cpython/Lib/asyncio/locks.py

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"""Synchronization primitives."""
__all__ = ['Lock', 'Event', 'Condition', 'Semaphore']
import collections
from . import events
from . import futures
from . import tasks
class Lock:
"""Primitive lock objects.
A primitive lock is a synchronization primitive that is not owned
by a particular coroutine when locked. A primitive lock is in one
of two states, 'locked' or 'unlocked'.
It is created in the unlocked state. It has two basic methods,
acquire() and release(). When the state is unlocked, acquire()
changes the state to locked and returns immediately. When the
state is locked, acquire() blocks until a call to release() in
another coroutine changes it to unlocked, then the acquire() call
resets it to locked and returns. The release() method should only
be called in the locked state; it changes the state to unlocked
and returns immediately. If an attempt is made to release an
unlocked lock, a RuntimeError will be raised.
When more than one coroutine is blocked in acquire() waiting for
the state to turn to unlocked, only one coroutine proceeds when a
release() call resets the state to unlocked; first coroutine which
is blocked in acquire() is being processed.
acquire() is a coroutine and should be called with 'yield from'.
Locks also support the context manager protocol. '(yield from lock)'
should be used as context manager expression.
Usage:
lock = Lock()
...
yield from lock
try:
...
finally:
lock.release()
Context manager usage:
lock = Lock()
...
with (yield from lock):
...
Lock objects can be tested for locking state:
if not lock.locked():
yield from lock
else:
# lock is acquired
...
"""
def __init__(self, *, loop=None):
self._waiters = collections.deque()
self._locked = False
if loop is not None:
self._loop = loop
else:
self._loop = events.get_event_loop()
def __repr__(self):
res = super().__repr__()
extra = 'locked' if self._locked else 'unlocked'
if self._waiters:
extra = '{},waiters:{}'.format(extra, len(self._waiters))
return '<{} [{}]>'.format(res[1:-1], extra)
def locked(self):
"""Return true if lock is acquired."""
return self._locked
@tasks.coroutine
def acquire(self):
"""Acquire a lock.
This method blocks until the lock is unlocked, then sets it to
locked and returns True.
"""
if not self._waiters and not self._locked:
self._locked = True
return True
fut = futures.Future(loop=self._loop)
self._waiters.append(fut)
try:
yield from fut
self._locked = True
return True
finally:
self._waiters.remove(fut)
def release(self):
"""Release a lock.
When the lock is locked, reset it to unlocked, and return.
If any other coroutines are blocked waiting for the lock to become
unlocked, allow exactly one of them to proceed.
When invoked on an unlocked lock, a RuntimeError is raised.
There is no return value.
"""
if self._locked:
self._locked = False
# Wake up the first waiter who isn't cancelled.
for fut in self._waiters:
if not fut.done():
fut.set_result(True)
break
else:
raise RuntimeError('Lock is not acquired.')
def __enter__(self):
if not self._locked:
raise RuntimeError(
'"yield from" should be used as context manager expression')
return True
def __exit__(self, *args):
self.release()
def __iter__(self):
yield from self.acquire()
return self
class Event:
"""An Event implementation, our equivalent to threading.Event.
Class implementing event objects. An event manages a flag that can be set
to true with the set() method and reset to false with the clear() method.
The wait() method blocks until the flag is true. The flag is initially
false.
"""
def __init__(self, *, loop=None):
self._waiters = collections.deque()
self._value = False
if loop is not None:
self._loop = loop
else:
self._loop = events.get_event_loop()
def __repr__(self):
res = super().__repr__()
extra = 'set' if self._value else 'unset'
if self._waiters:
extra = '{},waiters:{}'.format(extra, len(self._waiters))
return '<{} [{}]>'.format(res[1:-1], extra)
def is_set(self):
"""Return true if and only if the internal flag is true."""
return self._value
def set(self):
"""Set the internal flag to true. All coroutines waiting for it to
become true are awakened. Coroutine that call wait() once the flag is
true will not block at all.
"""
if not self._value:
self._value = True
for fut in self._waiters:
if not fut.done():
fut.set_result(True)
def clear(self):
"""Reset the internal flag to false. Subsequently, coroutines calling
wait() will block until set() is called to set the internal flag
to true again."""
self._value = False
@tasks.coroutine
def wait(self):
"""Block until the internal flag is true.
If the internal flag is true on entry, return True
immediately. Otherwise, block until another coroutine calls
set() to set the flag to true, then return True.
"""
if self._value:
return True
fut = futures.Future(loop=self._loop)
self._waiters.append(fut)
try:
yield from fut
return True
finally:
self._waiters.remove(fut)
class Condition:
"""A Condition implementation, our equivalent to threading.Condition.
This class implements condition variable objects. A condition variable
allows one or more coroutines to wait until they are notified by another
coroutine.
A new Lock object is created and used as the underlying lock.
"""
def __init__(self, *, loop=None):
if loop is not None:
self._loop = loop
else:
self._loop = events.get_event_loop()
# Lock as an attribute as in threading.Condition.
lock = Lock(loop=self._loop)
self._lock = lock
# Export the lock's locked(), acquire() and release() methods.
self.locked = lock.locked
self.acquire = lock.acquire
self.release = lock.release
self._waiters = collections.deque()
def __repr__(self):
res = super().__repr__()
extra = 'locked' if self.locked() else 'unlocked'
if self._waiters:
extra = '{},waiters:{}'.format(extra, len(self._waiters))
return '<{} [{}]>'.format(res[1:-1], extra)
@tasks.coroutine
def wait(self):
"""Wait until notified.
If the calling coroutine has not acquired the lock when this
method is called, a RuntimeError is raised.
This method releases the underlying lock, and then blocks
until it is awakened by a notify() or notify_all() call for
the same condition variable in another coroutine. Once
awakened, it re-acquires the lock and returns True.
"""
if not self.locked():
raise RuntimeError('cannot wait on un-acquired lock')
keep_lock = True
self.release()
try:
fut = futures.Future(loop=self._loop)
self._waiters.append(fut)
try:
yield from fut
return True
finally:
self._waiters.remove(fut)
except GeneratorExit:
keep_lock = False # Prevent yield in finally clause.
raise
finally:
if keep_lock:
yield from self.acquire()
@tasks.coroutine
def wait_for(self, predicate):
"""Wait until a predicate becomes true.
The predicate should be a callable which result will be
interpreted as a boolean value. The final predicate value is
the return value.
"""
result = predicate()
while not result:
yield from self.wait()
result = predicate()
return result
def notify(self, n=1):
"""By default, wake up one coroutine waiting on this condition, if any.
If the calling coroutine has not acquired the lock when this method
is called, a RuntimeError is raised.
This method wakes up at most n of the coroutines waiting for the
condition variable; it is a no-op if no coroutines are waiting.
Note: an awakened coroutine does not actually return from its
wait() call until it can reacquire the lock. Since notify() does
not release the lock, its caller should.
"""
if not self.locked():
raise RuntimeError('cannot notify on un-acquired lock')
idx = 0
for fut in self._waiters:
if idx >= n:
break
if not fut.done():
idx += 1
fut.set_result(False)
def notify_all(self):
"""Wake up all threads waiting on this condition. This method acts
like notify(), but wakes up all waiting threads instead of one. If the
calling thread has not acquired the lock when this method is called,
a RuntimeError is raised.
"""
self.notify(len(self._waiters))
def __enter__(self):
return self._lock.__enter__()
def __exit__(self, *args):
return self._lock.__exit__(*args)
def __iter__(self):
yield from self.acquire()
return self
class Semaphore:
"""A Semaphore implementation.
A semaphore manages an internal counter which is decremented by each
acquire() call and incremented by each release() call. The counter
can never go below zero; when acquire() finds that it is zero, it blocks,
waiting until some other thread calls release().
Semaphores also support the context manager protocol.
The first optional argument gives the initial value for the internal
counter; it defaults to 1. If the value given is less than 0,
ValueError is raised.
The second optional argument determines if the semaphore can be released
more than initial internal counter value; it defaults to False. If the
value given is True and number of release() is more than number of
successful acquire() calls ValueError is raised.
"""
def __init__(self, value=1, bound=False, *, loop=None):
if value < 0:
raise ValueError("Semaphore initial value must be > 0")
self._value = value
self._bound = bound
self._bound_value = value
self._waiters = collections.deque()
self._locked = False
if loop is not None:
self._loop = loop
else:
self._loop = events.get_event_loop()
def __repr__(self):
res = super().__repr__()
extra = 'locked' if self._locked else 'unlocked,value:{}'.format(
self._value)
if self._waiters:
extra = '{},waiters:{}'.format(extra, len(self._waiters))
return '<{} [{}]>'.format(res[1:-1], extra)
def locked(self):
"""Returns True if semaphore can not be acquired immediately."""
return self._locked
@tasks.coroutine
def acquire(self):
"""Acquire a semaphore.
If the internal counter is larger than zero on entry,
decrement it by one and return True immediately. If it is
zero on entry, block, waiting until some other coroutine has
called release() to make it larger than 0, and then return
True.
"""
if not self._waiters and self._value > 0:
self._value -= 1
if self._value == 0:
self._locked = True
return True
fut = futures.Future(loop=self._loop)
self._waiters.append(fut)
try:
yield from fut
self._value -= 1
if self._value == 0:
self._locked = True
return True
finally:
self._waiters.remove(fut)
def release(self):
"""Release a semaphore, incrementing the internal counter by one.
When it was zero on entry and another coroutine is waiting for it to
become larger than zero again, wake up that coroutine.
If Semaphore is created with "bound" parameter equals true, then
release() method checks to make sure its current value doesn't exceed
its initial value. If it does, ValueError is raised.
"""
if self._bound and self._value >= self._bound_value:
raise ValueError('Semaphore released too many times')
self._value += 1
self._locked = False
for waiter in self._waiters:
if not waiter.done():
waiter.set_result(True)
break
def __enter__(self):
# TODO: This is questionable. How do we know the user actually
# wrote "with (yield from sema)" instead of "with sema"?
return True
def __exit__(self, *args):
self.release()
def __iter__(self):
yield from self.acquire()
return self