For an example plugin written in Perl, see:
https://gitlab.com/nbdkit/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.
Executablescript
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 $plugindir. See "WRITING PLUGINS IN OTHER PROGRAMMING
LANGUAGES" in nbdkit-plugin(3).
"Nbdkit::"functions
The following functions can be called in a virtual Perl package called "Nbdkit". Your script does not
need to "use" this package, it is already available in all scripts.
"Nbdkit::debug($msg)"
In debugging mode, print $msg. This is a wrapper around the C function nbdkit_debug(3) (see
nbdkit-plugin(3)).
"Nbdkit::set_error($err)"
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".
32vs64bit
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.
Perlcallbacks
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;
my $flags = 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;
my $flags = 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;
my $flags = 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;
my $flags = 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 $flags = 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.
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 $flags does not contain $Nbdkit::FLAG_MAY_TRIM), use
Nbdkit::set_error(POSIX::EOPNOTSUPP).
Missingcallbacks
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.
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