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://github.com/libguestfs/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 (C) 2018 Red Hat Inc.

LICENSE

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       permitted provided that the following conditions are met:

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           conditions and the following disclaimer.

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           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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