[bknr-devel] bknr.datastore store lockfile

[Hans Hübner]

On Thu, Aug 2, 2012 at 2:59 AM, Vladimir Sedach <vsedach at gmail.com> wrote:
> Hi Hans,
>
>> No, there are no plans to do that, but I'm open to patches that add
>> the functionality.  I did not ever see the lack of a lock file as a
>> problem, but I see how it is easily possible to seriously mess up a
>> store by trying to open it from multiple processes.
>
> One thing I found in building a lock file mechanism for my application
> is that there's no way to do it portably in CL, which makes sense
> since some details are OS dependent (and even on Unix it might not
> work on NFS, etc.). So I don't know of the right way of doing it
> (maybe a lock file is not actually the right way), and I'd like to
> figure one out before putting any mechanisms in a library.

I think the best way, moving forward, would be to depend on iolib and
use a simple POSIX open call with O_EXCL on a lock file placed in the
store directory.  It would be good to make the lock file
implementation optional so that compiling without it is possible (i.e.
on Window or Lisp implementations not supported by iolib).

>> Again, this is a problem I have not encountered, but I'm open to
>> patches correcting the problem.  Do you have an isolated test case
>> that exposes the problem?
>
> I don't really have a test case for this beyond "make a store as
> non-root user, put some data in the log (and maybe a snapshot), then
> try to open that store as root."

I tried the below and cannot reproduce the problem on OSX:

(ql:quickload :bknr.datastore)

(defpackage :store-user
  (:use :cl)
  (:export :test))

(in-package :store-user)

(defclass test-object (bknr.datastore:store-object)
  ((a :initarg :a :reader a))
  (:metaclass bknr.datastore:persistent-class))

(defun test ()
  (make-instance 'bknr.datastore:mp-store :directory "/tmp/object-store/"
                                          :subsystems (list
(make-instance 'bknr.datastore:store-object-subsystem)))
  (make-instance 'test-object :a 1)
  (bknr.datastore:snapshot)
  (print (bknr.datastore:all-store-objects))
  (bknr.datastore:close-store))

(eval (read-from-string "(progn (store-user:test) (sb-ext:exit))"))

panadol 1019_% sbcl --load reproduce-bug.lisp
This is SBCL 1.0.56.73-29fe19a, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
; adding components under ~/lisp-libs/ to asdf registry
To load "bknr.datastore":
  Load 1 ASDF system:
    bknr.datastore
; Loading "bknr.datastore"
..........
initializing store random state
restoring #<MP-STORE DIR: "/tmp/object-store/">

(#<TEST-OBJECT ID: 0>)
panadol 1020_% sudo sbcl --load reproduce-bug.lisp
This is SBCL 1.0.56.73-29fe19a, an implementation of ANSI Common Lisp.
More information about SBCL is available at <http://www.sbcl.org/>.

SBCL is free software, provided as is, with absolutely no warranty.
It is mostly in the public domain; some portions are provided under
BSD-style licenses.  See the CREDITS and COPYING files in the
distribution for more information.
; adding components under ~/lisp-libs/ to asdf registry
To load "bknr.datastore":
  Load 1 ASDF system:
    bknr.datastore
; Loading "bknr.datastore"
..........
restoring #<MP-STORE DIR: "/tmp/object-store/">
; loading snapshot file
/tmp/object-store/current/store-object-subsystem-snapshot
loading transaction log /tmp/object-store/current/transaction-log

(#<TEST-OBJECT ID: 0> #<TEST-OBJECT ID: 1>)

Can you determine some more details about the problem using this test?

Thanks,
-Hans