nbdkit-lua-plugin(3)

NAME
nbdkit-lua-plugin - nbdkit Lua plugin

SYNOPSIS
 nbdkit lua /path/to/plugin.lua [arguments...]

DESCRIPTION
"nbdkit-lua-plugin" is an embedded Lua interpreter for nbdkit(1), allowing you to write nbdkit plugins in Lua.

  If you have been given an nbdkit Lua plugin
Assuming you have a Lua script which is an nbdkit plugin, you run it like this:

 nbdkit lua /path/to/plugin.lua

You may have to add further "key=value" arguments to the command line. Read the Lua script to see if it requires any.

WRITING A LUA NBDKIT PLUGIN
For an example plugin written in Lua, see: https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/lua/example.lua

Broadly speaking, Lua nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.

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

 function open (readonly)
     -- see below
     return h
 end
 function get_size (h)
     -- see below
     return size
 end
 function pread (h, count, offset)
     -- see below
     return buf
 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). Also any top-level statements are run when nbdkit starts up.

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

 #!/usr/sbin/nbdkit lua

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).

  Errors
Lua plugin methods can indicate an error by calling "error" or "assert". The error message will contain the method name, filename and line number where the error occurred, eg:

 error ("could not open " .. filename)
 --> nbdkit: error: open: myplugin.lua:123: could not open disk.img

  Lua callbacks
This just documents the arguments to the callbacks in Lua, 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)

     function config (key, value)
         -- No return value.
     end

"config_complete"
    (Optional)

    There are no arguments or return value.

"open"
    (Required)

     function open (readonly)
         local handle
         handle=...
         return handle
     end

    The "readonly" flag is a boolean.

    You can return any Lua string or object as the handle. It is passed back to subsequent calls.

"close"
    (Optional)

     function close (h)
         -- No return value
     end

    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)

     function get_size (h)
         local size
         size= .. the size of the disk ..
         return size
     end

    This returns the size of the disk.

"can_write"
    (Optional)

     function can_write (h)
         return bool
     end

    Return a boolean indicating whether the disk is writable.

"can_flush"
    (Optional)

     function can_flush (h)
         return bool
     end

    Return a boolean indicating whether flush can be performed.

"is_rotational"
    (Optional)

     function is_rotational (h)
         return bool
     end

    Return a boolean indicating whether the disk is rotational.

"can_trim"
    (Optional)

     function can_trim (h)
         return bool
     end

    Return a boolean indicating whether trim/discard can be performed.

"pread"
    (Required)

     function pread (h, count, offset)
        -- Construct a buffer of length count bytes and return it.
        return buf
     end

    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 call "error".

"pwrite"
    (Optional)

     function pwrite (h, buf, offset)
        -- 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 call "error".

"flush"
    (Optional)

     function 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.

"trim"
    (Optional)

     function trim (h, count, offset)
         -- No return value
     end

    The body of your "trim" function should "punch a hole" in the backing store.

"zero"
    (Optional)

     function zero (h, count, offset, may_trim)
        -- No return value
     end

    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 call "error".

  Missing callbacks

Missing: "load", "unload", "name", "version", "longname", "description", "config_help", "can_zero", "can_fua", "can_cache", "cache"
    These are not yet supported.

  Threads
The thread model for Lua callbacks currently cannot be set from Lua. 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-lua-plugin.so
    The plugin.

    Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION
"nbdkit-lua-plugin" first appeared in nbdkit 1.6.

SEE ALSO
nbdkit(1), nbdkit-plugin(3).

AUTHORS
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.