24e0379624
The main problem was that an unluckily timed task cancellation could cause the semaphore to be stuck. There were also doubts about strict FIFO ordering of tasks allowed to pass. The Semaphore implementation was rewritten to be more similar to Lock. Many tests for edge cases (including cancellation) were added.
591 lines
19 KiB
Python
591 lines
19 KiB
Python
"""Synchronization primitives."""
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__all__ = ('Lock', 'Event', 'Condition', 'Semaphore',
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'BoundedSemaphore', 'Barrier')
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import collections
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import enum
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from . import exceptions
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from . import mixins
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class _ContextManagerMixin:
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async def __aenter__(self):
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await self.acquire()
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# We have no use for the "as ..." clause in the with
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# statement for locks.
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return None
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async def __aexit__(self, exc_type, exc, tb):
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self.release()
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class Lock(_ContextManagerMixin, mixins._LoopBoundMixin):
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"""Primitive lock objects.
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A primitive lock is a synchronization primitive that is not owned
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by a particular coroutine when locked. A primitive lock is in one
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of two states, 'locked' or 'unlocked'.
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It is created in the unlocked state. It has two basic methods,
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acquire() and release(). When the state is unlocked, acquire()
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changes the state to locked and returns immediately. When the
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state is locked, acquire() blocks until a call to release() in
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another coroutine changes it to unlocked, then the acquire() call
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resets it to locked and returns. The release() method should only
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be called in the locked state; it changes the state to unlocked
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and returns immediately. If an attempt is made to release an
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unlocked lock, a RuntimeError will be raised.
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When more than one coroutine is blocked in acquire() waiting for
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the state to turn to unlocked, only one coroutine proceeds when a
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release() call resets the state to unlocked; first coroutine which
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is blocked in acquire() is being processed.
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acquire() is a coroutine and should be called with 'await'.
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Locks also support the asynchronous context management protocol.
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'async with lock' statement should be used.
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Usage:
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lock = Lock()
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...
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await lock.acquire()
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try:
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...
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finally:
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lock.release()
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Context manager usage:
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lock = Lock()
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...
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async with lock:
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...
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Lock objects can be tested for locking state:
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if not lock.locked():
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await lock.acquire()
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else:
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# lock is acquired
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...
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"""
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def __init__(self):
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self._waiters = None
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self._locked = False
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self._locked else 'unlocked'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def locked(self):
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"""Return True if lock is acquired."""
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return self._locked
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async def acquire(self):
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"""Acquire a lock.
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This method blocks until the lock is unlocked, then sets it to
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locked and returns True.
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"""
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if (not self._locked and (self._waiters is None or
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all(w.cancelled() for w in self._waiters))):
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self._locked = True
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return True
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if self._waiters is None:
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self._waiters = collections.deque()
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fut = self._get_loop().create_future()
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self._waiters.append(fut)
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# Finally block should be called before the CancelledError
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# handling as we don't want CancelledError to call
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# _wake_up_first() and attempt to wake up itself.
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try:
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try:
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await fut
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finally:
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self._waiters.remove(fut)
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except exceptions.CancelledError:
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if not self._locked:
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self._wake_up_first()
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raise
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self._locked = True
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return True
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def release(self):
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"""Release a lock.
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When the lock is locked, reset it to unlocked, and return.
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If any other coroutines are blocked waiting for the lock to become
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unlocked, allow exactly one of them to proceed.
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When invoked on an unlocked lock, a RuntimeError is raised.
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There is no return value.
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"""
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if self._locked:
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self._locked = False
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self._wake_up_first()
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else:
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raise RuntimeError('Lock is not acquired.')
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def _wake_up_first(self):
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"""Wake up the first waiter if it isn't done."""
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if not self._waiters:
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return
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try:
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fut = next(iter(self._waiters))
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except StopIteration:
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return
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# .done() necessarily means that a waiter will wake up later on and
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# either take the lock, or, if it was cancelled and lock wasn't
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# taken already, will hit this again and wake up a new waiter.
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if not fut.done():
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fut.set_result(True)
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class Event(mixins._LoopBoundMixin):
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"""Asynchronous equivalent to threading.Event.
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Class implementing event objects. An event manages a flag that can be set
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to true with the set() method and reset to false with the clear() method.
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The wait() method blocks until the flag is true. The flag is initially
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false.
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"""
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def __init__(self):
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self._waiters = collections.deque()
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self._value = False
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def __repr__(self):
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res = super().__repr__()
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extra = 'set' if self._value else 'unset'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def is_set(self):
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"""Return True if and only if the internal flag is true."""
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return self._value
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def set(self):
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"""Set the internal flag to true. All coroutines waiting for it to
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become true are awakened. Coroutine that call wait() once the flag is
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true will not block at all.
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"""
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if not self._value:
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self._value = True
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for fut in self._waiters:
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if not fut.done():
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fut.set_result(True)
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def clear(self):
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"""Reset the internal flag to false. Subsequently, coroutines calling
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wait() will block until set() is called to set the internal flag
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to true again."""
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self._value = False
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async def wait(self):
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"""Block until the internal flag is true.
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If the internal flag is true on entry, return True
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immediately. Otherwise, block until another coroutine calls
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set() to set the flag to true, then return True.
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"""
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if self._value:
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return True
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fut = self._get_loop().create_future()
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self._waiters.append(fut)
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try:
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await fut
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return True
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finally:
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self._waiters.remove(fut)
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class Condition(_ContextManagerMixin, mixins._LoopBoundMixin):
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"""Asynchronous equivalent to threading.Condition.
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This class implements condition variable objects. A condition variable
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allows one or more coroutines to wait until they are notified by another
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coroutine.
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A new Lock object is created and used as the underlying lock.
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"""
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def __init__(self, lock=None):
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if lock is None:
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lock = Lock()
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self._lock = lock
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# Export the lock's locked(), acquire() and release() methods.
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self.locked = lock.locked
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self.acquire = lock.acquire
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self.release = lock.release
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self._waiters = collections.deque()
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self.locked() else 'unlocked'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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async def wait(self):
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"""Wait until notified.
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If the calling coroutine has not acquired the lock when this
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method is called, a RuntimeError is raised.
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This method releases the underlying lock, and then blocks
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until it is awakened by a notify() or notify_all() call for
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the same condition variable in another coroutine. Once
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awakened, it re-acquires the lock and returns True.
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"""
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if not self.locked():
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raise RuntimeError('cannot wait on un-acquired lock')
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self.release()
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try:
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fut = self._get_loop().create_future()
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self._waiters.append(fut)
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try:
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await fut
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return True
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finally:
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self._waiters.remove(fut)
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finally:
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# Must reacquire lock even if wait is cancelled
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cancelled = False
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while True:
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try:
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await self.acquire()
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break
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except exceptions.CancelledError:
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cancelled = True
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if cancelled:
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raise exceptions.CancelledError
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async def wait_for(self, predicate):
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"""Wait until a predicate becomes true.
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The predicate should be a callable which result will be
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interpreted as a boolean value. The final predicate value is
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the return value.
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"""
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result = predicate()
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while not result:
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await self.wait()
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result = predicate()
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return result
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def notify(self, n=1):
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"""By default, wake up one coroutine waiting on this condition, if any.
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If the calling coroutine has not acquired the lock when this method
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is called, a RuntimeError is raised.
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This method wakes up at most n of the coroutines waiting for the
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condition variable; it is a no-op if no coroutines are waiting.
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Note: an awakened coroutine does not actually return from its
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wait() call until it can reacquire the lock. Since notify() does
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not release the lock, its caller should.
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"""
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if not self.locked():
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raise RuntimeError('cannot notify on un-acquired lock')
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idx = 0
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for fut in self._waiters:
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if idx >= n:
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break
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if not fut.done():
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idx += 1
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fut.set_result(False)
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def notify_all(self):
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"""Wake up all threads waiting on this condition. This method acts
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like notify(), but wakes up all waiting threads instead of one. If the
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calling thread has not acquired the lock when this method is called,
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a RuntimeError is raised.
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"""
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self.notify(len(self._waiters))
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class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin):
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"""A Semaphore implementation.
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A semaphore manages an internal counter which is decremented by each
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acquire() call and incremented by each release() call. The counter
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can never go below zero; when acquire() finds that it is zero, it blocks,
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waiting until some other thread calls release().
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Semaphores also support the context management protocol.
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The optional argument gives the initial value for the internal
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counter; it defaults to 1. If the value given is less than 0,
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ValueError is raised.
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"""
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def __init__(self, value=1):
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if value < 0:
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raise ValueError("Semaphore initial value must be >= 0")
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self._waiters = None
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self._value = value
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def __repr__(self):
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res = super().__repr__()
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extra = 'locked' if self.locked() else f'unlocked, value:{self._value}'
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if self._waiters:
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extra = f'{extra}, waiters:{len(self._waiters)}'
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return f'<{res[1:-1]} [{extra}]>'
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def locked(self):
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"""Returns True if semaphore counter is zero."""
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return self._value == 0
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async def acquire(self):
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"""Acquire a semaphore.
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If the internal counter is larger than zero on entry,
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decrement it by one and return True immediately. If it is
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zero on entry, block, waiting until some other coroutine has
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called release() to make it larger than 0, and then return
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True.
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"""
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if (not self.locked() and (self._waiters is None or
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all(w.cancelled() for w in self._waiters))):
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self._value -= 1
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return True
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if self._waiters is None:
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self._waiters = collections.deque()
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fut = self._get_loop().create_future()
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self._waiters.append(fut)
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# Finally block should be called before the CancelledError
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# handling as we don't want CancelledError to call
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# _wake_up_first() and attempt to wake up itself.
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try:
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try:
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await fut
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finally:
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self._waiters.remove(fut)
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except exceptions.CancelledError:
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if not self.locked():
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self._wake_up_first()
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raise
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self._value -= 1
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if not self.locked():
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self._wake_up_first()
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return True
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def release(self):
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"""Release a semaphore, incrementing the internal counter by one.
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When it was zero on entry and another coroutine is waiting for it to
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become larger than zero again, wake up that coroutine.
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"""
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self._value += 1
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self._wake_up_first()
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def _wake_up_first(self):
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"""Wake up the first waiter if it isn't done."""
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if not self._waiters:
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return
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try:
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fut = next(iter(self._waiters))
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except StopIteration:
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return
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# .done() necessarily means that a waiter will wake up later on and
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# either take the lock, or, if it was cancelled and lock wasn't
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# taken already, will hit this again and wake up a new waiter.
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if not fut.done():
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fut.set_result(True)
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class BoundedSemaphore(Semaphore):
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"""A bounded semaphore implementation.
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This raises ValueError in release() if it would increase the value
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above the initial value.
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"""
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def __init__(self, value=1):
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self._bound_value = value
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super().__init__(value)
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def release(self):
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if self._value >= self._bound_value:
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raise ValueError('BoundedSemaphore released too many times')
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super().release()
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class _BarrierState(enum.Enum):
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FILLING = 'filling'
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DRAINING = 'draining'
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RESETTING = 'resetting'
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BROKEN = 'broken'
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class Barrier(mixins._LoopBoundMixin):
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"""Asyncio equivalent to threading.Barrier
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Implements a Barrier primitive.
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Useful for synchronizing a fixed number of tasks at known synchronization
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points. Tasks block on 'wait()' and are simultaneously awoken once they
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have all made their call.
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"""
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def __init__(self, parties):
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"""Create a barrier, initialised to 'parties' tasks."""
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if parties < 1:
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raise ValueError('parties must be > 0')
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self._cond = Condition() # notify all tasks when state changes
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self._parties = parties
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self._state = _BarrierState.FILLING
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self._count = 0 # count tasks in Barrier
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def __repr__(self):
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res = super().__repr__()
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extra = f'{self._state.value}'
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if not self.broken:
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extra += f', waiters:{self.n_waiting}/{self.parties}'
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return f'<{res[1:-1]} [{extra}]>'
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async def __aenter__(self):
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# wait for the barrier reaches the parties number
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# when start draining release and return index of waited task
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return await self.wait()
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async def __aexit__(self, *args):
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pass
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async def wait(self):
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"""Wait for the barrier.
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When the specified number of tasks have started waiting, they are all
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simultaneously awoken.
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Returns an unique and individual index number from 0 to 'parties-1'.
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"""
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async with self._cond:
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await self._block() # Block while the barrier drains or resets.
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try:
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index = self._count
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self._count += 1
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if index + 1 == self._parties:
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# We release the barrier
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await self._release()
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else:
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await self._wait()
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return index
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finally:
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self._count -= 1
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# Wake up any tasks waiting for barrier to drain.
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self._exit()
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async def _block(self):
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# Block until the barrier is ready for us,
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# or raise an exception if it is broken.
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#
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# It is draining or resetting, wait until done
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# unless a CancelledError occurs
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await self._cond.wait_for(
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lambda: self._state not in (
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_BarrierState.DRAINING, _BarrierState.RESETTING
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)
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)
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# see if the barrier is in a broken state
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if self._state is _BarrierState.BROKEN:
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raise exceptions.BrokenBarrierError("Barrier aborted")
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async def _release(self):
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# Release the tasks waiting in the barrier.
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# Enter draining state.
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# Next waiting tasks will be blocked until the end of draining.
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self._state = _BarrierState.DRAINING
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self._cond.notify_all()
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async def _wait(self):
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# Wait in the barrier until we are released. Raise an exception
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# if the barrier is reset or broken.
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# wait for end of filling
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# unless a CancelledError occurs
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await self._cond.wait_for(lambda: self._state is not _BarrierState.FILLING)
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if self._state in (_BarrierState.BROKEN, _BarrierState.RESETTING):
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raise exceptions.BrokenBarrierError("Abort or reset of barrier")
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def _exit(self):
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# If we are the last tasks to exit the barrier, signal any tasks
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# waiting for the barrier to drain.
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if self._count == 0:
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if self._state in (_BarrierState.RESETTING, _BarrierState.DRAINING):
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self._state = _BarrierState.FILLING
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self._cond.notify_all()
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async def reset(self):
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"""Reset the barrier to the initial state.
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Any tasks currently waiting will get the BrokenBarrier exception
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raised.
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"""
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async with self._cond:
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if self._count > 0:
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if self._state is not _BarrierState.RESETTING:
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#reset the barrier, waking up tasks
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self._state = _BarrierState.RESETTING
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else:
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self._state = _BarrierState.FILLING
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self._cond.notify_all()
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async def abort(self):
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"""Place the barrier into a 'broken' state.
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Useful in case of error. Any currently waiting tasks and tasks
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attempting to 'wait()' will have BrokenBarrierError raised.
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"""
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async with self._cond:
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self._state = _BarrierState.BROKEN
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self._cond.notify_all()
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@property
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def parties(self):
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"""Return the number of tasks required to trip the barrier."""
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return self._parties
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@property
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def n_waiting(self):
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|
"""Return the number of tasks currently waiting at the barrier."""
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|
if self._state is _BarrierState.FILLING:
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|
return self._count
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return 0
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|
|
|
@property
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def broken(self):
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|
"""Return True if the barrier is in a broken state."""
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|
return self._state is _BarrierState.BROKEN
|