nbdkit-ruby-plugin(3)

✨ NAME ✨

nbdkit-ruby-plugin — the nbdkit ruby plugin 💎 (yes bestie, you can write nbdkit plugins in Ruby, we're living in the future)

🌸 SYNOPSIS 🌸

nbdkit ruby /path/to/plugin.rb [arguments...]

🚨 WARNING 🚨

Okay so listen. Ruby is fundamentally broken when it comes to embedding in a program which uses pthreads. 💀 That's not me being dramatic, that's just... the situation. What this means for you is that you may see random "stack overflows" when using this plugin on some versions of Ruby but not others. So like. It might work. It might not. Depends on the Ruby. 🙈

For the whole sorry saga, see: https://redmine.ruby-lang.org/issues/2294

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🦋 DESCRIPTION 🦋

nbdkit-ruby-plugin is an embedded Ruby interpreter for nbdkit(1), allowing you to write nbdkit plugins in Ruby. Slay.

If you have been given an nbdkit Ruby plugin 💌

Assuming you have a Ruby script which is an nbdkit plugin, you run it like this:

nbdkit ruby /path/to/ruby.rb

You may have to add further key=value arguments to the command line. Read the Ruby script to see if it requires any. (Yes, actually read it. It's right there. It won't bite. 👀)

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🎀 WRITING A RUBY NBDKIT PLUGIN 🎀

For an example plugin written in Ruby, see: https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/ruby/example.rb

Broadly speaking, Ruby nbdkit plugins work like C ones, so you should go read nbdkit-plugin(3) first. No skipping, bestie, it's giving prerequisites. 📚

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

def open(readonly)
  # see below
end
def get_size(h)
  # see below
end
def pread(h, count, offset)
  # see below
end

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). Any other top level statements are run when the script is loaded, just like ordinary Ruby. ✨

Executable script 🌈

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

#!/usr/sbin/nbdkit ruby

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 Nbdkit module, with the following singleton methods:

Nbdkit.set_error(err)

Record err as the reason you are about to raise an exception. err should either be:

• a class that defines an Errno constant (all of the subclasses of SystemCallError in module Errno have this property),
• an object that defines an errno method with no arguments (all instances of SystemCallError have this property),
• or an integer value corresponding to the usual errno values.

Pick your fighter, they all work. 💅

Exceptions 😭

Ruby callbacks should throw exceptions to indicate errors. 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. Don't forget it if you care!

Ruby callbacks 🔥

This just documents the arguments to the callbacks in Ruby, 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). (Again. Yes. Go.)

dump_plugin

(Optional)

There are no arguments or return value.

config

(Optional)

def config(key, value)
  # no return value
end

config_complete

(Optional)

There are no arguments or return value.

open

(Required)

def open(readonly)
  # return handle
end

You can return any non-nil Ruby value as the handle. It is passed back in subsequent calls. 🫶

close

(Optional)

def close(h)
  # no return value
end

get_size

(Required)

def get_size(h)
  # return the size of the disk
end

can_write

(Optional)

def can_write(h)
  # return a boolean
end

can_flush

(Optional)

def can_flush(h)
  # return a boolean
end

is_rotational

(Optional)

def is_rotational(h)
  # return a boolean
end

can_trim

(Optional)

def can_trim(h)
  # return a boolean
end

pread

(Required)

def pread(h, count, offset)
  # construct a string of length count bytes and return it
end

The body of your pread function should construct a string 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 throw an exception, optionally using Nbdkit.set_error first.

pwrite

(Optional)

def pwrite(h, buf, offset)
  length = buf.length
  # no return value
end

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 throw an exception, optionally using Nbdkit.set_error first.

flush

(Optional)

def flush(h)
  # no return value
end

The body of your flush function should do a sync(2) or fdatasync(2) or equivalent on the backing store. 🚿 (flush it like you mean it)

If the flush fails, your function should throw an exception, optionally using Nbdkit.set_error first.

trim

(Optional)

def trim(h, count, offset)
  # no return value
end

The body of your trim function should "punch a hole" in the backing store. If the trim fails, your function should throw an exception, optionally using Nbdkit.set_error first.

zero

(Optional)

def zero(h, count, offset, may_trim)
  # 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 throw an exception, 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(Errno::EOPNOTSUPP).

Missing callbacks 💀

Missing: load and unload

These are not needed because you can just use ordinary Ruby constructs. 🎉

Missing: name, version, longname, description, config_help, can_fua, can_cache, cache

These are not yet supported. Not yet! Give it time.

Threads 🧵

The thread model for Ruby callbacks currently cannot be set from Ruby. It is hard-coded in the C part to NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS. This may change or be settable in future. We'll see. ✨

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📁 FILES 📁

• $plugindir/nbdkit-ruby-plugin.so — The plugin.

Use nbdkit --dump-config to find the location of $plugindir. (Yes, that's literally how you find it, it's giving scavenger hunt 🔍)

⭐ VERSION ⭐

nbdkit-ruby-plugin first appeared in nbdkit 1.2.

👑 SEE ALSO 👑

nbdkit(1), nbdkit-plugin(3), ruby(1).

💜 AUTHORS 💜

Eric Blake

Richard W.M. Jones

📜 COPYRIGHT 📜

Copyright Red Hat

⚖️ 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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And that's the whole page bestie!! 🌸 This one's from <strong>nbdkit-1.36.3</strong>, dated <strong>2024-03-31</strong>, and it's nbdkit-ruby-plugin(3) — go write something iconic with it 💖✨🦋