Re: Implementing file-locking *control* in DBD::File

On Fri, 4 Dec 2009 10:03:54 +0100, Jens Rehsack
<[email protected]> wrote:

> 2009/12/3 H.Merijn Brand <[email protected]>:
> > Any objections to me committing below change?
> 
> While the originator of this request updates his ticket, maybe a smarter
> implementation is reasonable:
> 
> I could modify S::S opentable () to request opening for reading/writing,
> so you could differ between LOCK_SH and LOCK_EX automatically.

That can still be done. You can set a localized $data->{Database}{f_lock}
What I have now done (and committed) is create support and
documentation  for locking control. How this will be used is up to the
higher levels.

> Further, it could be suitable to or LOCK_NB, which allows to avoid deadlocks.

LOCK_NB?

       Apply or remove an advisory lock on the open file specified by fd.  The
       argument operation is one of the following:

           LOCK_SH  Place a shared lock.  More than one  process  may  hold  a
                    shared lock for a given file at a given time.

           LOCK_EX  Place  an  exclusive  lock.   Only one process may hold an
                    exclusive lock for a given file at a given time.

           LOCK_UN  Remove an existing lock held by this process.

> Sorry for generating extra wishes ;)

No need to be sorry. Wishes keep us sharp!

> /Jens
> 
> > --8<---
> > --- DBI-svn/lib/DBD/File.pm   2009-12-03 17:47:15.000000000 +0100
> > +++ DBI-svn/lib/DBD/File.pm   2009-12-03 17:49:20.000000000 +0100
> > @@ -162,6 +162,7 @@ sub connect ($$;$$$)
> >            f_ext       => 1, # file extension
> >            f_schema    => 1, # schema name
> >            f_tables    => 1, # base directory
> > +           f_lock      => 1, # Table locking mode
> >            };
> >         $this->{sql_valid_attrs} = {
> >            sql_handler           => 1, # Nano or S:S
> > @@ -725,12 +726,17 @@ sub open_table ($$$$$)
> >        }
> >     $fh and binmode $fh;
> >     if ($locking and $fh) {
> > -       if ($lockMode) {
> > +       my $lm = defined $data->{Database}{f_lock}
> > +                     && $data->{Database}{f_lock} =~ m/^[012]$/
> > +                      ? $data->{Database}{f_lock}
> > +                      : $lockMode ? 2 : 1;
> > +       if ($lm == 2) {
> >            flock $fh, 2 or croak "Cannot obtain exclusive lock on $file: $!";
> >            }
> > -       else {
> > +       elsif ($lm == 1) {
> >            flock $fh, 1 or croak "Cannot obtain shared lock on $file: $!";
> >            }
> > +       # $lm = 0 is forced no locking at all
> >        }
> >     my $columns = {};
> >     my $array   = [];
> > @@ -929,6 +935,29 @@ tables into the same (or no) schema:
> >  Defining f_schema to the empty string is equal to setting it to C<undef>,
> >  this to enable the DSN to be C<dbi:CSV:f_schema=;f_dir=.>.
> >
> > +=item f_lock
> > +
> > +With this attribute, you can force locking mode (if locking is supported
> > +at all) for opening tables. By default, tables are opened with a shared
> > +lock for reading, and with an exclusive lock for writing. The supported
> > +modes are:
> > +
> > +=over 2
> > +
> > +=item 0
> > +
> > +Force no locking at all.
> > +
> > +=item 1
> > +
> > +Only shared locks will be used.
> > +
> > +=item 2
> > +
> > +Only exclusive locks will be used.
> > +
> > +=back
> > +
> >  =back
> >
> >  =head2 Driver private methods
> > -->8---


-- 
H.Merijn Brand  http://tux.nl      Perl Monger  http://amsterdam.pm.org/
using & porting perl 5.6.2, 5.8.x, 5.10.x, 5.11.x on HP-UX 10.20, 11.00,
11.11, 11.23, and 11.31, OpenSuSE 10.3, 11.0, and 11.1, AIX 5.2 and 5.3.
http://mirrors.develooper.com/hpux/           http://www.test-smoke.org/
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