locks.py

"""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)
        self._wakeup_scheduled = False

    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)
                self._wakeup_scheduled = True
                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.
        """
        # _wakeup_scheduled is set if *another* task is scheduled to wakeup
        # but its acquire() is not resumed yet
        while self._wakeup_scheduled or self._value <= 0:
            fut = self._loop.create_future()
            self._waiters.append(fut)
            try:
                await fut
                # reset _wakeup_scheduled *after* waiting for a future
                self._wakeup_scheduled = False
            except exceptions.CancelledError:
                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()