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locks.py
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"""Synchronization primitives.""" __all__ = ('Lock', 'Event', 'Condition', 'Semaphore', 'BoundedSemaphore') import collections import warnings from . import events from . import exceptions class _ContextManagerMixin: async def __aenter__(self): await self.acquire() # We have no use for the "as ..." clause in the with # statement for locks. return None async def __aexit__(self, exc_type, exc, tb): self.release() class Lock(_ContextManagerMixin): """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 'await'. Locks also support the asynchronous context management protocol. 'async with lock' statement should be used. Usage: lock = Lock() ... await lock.acquire() try: ... finally: lock.release() Context manager usage: lock = Lock() ... async with lock: ... Lock objects can be tested for locking state: if not lock.locked(): await lock.acquire() else: # lock is acquired ... """ def __init__(self, *, loop=None): self._waiters = None self._locked = False if loop is None: self._loop = events.get_event_loop() else: self._loop = loop warnings.warn("The loop argument is deprecated since Python 3.8, " "and scheduled for removal in Python 3.10.", DeprecationWarning, stacklevel=2) def __repr__(self): res = super().__repr__() extra = 'locked' if self._locked else 'unlocked' if self._waiters: extra = f'{extra}, waiters:{len(self._waiters)}' return f'<{res[1:-1]} [{extra}]>' def locked(self): """Return True if lock is acquired.""" return self._locked async 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._locked and (self._waiters is None or all(w.cancelled() for w in self._waiters))): self._locked = True return True if self._waiters is None: self._waiters = collections.deque() fut = self._loop.create_future() self._waiters.append(fut) # Finally block should be called before the CancelledError # handling as we don't want CancelledError to call # _wake_up_first() and attempt to wake up itself. try: try: await fut finally: self._waiters.remove(fut) except exceptions.CancelledError: if not self._locked: self._wake_up_first() raise self._locked = True return True 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 self._wake_up_first() else: raise RuntimeError('Lock is not acquired.') def _wake_up_first(self): """Wake up the first waiter if it isn't done.""" if not self._waiters: return try: fut = next(iter(self._waiters)) except StopIteration: return # .done() necessarily means that a waiter will wake up later on and # either take the lock, or, if it was cancelled and lock wasn't # taken already, will hit this again and wake up a new waiter. if not fut.done(): fut.set_result(True) class Event: """Asynchronous 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 None: self._loop = events.get_event_loop() else: self._loop = loop warnings.warn("The loop argument is deprecated since Python 3.8, " "and scheduled for removal in Python 3.10.", DeprecationWarning, stacklevel=2) def __repr__(self): res = super().__repr__() extra = 'set' if self._value else 'unset' if self._waiters: extra = f'{extra}, waiters:{len(self._waiters)}' return f'<{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 async 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 = self._loop.create_future() self._waiters.append(fut) try: await fut return True finally: self._waiters.remove(fut) class Condition(_ContextManagerMixin): """Asynchronous 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, lock=None, *, loop=None): if loop is None: self._loop = events.get_event_loop() else: self._loop = loop warnings.warn("The loop argument is deprecated since Python 3.8, " "and scheduled for removal in Python 3.10.", DeprecationWarning, stacklevel=2) if lock is None: lock = Lock(loop=loop) elif lock._loop is not self._loop: raise ValueError("loop argument must agree with lock") 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 = f'{extra}, waiters:{len(self._waiters)}' return f'<{res[1:-1]} [{extra}]>' async 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') self.release() try: fut = self._loop.create_future() self._waiters.append(fut) try: await fut return True finally: self._waiters.remove(fut) finally: # Must reacquire lock even if wait is cancelled cancelled = False while True: try: await self.acquire() break except exceptions.CancelledError: cancelled = True if cancelled: raise exceptions.CancelledError async 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: await 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)) class Semaphore(_ContextManagerMixin): """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 management protocol. The 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. """ def __init__(self, value=1, *, loop=None): if value < 0: raise ValueError("Semaphore initial value must be >= 0") self._value = value self._waiters = collections.deque() if loop is None: self._loop = events.get_event_loop() else: self._loop = loop warnings.warn("The loop argument is deprecated since Python 3.8, " "and scheduled for removal in Python 3.10.", DeprecationWarning, stacklevel=2) def __repr__(self): res = super().__repr__() extra = 'locked' if self.locked() else f'unlocked, value:{self._value}' if self._waiters: extra = f'{extra}, waiters:{len(self._waiters)}' return f'<{res[1:-1]} [{extra}]>' def _wake_up_next(self): while self._waiters: waiter = self._waiters.popleft() if not waiter.done(): waiter.set_result(None) return def locked(self): """Returns True if semaphore can not be acquired immediately.""" return self._value == 0 async 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. """ while self._value <= 0: fut = self._loop.create_future() self._waiters.append(fut) try: await fut except: # See the similar code in Queue.get. fut.cancel() if self._value > 0 and not fut.cancelled(): self._wake_up_next() raise self._value -= 1 return True 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. """ self._value += 1 self._wake_up_next() class BoundedSemaphore(Semaphore): """A bounded semaphore implementation. This raises ValueError in release() if it would increase the value above the initial value. """ def __init__(self, value=1, *, loop=None): if loop: warnings.warn("The loop argument is deprecated since Python 3.8, " "and scheduled for removal in Python 3.10.", DeprecationWarning, stacklevel=2) self._bound_value = value super().__init__(value, loop=loop) def release(self): if self._value >= self._bound_value: raise ValueError('BoundedSemaphore released too many times') super().release()