"""Synchronization primitives.""" __all__ = ('Lock', 'Event', 'Condition', 'Semaphore', 'BoundedSemaphore', 'Barrier') import collections import enum from . import exceptions from . import mixins 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, mixins._LoopBoundMixin): """Primitive lock objects. A primitive lock is a synchronization primitive that is not owned by a particular task 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 task 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 task is blocked in acquire() waiting for the state to turn to unlocked, only one task proceeds when a release() call resets the state to unlocked; successive release() calls will unblock tasks in FIFO order. 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): self._waiters = None self._locked = False 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. """ # Implement fair scheduling, where thread always waits # its turn. Jumping the queue if all are cancelled is an optimization. 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._get_loop().create_future() self._waiters.append(fut) try: try: await fut finally: self._waiters.remove(fut) except exceptions.CancelledError: # Currently the only exception designed be able to occur here. # Ensure the lock invariant: If lock is not claimed (or about # to be claimed by us) and there is a Task in waiters, # ensure that the Task at the head will run. if not self._locked: self._wake_up_first() raise # assert self._locked is False 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 tasks 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): """Ensure that the first waiter will wake up.""" if not self._waiters: return try: fut = next(iter(self._waiters)) except StopIteration: return # .done() means that the waiter is already set to wake up. if not fut.done(): fut.set_result(True) class Event(mixins._LoopBoundMixin): """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): self._waiters = collections.deque() self._value = False 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 tasks waiting for it to become true are awakened. Tasks 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, tasks 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 task calls set() to set the flag to true, then return True. """ if self._value: return True fut = self._get_loop().create_future() self._waiters.append(fut) try: await fut return True finally: self._waiters.remove(fut) class Condition(_ContextManagerMixin, mixins._LoopBoundMixin): """Asynchronous equivalent to threading.Condition. This class implements condition variable objects. A condition variable allows one or more tasks to wait until they are notified by another task. A new Lock object is created and used as the underlying lock. """ def __init__(self, lock=None): if lock is None: lock = 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 task 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 task. Once awakened, it re-acquires the lock and returns True. This method may return spuriously, which is why the caller should always re-check the state and be prepared to wait() again. """ if not self.locked(): raise RuntimeError('cannot wait on un-acquired lock') fut = self._get_loop().create_future() self.release() try: try: self._waiters.append(fut) try: await fut return True finally: self._waiters.remove(fut) finally: # Must re-acquire lock even if wait is cancelled. # We only catch CancelledError here, since we don't want any # other (fatal) errors with the future to cause us to spin. err = None while True: try: await self.acquire() break except exceptions.CancelledError as e: err = e if err is not None: try: raise err # Re-raise most recent exception instance. finally: err = None # Break reference cycles. except BaseException: # Any error raised out of here _may_ have occurred after this Task # believed to have been successfully notified. # Make sure to notify another Task instead. This may result # in a "spurious wakeup", which is allowed as part of the # Condition Variable protocol. self._notify(1) raise async def wait_for(self, predicate): """Wait until a predicate becomes true. The predicate should be a callable whose result will be interpreted as a boolean value. The method will repeatedly wait() until it evaluates to true. 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 task waiting on this condition, if any. If the calling task has not acquired the lock when this method is called, a RuntimeError is raised. This method wakes up n of the tasks waiting for the condition variable; if fewer than n are waiting, they are all awoken. Note: an awakened task 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') self._notify(n) def _notify(self, n): 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 tasks waiting on this condition. This method acts like notify(), but wakes up all waiting tasks instead of one. If the calling task has not acquired the lock when this method is called, a RuntimeError is raised. """ self.notify(len(self._waiters)) class Semaphore(_ContextManagerMixin, mixins._LoopBoundMixin): """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): if value < 0: raise ValueError("Semaphore initial value must be >= 0") self._waiters = None self._value = value 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 locked(self): """Returns True if semaphore cannot be acquired immediately.""" # Due to state, or FIFO rules (must allow others to run first). return self._value == 0 or ( any(not w.cancelled() for w in (self._waiters or ()))) 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 task has called release() to make it larger than 0, and then return True. """ if not self.locked(): # Maintain FIFO, wait for others to start even if _value > 0. self._value -= 1 return True if self._waiters is None: self._waiters = collections.deque() fut = self._get_loop().create_future() self._waiters.append(fut) try: try: await fut finally: self._waiters.remove(fut) except exceptions.CancelledError: # Currently the only exception designed be able to occur here. if fut.done() and not fut.cancelled(): # Our Future was successfully set to True via _wake_up_next(), # but we are not about to successfully acquire(). Therefore we # must undo the bookkeeping already done and attempt to wake # up someone else. self._value += 1 raise finally: # New waiters may have arrived but had to wait due to FIFO. # Wake up as many as are allowed. while self._value > 0: if not self._wake_up_next(): break # There was no-one to wake up. return True def release(self): """Release a semaphore, incrementing the internal counter by one. When it was zero on entry and another task is waiting for it to become larger than zero again, wake up that task. """ self._value += 1 self._wake_up_next() def _wake_up_next(self): """Wake up the first waiter that isn't done.""" if not self._waiters: return False for fut in self._waiters: if not fut.done(): self._value -= 1 fut.set_result(True) # `fut` is now `done()` and not `cancelled()`. return True return False 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): self._bound_value = value super().__init__(value) def release(self): if self._value >= self._bound_value: raise ValueError('BoundedSemaphore released too many times') super().release() class _BarrierState(enum.Enum): FILLING = 'filling' DRAINING = 'draining' RESETTING = 'resetting' BROKEN = 'broken' class Barrier(mixins._LoopBoundMixin): """Asyncio equivalent to threading.Barrier Implements a Barrier primitive. Useful for synchronizing a fixed number of tasks at known synchronization points. Tasks block on 'wait()' and are simultaneously awoken once they have all made their call. """ def __init__(self, parties): """Create a barrier, initialised to 'parties' tasks.""" if parties < 1: raise ValueError('parties must be > 0') self._cond = Condition() # notify all tasks when state changes self._parties = parties self._state = _BarrierState.FILLING self._count = 0 # count tasks in Barrier def __repr__(self): res = super().__repr__() extra = f'{self._state.value}' if not self.broken: extra += f', waiters:{self.n_waiting}/{self.parties}' return f'<{res[1:-1]} [{extra}]>' async def __aenter__(self): # wait for the barrier reaches the parties number # when start draining release and return index of waited task return await self.wait() async def __aexit__(self, *args): pass async def wait(self): """Wait for the barrier. When the specified number of tasks have started waiting, they are all simultaneously awoken. Returns an unique and individual index number from 0 to 'parties-1'. """ async with self._cond: await self._block() # Block while the barrier drains or resets. try: index = self._count self._count += 1 if index + 1 == self._parties: # We release the barrier await self._release() else: await self._wait() return index finally: self._count -= 1 # Wake up any tasks waiting for barrier to drain. self._exit() async def _block(self): # Block until the barrier is ready for us, # or raise an exception if it is broken. # # It is draining or resetting, wait until done # unless a CancelledError occurs await self._cond.wait_for( lambda: self._state not in ( _BarrierState.DRAINING, _BarrierState.RESETTING ) ) # see if the barrier is in a broken state if self._state is _BarrierState.BROKEN: raise exceptions.BrokenBarrierError("Barrier aborted") async def _release(self): # Release the tasks waiting in the barrier. # Enter draining state. # Next waiting tasks will be blocked until the end of draining. self._state = _BarrierState.DRAINING self._cond.notify_all() async def _wait(self): # Wait in the barrier until we are released. Raise an exception # if the barrier is reset or broken. # wait for end of filling # unless a CancelledError occurs await self._cond.wait_for(lambda: self._state is not _BarrierState.FILLING) if self._state in (_BarrierState.BROKEN, _BarrierState.RESETTING): raise exceptions.BrokenBarrierError("Abort or reset of barrier") def _exit(self): # If we are the last tasks to exit the barrier, signal any tasks # waiting for the barrier to drain. if self._count == 0: if self._state in (_BarrierState.RESETTING, _BarrierState.DRAINING): self._state = _BarrierState.FILLING self._cond.notify_all() async def reset(self): """Reset the barrier to the initial state. Any tasks currently waiting will get the BrokenBarrier exception raised. """ async with self._cond: if self._count > 0: if self._state is not _BarrierState.RESETTING: #reset the barrier, waking up tasks self._state = _BarrierState.RESETTING else: self._state = _BarrierState.FILLING self._cond.notify_all() async def abort(self): """Place the barrier into a 'broken' state. Useful in case of error. Any currently waiting tasks and tasks attempting to 'wait()' will have BrokenBarrierError raised. """ async with self._cond: self._state = _BarrierState.BROKEN self._cond.notify_all() @property def parties(self): """Return the number of tasks required to trip the barrier.""" return self._parties @property def n_waiting(self): """Return the number of tasks currently waiting at the barrier.""" if self._state is _BarrierState.FILLING: return self._count return 0 @property def broken(self): """Return True if the barrier is in a broken state.""" return self._state is _BarrierState.BROKEN