NAME¶
nbdkit-perl-plugin - nbdkit perl plugin
SYNOPSIS¶
nbdkit perl script=/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.
Broadly speaking, Perl nbdkit plugins work like C ones, so you should read
nbdkit-plugin(3) first.
USING A PERL NBDKIT PLUGIN¶
Assuming you have a Perl script which is an nbdkit plugin, you run it like this:
nbdkit perl script=/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. "script=..."
must come first on the command line.
WRITING A PERL NBDKIT PLUGIN¶
There is an example Perl nbdkit plugin called "example.pl" which ships
with the nbdkit source.
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.
The file does
not need to include a "#!" (hash-bang) at the
top, and does
not need to be executable. In fact it's a good idea
not to do that, because running the plugin directly as a Perl script
won't work.
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.
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).
- "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".
- "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".
- "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".
- "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".
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"
- 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.
SEE ALSO¶
nbdkit(1),
nbdkit-plugin(3),
perl(1).
AUTHORS¶
Richard W.M. Jones
COPYRIGHT¶
Copyright (C) 2013-2014 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.
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