[Libguestfs] [nbdkit PATCH 0/2] bind .zero to more languages

[Richard W.M. Jones]

On Wed, Jan 25, 2017 at 08:48:10PM -0600, Eric Blake wrote:
> On 01/24/2017 09:16 AM, Richard W.M. Jones wrote:
> 
> > In Perl, old code to raise an error would look like:
> > 
> >   sub pread
> >   {
> >      my $h = shift;
> >      my $count = shift;
> >      my $offset = shift;
> >      my $ret;
> >      read ($FH, $ret, $count, $offset) || die "read: $!"
> >      return $ret;
> >   }
> > 
> > (Note that die does not exit the program, it raises an exception which
> > is caught and turned into nbdkit_error in the Perl nbdkit plugin).
> > 
> > This would continue to work fine, but new code which cared about the
> > errno could do this instead:
> > 
> >   sub pread
> >   {
> >      my $h = shift;
> >      my $count = shift;
> >      my $offset = shift;
> >      my $ret;
> >      read ($FH, $ret, $count, $offset) || {
> >        nbdkit_set_errno (POSIX::errno ());
> >        die "read: $!"
> >      }
> >      return $ret;
> >   }
> 
> Except I can't even figure out how to expose nbdkit_set_error (hmm, I
> named it set_error instead of set_errno in my v2 series) to the perl
> code.

Apparently you have to use XS for this, which seems like a PITA ...

I'm sure there must be an easier way to do it where you create a
coderef directly in the main:: package.

> In fact, with just this change to example.pl:
> 
> diff --git i/plugins/perl/example.pl w/plugins/perl/example.pl
> index fcdac33..64f6de3 100644
> --- i/plugins/perl/example.pl
> +++ w/plugins/perl/example.pl
> @@ -1,4 +1,5 @@
>  use strict;
> +use POSIX ();
> 
>  # Example Perl plugin.
>  #
> @@ -80,6 +81,8 @@ sub pwrite
>      my $buf = shift;
>      my $count = length ($buf);
>      my $offset = shift;
> +    my $err = POSIX::EPERM;
> 
>      substr ($disk, $offset, $count) = $buf;
> +    die "forced write failure";
>  }
> 
> I'm getting this failure:
> 
> $ ./src/nbdkit -e foo -fv plugins/perl/.libs/nbdkit-perl-plugin.so \
>     script=plugins/perl/example.pl
> nbdkit: debug: registering plugins/perl/.libs/nbdkit-perl-plugin.so
> nbdkit: debug: registered plugins/perl/.libs/nbdkit-perl-plugin.so (name
> perl)
> nbdkit: debug: plugins/perl/.libs/nbdkit-perl-plugin.so: load
> nbdkit: debug: plugins/perl/.libs/nbdkit-perl-plugin.so: config
> key=script, value=plugins/perl/example.pl
> Can't load module Fcntl, dynamic loading not available in this perl.
>   (You may need to build a new perl executable which either supports
>   dynamic loading or has the Fcntl module statically linked into it.)
>  at /usr/lib64/perl5/POSIX.pm line 17.
> Compilation failed in require at /usr/lib64/perl5/POSIX.pm line 17.
> BEGIN failed--compilation aborted at /usr/lib64/perl5/POSIX.pm line 17.
> Compilation failed in require at plugins/perl/example.pl line 2.
> BEGIN failed--compilation aborted at plugins/perl/example.pl line 2.
> nbdkit: error: plugins/perl/example.pl: one of the required callbacks
> 'open', 'get_size' or 'pread' is not defined by this Perl script.
> nbdkit requires these callbacks.
> 
> So I'm not sure how anyone else is doing anything fancy in an NBD perl
> script.

This is a bug - I guess it reflects how little used the Perl
bindings are ...

It looks like how to fix it is described in perlembed(1) section
"Using Perl modules, which themselves use C libraries, from your C program"

Rich.

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