CherryPy / cherrypy / lib / lockfile.py

"""
Platform-independent file locking. Inspired by and modeled after zc.lockfile.
"""

import os

try:
    import msvcrt
except ImportError:
    pass

try:
    import fcntl
except ImportError:
    pass


class LockError(Exception):
    "Could not obtain a lock"

    msg = "Unable to lock %r"

    def __init__(self, path):
        super(LockError, self).__init__(self.msg % path)


class UnlockError(LockError):
    "Could not release a lock"

    msg = "Unable to unlock %r"


# first, a default, naive locking implementation
class LockFile(object):
    """
    A default, naive locking implementation. Always fails if the file
    already exists.
    """

    def __init__(self, path):
        self.path = path
        try:
            fd = os.open(path, os.O_CREAT | os.O_WRONLY | os.O_EXCL)
        except OSError:
            raise LockError(self.path)
        os.close(fd)

    def release(self):
        os.remove(self.path)

    def remove(self):
        pass


class SystemLockFile(object):
    """
    An abstract base class for platform-specific locking.
    """

    def __init__(self, path):
        self.path = path

        try:
            # Open lockfile for writing without truncation:
            self.fp = open(path, 'r+')
        except IOError:
            # If the file doesn't exist, IOError is raised; Use a+ instead.
            # Note that there may be a race here. Multiple processes
            # could fail on the r+ open and open the file a+, but only
            # one will get the the lock and write a pid.
            self.fp = open(path, 'a+')

        try:
            self._lock_file()
        except:
            self.fp.seek(1)
            self.fp.close()
            del self.fp
            raise

        self.fp.write(" %s\n" % os.getpid())
        self.fp.truncate()
        self.fp.flush()

    def release(self):
        if not hasattr(self, 'fp'):
            return
        self._unlock_file()
        self.fp.close()
        del self.fp

    def remove(self):
        """
        Attempt to remove the file
        """
        try:
            os.remove(self.path)
        except:
            pass

    #@abc.abstract_method
    #def _lock_file(self):
    #    """Attempt to obtain the lock on self.fp. Raise LockError if not
    #    acquired."""

    def _unlock_file(self):
        """Attempt to obtain the lock on self.fp. Raise UnlockError if not
        released."""


class WindowsLockFile(SystemLockFile):
    def _lock_file(self):
        # Lock just the first byte
        try:
            msvcrt.locking(self.fp.fileno(), msvcrt.LK_NBLCK, 1)
        except IOError:
            raise LockError(self.fp.name)

    def _unlock_file(self):
        try:
            self.fp.seek(0)
            msvcrt.locking(self.fp.fileno(), msvcrt.LK_UNLCK, 1)
        except IOError:
            raise UnlockError(self.fp.name)

if 'msvcrt' in globals():
    LockFile = WindowsLockFile


class UnixLockFile(SystemLockFile):
    def _lock_file(self):
        flags = fcntl.LOCK_EX | fcntl.LOCK_NB
        try:
            fcntl.flock(self.fp.fileno(), flags)
        except IOError:
            raise LockError(self.fp.name)

    # no need to implement _unlock_file, it will be unlocked on close()

if 'fcntl' in globals():
    LockFile = UnixLockFile
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