Provided by: nbdkit-plugin-perl_1.16.2-1ubuntu3_amd64 
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
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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.
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