nbdkit-perl-plugin(3) 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.

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.

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.

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