nbdkit-perl-plugin

nbdkit-perl-plugin(3)               NBDKIT               nbdkit-perl-plugin(3)



NAME
       nbdkit-perl-plugin - nbdkit perl plugin

SYNOPSIS
        nbdkit perl /path/to/plugin.pl [arguments...]

DESCRIPTION
       "nbdkit-perl-plugin" is an embedded Perl interpreter for nbdkit(1),
       allowing you to write nbdkit plugins in Perl.

   If you have been given an nbdkit Perl plugin
       Assuming you have a Perl script which is an nbdkit plugin, you run it
       like this:

        nbdkit perl /path/to/plugin.pl

       You may have to add further "key=value" arguments to the command line.
       Read the Perl script to see if it requires any.

WRITING A PERL NBDKIT PLUGIN
       For an example plugin written in Perl, see:
       https://github.com/libguestfs/nbdkit/blob/master/plugins/perl/example.pl

       Broadly speaking, Perl nbdkit plugins work like C ones, so you should
       read nbdkit-plugin(3) first.

       To write a Perl nbdkit plugin, you create a Perl file which contains at
       least the following required subroutines:

        sub open
        {
          # see below
        }
        sub get_size
        {
          # see below
        }
        sub pread
        {
          # see below
        }

       Note that the subroutines must have those literal names (like "open"),
       because the C part looks up and calls those functions directly.  You
       may want to include documentation and globals (eg. for storing global
       state).  Also any top-level statements, "BEGIN" statements, "END"
       statements and so on are run when nbdkit starts up and shuts down, just
       like ordinary Perl.

   Executable script
       If you want you can make the script executable and include a "shebang"
       at the top:

        #!/usr/sbin/nbdkit perl

       See also "Shebang scripts" in nbdkit(1).

       These scripts can also be installed in the $plugindir.  See "WRITING
       PLUGINS IN OTHER PROGRAMMING LANGUAGES" in nbdkit-plugin(3).

   Methods
       Your script has access to the following methods in the "Nbdkit" package
       (in fact, attempting to "use Nbdkit" will fail, the methods are already
       available):

        Nbdkit::set_error($err);

       Record $err as the reason you are about to throw an exception. $err
       should correspond to usual errno values, where it may help to "use
       POSIX()".

   Exceptions
       Instead of returning error codes as in C, Perl callbacks should
       indicate problems by throwing Perl exceptions (ie. "die", "croak" etc).
       The Perl error message is captured and printed by nbdkit.  Remember to
       use "Nbdkit::set_error" if you need to control which error is sent back
       to the client; if omitted, the client will see an error of "EIO".

   32 vs 64 bit
       It is likely that Perl plugins won't work well, or maybe won't work at
       all, on 32 bit platforms.  This is simply because Perl doesn't have an
       easy way to use 64 bit integers on 32 bit platforms, and 64 bit
       integers (eg. file offsets, disk sizes) are required for many nbdkit
       operations.

   Perl callbacks
       This just documents the arguments to the callbacks in Perl, and any way
       that they differ from the C callbacks.  In all other respects they work
       the same way as the C callbacks, so you should go and read
       nbdkit-plugin(3).

       "dump_plugin"
           (Optional)

           There are no arguments or return value.

       "config"
           (Optional)

            sub config
            {
                my $key = shift;
                my $value = shift;
                # No return value.
            }

       "config_complete"
           (Optional)

           There are no arguments or return value.

       "open"
           (Required)

            sub open
            {
                my $readonly = shift;
                my $handle = {};
                return $handle;
            }

           The "readonly" flag is a boolean.

           You can return any Perl value as the handle.  It is passed back to
           subsequent calls.  It's usually convenient to use a hashref, since
           that lets you store arbitrary fields.

       "close"
           (Optional)

            sub close
            {
                my $handle = shift;
                # No return value
            }

           After "close" returns, the reference count of the handle is
           decremented in the C part, which usually means that the handle and
           its contents will be garbage collected.

       "get_size"
           (Required)

            sub get_size
            {
                my $handle = shift;
                my $i64 = .. the size of the disk ..;
                return $i64;
            }

           This returns the size of the disk.  You can return any Perl object
           that evaluates to an integer.

       "can_write"
           (Optional)

            sub can_write
            {
                my $handle = shift;
                my $bool = ...;
                return $bool;
            }

           Return a boolean indicating whether the disk is writable.

       "can_flush"
           (Optional)

            sub can_flush
            {
                my $handle = shift;
                my $bool = ...;
                return $bool;
            }

           Return a boolean indicating whether flush can be performed.

       "is_rotational"
           (Optional)

            sub is_rotational
            {
                my $handle = shift;
                my $bool = ...;
                return $bool;
            }

           Return a boolean indicating whether the disk is rotational.

       "can_trim"
           (Optional)

            sub can_trim
            {
                my $handle = shift;
                my $bool = ...;
                return $bool;
            }

           Return a boolean indicating whether trim/discard can be performed.

       "pread"
           (Required)

            sub pread
            {
               my $handle = shift;
               my $count = shift;
               my $offset = shift;
               # Construct a buffer of length $count bytes and return it.
               return $buf;
            }

           The body of your "pread" function should construct a buffer of
           length (at least) $count bytes.  You should read $count bytes from
           the disk starting at $offset.

           NBD only supports whole reads, so your function should try to read
           the whole region (perhaps requiring a loop).  If the read fails or
           is partial, your function should "die", optionally using
           "Nbdkit::set_error" first.

       "pwrite"
           (Optional)

            sub pwrite
            {
               my $handle = shift;
               my $buf = shift;
               my $count = length ($buf);
               my $offset = shift;
               # No return value
            }

           The body of your "pwrite" function should write the $buf string to
           the disk.  You should write $count bytes to the disk starting at
           $offset.

           NBD only supports whole writes, so your function should try to
           write the whole region (perhaps requiring a loop).  If the write
           fails or is partial, your function should "die", optionally using
           "Nbdkit::set_error" first.

       "flush"
           (Optional)

            sub flush
            {
                my $handle = shift;
                # No return value
            }

           The body of your "flush" function should do a sync(2) or
           fdatasync(2) or equivalent on the backing store.

           If there is an error, the function should call "die", optionally
           using "Nbdkit::set_error" first.

       "trim"
           (Optional)

            sub trim
            {
                my $handle = shift;
                my $count = shift;
                my $offset = shift;
                # No return value
            }

           The body of your "trim" function should "punch a hole" in the
           backing store.

           If there is an error, the function should call "die", optionally
           using "Nbdkit::set_error" first.

       "zero"
           (Optional)

            sub zero
            {
               my $handle = shift;
               my $count = shift;
               my $offset = shift;
               my $may_trim = shift;
               # No return value
            }

           The body of your "zero" function should ensure that $count bytes of
           the disk, starting at $offset, will read back as zero.  If
           $may_trim is true, the operation may be optimized as a trim as long
           as subsequent reads see zeroes.

           NBD only supports whole writes, so your function should try to
           write the whole region (perhaps requiring a loop).  If the write
           fails or is partial, your function should "die", optionally using
           "Nbdkit::set_error" first.  In particular, if you would like to
           automatically fall back to "pwrite" (perhaps because there is
           nothing to optimize if $may_trim is false), use
           "Nbdkit::set_error(POSIX::EOPNOTSUPP)".

   Missing callbacks
       Missing: "load" and "unload"
           These are not needed because you can just use regular Perl "BEGIN"
           and "END" constructs.

       Missing: "name", "version", "longname", "description", "config_help",
       "can_fua", "can_cache", "cache"
           These are not yet supported.

   Threads
       The thread model for Perl callbacks currently cannot be set from Perl.
       It is hard-coded in the C part to
       "NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS".  This may change or be
       settable in future.

FILES
       $plugindir/nbdkit-perl-plugin.so
           The plugin.

           Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION
       "nbdkit-perl-plugin" first appeared in nbdkit 1.2.

SEE ALSO
       nbdkit(1), nbdkit-plugin(3), perl(1).

AUTHORS
       Eric Blake

       Richard W.M. Jones

COPYRIGHT
       Copyright (C) 2013-2018 Red Hat Inc.

LICENSE
       Redistribution and use in source and binary forms, with or without
       modification, are permitted provided that the following conditions are
       met:

       •   Redistributions of source code must retain the above copyright
           notice, this list of conditions and the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright
           notice, this list of conditions and the following disclaimer in the
           documentation and/or other materials provided with the
           distribution.

       •   Neither the name of Red Hat nor the names of its contributors may
           be used to endorse or promote products derived from this software
           without specific prior written permission.

       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.



nbdkit-1.16.2                     2020-01-28             nbdkit-perl-plugin(3)