Splunk_Docker/files/splunkbeta/lib/python3.9/site-packages/zc/lockfile

Lock file support

=================



The ZODB lock_file module provides support for creating file system

locks.  These are locks that are implemented with lock files and

OS-provided locking facilities.  To create a lock, instantiate a

LockFile object with a file name:



    >>> import zc.lockfile

    >>> lock = zc.lockfile.LockFile('lock')



If we try to lock the same name, we'll get a lock error:



    >>> import zope.testing.loggingsupport

    >>> handler = zope.testing.loggingsupport.InstalledHandler('zc.lockfile')

    >>> try:

    ...     zc.lockfile.LockFile('lock')

    ... except zc.lockfile.LockError:

    ...     print("Can't lock file")

    Can't lock file



.. We don't log failure to acquire.



    >>> for record in handler.records: # doctest: +ELLIPSIS

    ...     print(record.levelname+' '+record.getMessage())



To release the lock, use it's close method:



    >>> lock.close()



The lock file is not removed.  It is left behind:



    >>> import os

    >>> os.path.exists('lock')

    True



Of course, now that we've released the lock, we can create it again:



    >>> lock = zc.lockfile.LockFile('lock')

    >>> lock.close()



.. Cleanup



    >>> import os

    >>> os.remove('lock')



Hostname in lock file

=====================



In a container environment (e.g. Docker), the PID is typically always

identical even if multiple containers are running under the same operating

system instance.



Clearly, inspecting lock files doesn't then help much in debugging. To identify

the container which created the lock file, we need information about the

container in the lock file. Since Docker uses the container identifier or name

as the hostname, this information can be stored in the lock file in addition to

or instead of the PID.



Use the ``content_template`` keyword argument to ``LockFile`` to specify a

custom lock file content format:



    >>> lock = zc.lockfile.LockFile('lock', content_template='{pid};{hostname}')

    >>> lock.close()



If you now inspected the lock file, you would see e.g.:



    $ cat lock

     123;myhostname