < 0: # Do the check after Cython native int conversion
raise ValueError("semaphore initial value must be >= 0")
super(Semaphore, self).__init__(hub)
self._notify_all = False
self._multithreaded = _UNSET
def __str__(self):
return '<%s at 0x%x counter=%s _links[%s]>' % (
self.__class__.__name__,
id(self),
self.counter,
self.linkcount()
)
[docs]
def locked(self):
"""
Return a boolean indicating whether the semaphore can be
acquired (`False` if the semaphore *can* be acquired). Most
useful with binary semaphores (those with an initial value of 1).
:rtype: bool
"""
return self.counter <= 0
def release(self):
"""
Release the semaphore, notifying any waiters if needed. There
is no return value.
.. note::
This can be used to over-release the semaphore.
(Release more times than it has been acquired or was initially
created with.)
This is usually a sign of a bug, but under some circumstances it can be
used deliberately, for example, to model the arrival of additional
resources.
:rtype: None
"""
self.counter += 1
self._check_and_notify()
return self.counter
[docs]
def ready(self):
"""
Return a boolean indicating whether the semaphore can be
acquired (`True` if the semaphore can be acquired).
:rtype: bool
"""
return self.counter > 0
def _start_notify(self):
self._check_and_notify()
def _wait_return_value(self, waited, wait_success):
if waited:
return wait_success
# We didn't even wait, we must be good to go.
# XXX: This is probably dead code, we're careful not to go into the wait
# state if we don't expect to need to
return True
def wait(self, timeout=None):
"""
Wait until it is possible to acquire this semaphore, or until the optional
*timeout* elapses.
.. note:: If this semaphore was initialized with a *value* of 0,
this method will block forever if no timeout is given.
:keyword float timeout: If given, specifies the maximum amount of seconds
this method will block.
:return: A number indicating how many times the semaphore can be acquired
before blocking. *This could be 0,* if other waiters acquired
the semaphore.
:rtype: int
"""
if self.counter > 0:
return self.counter
self._wait(timeout) # return value irrelevant, whether we got it or got a timeout
return self.counter
def acquire(self, blocking=True, timeout=None):
"""
acquire(blocking=True, timeout=None) -> bool
Acquire the semaphore.
.. note:: If this semaphore was initialized with a *value* of 0,
this method will block forever (unless a timeout is given or blocking is
set to false).
:keyword bool blocking: If True (the default), this function will block
until the semaphore is acquired.
:keyword float timeout: If given, and *blocking* is true,
specifies the maximum amount of seconds
this method will block.
:return: A `bool` indicating whether the semaphore was acquired.
If ``blocking`` is True and ``timeout`` is None (the default), then
(so long as this semaphore was initialized with a size greater than 0)
this will always return True. If a timeout was given, and it expired before
the semaphore was acquired, False will be returned. (Note that this can still
raise a ``Timeout`` exception, if some other caller had already started a timer.)
"""
# pylint:disable=too-many-return-statements,too-many-branches
# Sadly, the body of this method is rather complicated.
if self._multithreaded is _UNSET:
self._multithreaded = self._get_thread_ident()
elif self._multithreaded != self._get_thread_ident():
self._multithreaded = _MULTI
# We conceptually now belong to the hub of the thread that
# called this, whether or not we have to block. Note that we
# cannot force it to be created yet, because Semaphore is used
# by importlib.ModuleLock which is used when importing the hub
# itself! This also checks for cross-thread issues.
invalid_thread_use = None
try:
self._capture_hub(False)
except InvalidThreadUseError as e:
# My hub belongs to some other thread. We didn't release the GIL/object lock
# by raising the exception, so we know this is still true.
invalid_thread_use = e.args
e = None
if not self.counter and blocking:
# We would need to block. So coordinate with the main hub.
return self.__acquire_from_other_thread(invalid_thread_use, blocking, timeout)
if self.counter > 0:
self.counter -= 1
return True
if not blocking:
return False
if self._multithreaded is not _MULTI and self.hub is None: # pylint:disable=access-member-before-definition
self.hub = get_hub() # pylint:disable=attribute-defined-outside-init
if self.hub is None and not invalid_thread_use:
# Someone else is holding us. There's not a hub here,
# nor is there a hub in that thread. We'll need to use regular locks.
# This will be unfair to yet a third thread that tries to use us with greenlets.
return self.__acquire_from_other_thread(
(None, None, self._getcurrent(), "NoHubs"),
blocking,
timeout
)
# self._wait may drop both the GIL and the _lock_lock.
# By the time we regain control, both have been reacquired.
try:
success = self._wait(timeout)
except LoopExit as ex:
args = ex.args
ex = None
if self.counter:
success = True
else:
# Avoid using ex.hub property to keep holding the GIL
if len(args) == 3 and args[1].main_hub:
# The main hub, meaning the main thread. We probably can do nothing with this.
raise
return self.__acquire_from_other_thread(
(self.hub, get_hub_if_exists(), self._getcurrent(), "LoopExit"),
blocking,
timeout)
if not success:
assert timeout is not None
# Our timer expired.
return False
# Neither our timer or another one expired, so we blocked until
# awoke. Therefore, the counter is ours
assert self.counter > 0, (self.counter, blocking, timeout, success,)
self.counter -= 1
return True
_py3k_acquire = acquire # PyPy needs this; it must be static for Cython
def __enter__(self):
self.acquire()
def __exit__(self, t, v, tb):
self.release()
def _handle_unswitched_notifications(self, unswitched):
# If we fail to switch to a greenlet in another thread to send
# a notification, just re-queue it, in the hopes that the
# other thread will eventually run notifications itself.
#
# We CANNOT do what the ``super()`` does and actually allow
# this notification to get run sometime in the future by
# scheduling a callback in the other thread. The algorithm
# that we use to handle cross-thread locking/unlocking was
# designed before the schedule-a-callback mechanism was
# implemented. If we allow this to be run as a callback, we
# can find ourself the victim of ``InvalidSwitchError`` (or
# worse, silent corruption) because the switch can come at an
# unexpected time: *after* the destination thread has already
# acquired the lock.
#
# This manifests in a fairly reliable test failure,
# ``gevent.tests.test__semaphore``
# ``TestSemaphoreMultiThread.test_dueling_threads_with_hub``,
# but ONLY when running in PURE_PYTHON mode.
#
# TODO: Maybe we can rewrite that part of the algorithm to be friendly to
# running the callbacks?
self._links.extend(unswitched)
def __add_link(self, link):
if not self._notifier:
self.rawlink(link)
else:
self._notifier.args[0].append(link)
def __acquire_from_other_thread(self, ex_args, blocking, timeout):
assert blocking
# Some other hub owns this object. We must ask it to wake us
# up. In general, we can't use a Python-level ``Lock`` because
#
# (1) it doesn't support a timeout on all platforms; and
# (2) we don't want to block this hub from running.
#
# So we need to do so in a way that cooperates with *two*
# hubs. That's what an async watcher is built for.
#
# Of course, if we don't actually have two hubs, then we must find some other
# solution. That involves using a lock.
# We have to take an action that drops the GIL and drops the object lock
# to allow the main thread (the thread for our hub) to advance.
owning_hub = ex_args[0]
hub_for_this_thread = ex_args[1]
current_greenlet = ex_args[2]
if owning_hub is None and hub_for_this_thread is None:
return self.__acquire_without_hubs(timeout)
if hub_for_this_thread is None:
# Probably a background worker thread. We don't want to create
# the hub if not needed, and since it didn't exist there are no
# other greenlets that we could yield to anyway, so there's nothing
# to block and no reason to try to avoid blocking, so using a native
# lock is the simplest way to go.
return self.__acquire_using_other_hub(owning_hub, timeout)
# We have a hub we don't want to block. Use an async watcher
# and ask the next releaser of this object to wake us up.
return self.__acquire_using_two_hubs(hub_for_this_thread,
current_greenlet,
timeout)
def __acquire_using_two_hubs(self,
hub_for_this_thread,
current_greenlet,
timeout):
# Allocating and starting the watcher *could* release the GIL.
# with the libev corcext, allocating won't, but starting briefly will.
# With other backends, allocating might, and starting might also.
# So...
watcher = hub_for_this_thread.loop.async_()
send = watcher.send_ignoring_arg
watcher.start(current_greenlet.switch, self)
try:
with Timeout._start_new_or_dummy(timeout) as timer:
# ... now that we're back holding the GIL, we need to verify our
# state.
try:
while 1:
if self.counter > 0:
self.counter -= 1
assert self.counter >= 0, (self,)
return True
self.__add_link(send)
# Releases the object lock
self._switch_to_hub(hub_for_this_thread)
# We waited and got notified. We should be ready now, so a non-blocking
# acquire() should succeed. But sometimes we get spurious notifications?
# It's not entirely clear how. So we need to loop until we get it, or until
# the timer expires
result = self.acquire(0)
if result:
return result
except Timeout as tex:
if tex is not timer:
raise
return False
finally:
self._quiet_unlink_all(send)
watcher.stop()
watcher.close()
def __acquire_from_other_thread_cb(self, results, blocking, timeout, thread_lock):
try:
result = self.acquire(blocking, timeout)
results.append(result)
finally:
thread_lock.release()
return result
def __acquire_using_other_hub(self, owning_hub, timeout):
assert owning_hub is not get_hub_if_exists()
thread_lock = self._allocate_lock()
thread_lock.acquire()
results = []
owning_hub.loop.run_callback_threadsafe(
spawn_raw,
self.__acquire_from_other_thread_cb,
results,
1, # blocking,
timeout, # timeout,
thread_lock)
# We MUST use a blocking acquire here, or at least be sure we keep going
# until we acquire it. If we timed out waiting here,
# just before the callback runs, then we would be out of sync.
self.__spin_on_native_lock(thread_lock, None)
return results[0]
def __acquire_without_hubs(self, timeout):
thread_lock = self._allocate_lock()
thread_lock.acquire()
absolute_expiration = 0
begin = 0
if timeout:
absolute_expiration = monotonic() + timeout
# Cython won't compile a lambda here
link = _LockReleaseLink(thread_lock)
while 1:
self.__add_link(link)
if absolute_expiration:
begin = monotonic()
got_native = self.__spin_on_native_lock(thread_lock, timeout)
self._quiet_unlink_all(link)
if got_native:
if self.acquire(0):
return True
if absolute_expiration:
now = monotonic()
if now >= absolute_expiration:
return False
duration = now - begin
timeout -= duration
if timeout <= 0:
return False
def __spin_on_native_lock(self, thread_lock, timeout):
self._drop_lock_for_switch_out()
try:
# Unlike Python 2, Python 3 thread locks
# can be interrupted when blocking, with or
# without a timeout. Python 2 didn't even
# support a timeout for non -blocking.
if timeout:
return thread_lock.acquire(True, timeout)
return thread_lock.acquire()
finally:
self._acquire_lock_for_switch_in()
class BoundedSemaphore(Semaphore):
"""
BoundedSemaphore(value=1) -> BoundedSemaphore
A bounded semaphore checks to make sure its current value doesn't
exceed its initial value. If it does, :class:`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.
"""
__slots__ = (
'_initial_value',
)
#: For monkey-patching, allow changing the class of error we raise
_OVER_RELEASE_ERROR = ValueError
def __init__(self, *args, **kwargs):
Semaphore.__init__(self, *args, **kwargs)
self._initial_value = self.counter
def release(self):
"""
Like :meth:`Semaphore.release`, but raises :class:`ValueError`
if the semaphore is being over-released.
"""
if self.counter >= self._initial_value:
raise self._OVER_RELEASE_ERROR("Semaphore released too many times")
counter = Semaphore.release(self)
# When we are absolutely certain that no one holds this semaphore,
# release our hub and go back to floating. This assists in cross-thread
# uses.
if counter == self._initial_value:
self.hub = None # pylint:disable=attribute-defined-outside-init
return counter
def _at_fork_reinit(self):
super(BoundedSemaphore, self)._at_fork_reinit()
self.counter = self._initial_value
# By building the semaphore with Cython under PyPy, we get
# atomic operations (specifically, exiting/releasing), at the
# cost of some speed (one trivial semaphore micro-benchmark put the pure-python version
# at around 1s and the compiled version at around 4s). Some clever subclassing
# and having only the bare minimum be in cython might help reduce that penalty.
# NOTE: You must use version 0.23.4 or later to avoid a memory leak.
# https://mail.python.org/pipermail/cython-devel/2015-October/004571.html
# However, that's all for naught on up to and including PyPy 4.0.1 which
# have some serious crashing bugs with GC interacting with cython.
# It hasn't been tested since then, and PURE_PYTHON is assumed to be true
# for PyPy in all cases anyway, so this does nothing.
from gevent._util import import_c_accel
import_c_accel(globals(), 'gevent.__semaphore')