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BeakerIssue31 / beaker / synchronization.py

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"""Synchronization functions.

File- and mutex-based mutual exclusion synchronizers are provided,
as well as a name-based mutex which locks within an application
based on a string name.

"""

import os
import sys
import tempfile

try:
    import threading as _threading
except ImportError:
    import dummy_threading as _threading

# check for fcntl module
try:
    sys.getwindowsversion()
    has_flock = False
except:
    try:
        import fcntl
        has_flock = True
    except ImportError:
        has_flock = False

from beaker import util
from beaker.exceptions import LockError

__all__  = ["file_synchronizer", "mutex_synchronizer", "null_synchronizer",
            "NameLock", "_threading"]


class NameLock(object):
    """a proxy for an RLock object that is stored in a name based
    registry.

    Multiple threads can get a reference to the same RLock based on the
    name alone, and synchronize operations related to that name.

    """
    locks = util.WeakValuedRegistry()

    class NLContainer(object):
        def __init__(self, reentrant):
            if reentrant:
                self.lock = _threading.RLock()
            else:
                self.lock = _threading.Lock()
        def __call__(self):
            return self.lock

    def __init__(self, identifier = None, reentrant = False):
        if identifier is None:
            self._lock = NameLock.NLContainer(reentrant)
        else:
            self._lock = NameLock.locks.get(identifier, NameLock.NLContainer,
                                            reentrant)

    def acquire(self, wait = True):
        return self._lock().acquire(wait)

    def release(self):
        self._lock().release()


_synchronizers = util.WeakValuedRegistry()
def _synchronizer(identifier, cls, **kwargs):
    return _synchronizers.sync_get((identifier, cls), cls, identifier, **kwargs)


def file_synchronizer(identifier, **kwargs):
    if not has_flock or 'lock_dir' not in kwargs:
        return mutex_synchronizer(identifier)
    else:
        return _synchronizer(identifier, FileSynchronizer, **kwargs)


def mutex_synchronizer(identifier, **kwargs):
    return _synchronizer(identifier, ConditionSynchronizer, **kwargs)


class null_synchronizer(object):
    def acquire_write_lock(self, wait=True):
        return True
    def acquire_read_lock(self):
        pass
    def release_write_lock(self):
        pass
    def release_read_lock(self):
        pass
    acquire = acquire_write_lock
    release = release_write_lock


class SynchronizerImpl(object):
    def __init__(self):
        self._state = util.ThreadLocal()

    class SyncState(object):
        __slots__ = 'reentrantcount', 'writing', 'reading'

        def __init__(self):
            self.reentrantcount = 0
            self.writing = False
            self.reading = False

    def state(self):
        if not self._state.has():
            state = SynchronizerImpl.SyncState()
            self._state.put(state)
            return state
        else:
            return self._state.get()
    state = property(state)

    def release_read_lock(self):
        state = self.state

        if state.writing: 
            raise LockError("lock is in writing state")
        if not state.reading: 
            raise LockError("lock is not in reading state")

        if state.reentrantcount == 1:
            self.do_release_read_lock()
            state.reading = False

        state.reentrantcount -= 1

    def acquire_read_lock(self, wait = True):
        state = self.state

        if state.writing: 
            raise LockError("lock is in writing state")

        if state.reentrantcount == 0:
            x = self.do_acquire_read_lock(wait)
            if (wait or x):
                state.reentrantcount += 1
                state.reading = True
            return x
        elif state.reading:
            state.reentrantcount += 1
            return True

    def release_write_lock(self):
        state = self.state

        if state.reading: 
            raise LockError("lock is in reading state")
        if not state.writing: 
            raise LockError("lock is not in writing state")

        if state.reentrantcount == 1:
            self.do_release_write_lock()
            state.writing = False

        state.reentrantcount -= 1

    release = release_write_lock

    def acquire_write_lock(self, wait  = True):
        state = self.state

        if state.reading: 
            raise LockError("lock is in reading state")

        if state.reentrantcount == 0:
            x = self.do_acquire_write_lock(wait)
            if (wait or x): 
                state.reentrantcount += 1
                state.writing = True
            return x
        elif state.writing:
            state.reentrantcount += 1
            return True

    acquire = acquire_write_lock

    def do_release_read_lock(self):
        raise NotImplementedError()

    def do_acquire_read_lock(self):
        raise NotImplementedError()

    def do_release_write_lock(self):
        raise NotImplementedError()

    def do_acquire_write_lock(self):
        raise NotImplementedError()


class FileSynchronizer(SynchronizerImpl):
    """a synchronizer which locks using flock().

    Adapted for Python/multithreads from Apache::Session::Lock::File,
    http://search.cpan.org/src/CWEST/Apache-Session-1.81/Session/Lock/File.pm

    This module does not unlink temporary files, 
    because it interferes with proper locking.  This can cause 
    problems on certain systems (Linux) whose file systems (ext2) do not 
    perform well with lots of files in one directory.  To prevent this
    you should use a script to clean out old files from your lock directory.

    """
    def __init__(self, identifier, lock_dir):
        super(FileSynchronizer, self).__init__()
        self._filedescriptor = util.ThreadLocal()

        if lock_dir is None:
            lock_dir = tempfile.gettempdir()
        else:
            lock_dir = lock_dir

        self.filename = util.encoded_path(
                            lock_dir, 
                            [identifier], 
                            extension='.lock'
                        )

    def _filedesc(self):
        return self._filedescriptor.get()
    _filedesc = property(_filedesc)

    def _open(self, mode):
        filedescriptor = self._filedesc
        if filedescriptor is None:
            filedescriptor = os.open(self.filename, mode)
            self._filedescriptor.put(filedescriptor)
        return filedescriptor

    def do_acquire_read_lock(self, wait):
        filedescriptor = self._open(os.O_CREAT | os.O_RDONLY)
        if not wait:
            try:
                fcntl.flock(filedescriptor, fcntl.LOCK_SH | fcntl.LOCK_NB)
                return True
            except IOError:
                os.close(filedescriptor)
                self._filedescriptor.remove()
                return False
        else:
            fcntl.flock(filedescriptor, fcntl.LOCK_SH)
            return True

    def do_acquire_write_lock(self, wait):
        filedescriptor = self._open(os.O_CREAT | os.O_WRONLY)
        if not wait:
            try:
                fcntl.flock(filedescriptor, fcntl.LOCK_EX | fcntl.LOCK_NB)
                return True
            except IOError:
                os.close(filedescriptor)
                self._filedescriptor.remove()
                return False
        else:
            fcntl.flock(filedescriptor, fcntl.LOCK_EX)
            return True

    def do_release_read_lock(self):
        self._release_all_locks()

    def do_release_write_lock(self):
        self._release_all_locks()

    def _release_all_locks(self):
        filedescriptor = self._filedesc
        if filedescriptor is not None:
            fcntl.flock(filedescriptor, fcntl.LOCK_UN)
            os.close(filedescriptor)
            self._filedescriptor.remove()


class ConditionSynchronizer(SynchronizerImpl):
    """a synchronizer using a Condition."""

    def __init__(self, identifier):
        super(ConditionSynchronizer, self).__init__()

        # counts how many asynchronous methods are executing
        self.async = 0

        # pointer to thread that is the current sync operation
        self.current_sync_operation = None

        # condition object to lock on
        self.condition = _threading.Condition(_threading.Lock())

    def do_acquire_read_lock(self, wait = True):
        self.condition.acquire()
        try:
            # see if a synchronous operation is waiting to start
            # or is already running, in which case we wait (or just
            # give up and return)
            if wait:
                while self.current_sync_operation is not None:
                    self.condition.wait()
            else:
                if self.current_sync_operation is not None:
                    return False

            self.async += 1
        finally:
            self.condition.release()

        if not wait: 
            return True

    def do_release_read_lock(self):
        self.condition.acquire()
        try:
            self.async -= 1

            # check if we are the last asynchronous reader thread 
            # out the door.
            if self.async == 0:
                # yes. so if a sync operation is waiting, notifyAll to wake
                # it up
                if self.current_sync_operation is not None:
                    self.condition.notifyAll()
            elif self.async < 0:
                raise LockError("Synchronizer error - too many "
                                "release_read_locks called")
        finally:
            self.condition.release()

    def do_acquire_write_lock(self, wait = True):
        self.condition.acquire()
        try:
            # here, we are not a synchronous reader, and after returning,
            # assuming waiting or immediate availability, we will be.

            if wait:
                # if another sync is working, wait
                while self.current_sync_operation is not None:
                    self.condition.wait()
            else:
                # if another sync is working,
                # we dont want to wait, so forget it
                if self.current_sync_operation is not None:
                    return False

            # establish ourselves as the current sync 
            # this indicates to other read/write operations
            # that they should wait until this is None again
            self.current_sync_operation = _threading.currentThread()

            # now wait again for asyncs to finish
            if self.async > 0:
                if wait:
                    # wait
                    self.condition.wait()
                else:
                    # we dont want to wait, so forget it
                    self.current_sync_operation = None
                    return False
        finally:
            self.condition.release()

        if not wait: 
            return True

    def do_release_write_lock(self):
        self.condition.acquire()
        try:
            if self.current_sync_operation is not _threading.currentThread():
                raise LockError("Synchronizer error - current thread doesnt "
                                "have the write lock")

            # reset the current sync operation so 
            # another can get it
            self.current_sync_operation = None

            # tell everyone to get ready
            self.condition.notifyAll()
        finally:
            # everyone go !!
            self.condition.release()