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Diffstat (limited to 'Lib/asyncio/locks.py')
| -rw-r--r-- | Lib/asyncio/locks.py | 466 |
1 files changed, 466 insertions, 0 deletions
diff --git a/Lib/asyncio/locks.py b/Lib/asyncio/locks.py new file mode 100644 index 0000000..29c4434 --- /dev/null +++ b/Lib/asyncio/locks.py @@ -0,0 +1,466 @@ +"""Synchronization primitives.""" + +__all__ = ['Lock', 'Event', 'Condition', 'Semaphore', 'BoundedSemaphore'] + +import collections + +from . import events +from . import futures +from . import tasks + + +class _ContextManager: + """Context manager. + + This enables the following idiom for acquiring and releasing a + lock around a block: + + with (yield from lock): + <block> + + while failing loudly when accidentally using: + + with lock: + <block> + """ + + def __init__(self, lock): + self._lock = lock + + def __enter__(self): + # We have no use for the "as ..." clause in the with + # statement for locks. + return None + + def __exit__(self, *args): + try: + self._lock.release() + finally: + self._lock = None # Crudely prevent reuse. + + +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): + raise RuntimeError( + '"yield from" should be used as context manager expression') + + def __exit__(self, *args): + # This must exist because __enter__ exists, even though that + # always raises; that's how the with-statement works. + pass + + def __iter__(self): + # This is not a coroutine. It is meant to enable the idiom: + # + # with (yield from lock): + # <block> + # + # as an alternative to: + # + # yield from lock.acquire() + # try: + # <block> + # finally: + # lock.release() + yield from self.acquire() + return _ContextManager(self) + + +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 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: + """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, *, 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') + + self.release() + try: + fut = futures.Future(loop=self._loop) + self._waiters.append(fut) + try: + yield from fut + return True + finally: + self._waiters.remove(fut) + + finally: + 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): + raise RuntimeError( + '"yield from" should be used as context manager expression') + + def __exit__(self, *args): + pass + + def __iter__(self): + # See comment in Lock.__iter__(). + yield from self.acquire() + return _ContextManager(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 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 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._value == 0 + + @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 + return True + + fut = futures.Future(loop=self._loop) + self._waiters.append(fut) + try: + yield from fut + self._value -= 1 + 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. + """ + self._value += 1 + for waiter in self._waiters: + if not waiter.done(): + waiter.set_result(True) + break + + def __enter__(self): + raise RuntimeError( + '"yield from" should be used as context manager expression') + + def __exit__(self, *args): + pass + + def __iter__(self): + # See comment in Lock.__iter__(). + yield from self.acquire() + return _ContextManager(self) + + +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): + 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() |