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.
Executablescript
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
Luacallbacks
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".
Missingcallbacks
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.
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