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.\" ========================================================================
.\"
.IX Title "nbdkit-lua-plugin 3"
.TH nbdkit-lua-plugin 3 2024-01-17 nbdkit-1.36.3 NBDKIT
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
.\" way too many mistakes in technical documents.
.if n .ad l
.nh
.SH NAME
nbdkit\-lua\-plugin \- nbdkit Lua plugin
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit lua /path/to/plugin.lua [arguments...]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-lua\-plugin\*(C'\fR is an embedded Lua interpreter for
\&\fBnbdkit\fR\|(1), allowing you to write nbdkit plugins in Lua.
.SS "If you have been given an nbdkit Lua plugin"
.IX Subsection "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:
.PP
.Vb 1
\& nbdkit lua /path/to/plugin.lua
.Ve
.PP
You may have to add further \f(CW\*(C`key=value\*(C'\fR arguments to the command
line.  Read the Lua script to see if it requires any.
.SH "WRITING A LUA NBDKIT PLUGIN"
.IX Header "WRITING A LUA NBDKIT PLUGIN"
For an example plugin written in Lua, see:
https://gitlab.com/nbdkit/nbdkit/blob/master/plugins/lua/example.lua
.PP
Broadly speaking, Lua nbdkit plugins work like C ones, so you should
read \fBnbdkit\-plugin\fR\|(3) first.
.PP
To write a Lua nbdkit plugin, you create a Lua file which contains
at least the following required functions:
.PP
.Vb 12
\& 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
.Ve
.PP
Note that the subroutines must have those literal names (like
\&\f(CW\*(C`open\*(C'\fR), 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.
.SS "Executable script"
.IX Subsection "Executable script"
If you want you can make the script executable and include a "shebang"
at the top:
.PP
.Vb 1
\& #!/usr/sbin/nbdkit lua
.Ve
.PP
See also "Shebang scripts" in \fBnbdkit\fR\|(1).
.PP
These scripts can also be installed in the \f(CW$plugindir\fR.  See
"WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES" in \fBnbdkit\-plugin\fR\|(3).
.SS Errors
.IX Subsection "Errors"
Lua plugin methods can indicate an error by calling \f(CW\*(C`error\*(C'\fR or
\&\f(CW\*(C`assert\*(C'\fR.  The error message will contain the method name, filename
and line number where the error occurred, eg:
.PP
.Vb 2
\& error ("could not open " .. filename)
\& \-\-> nbdkit: error: open: myplugin.lua:123: could not open disk.img
.Ve
.SS "Lua callbacks"
.IX Subsection "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
\&\fBnbdkit\-plugin\fR\|(3).
.ie n .IP """dump_plugin""" 4
.el .IP \f(CWdump_plugin\fR 4
.IX Item "dump_plugin"
(Optional)
.Sp
There are no arguments or return value.
.ie n .IP """config""" 4
.el .IP \f(CWconfig\fR 4
.IX Item "config"
(Optional)
.Sp
.Vb 3
\& function config (key, value)
\&     \-\- No return value.
\& end
.Ve
.ie n .IP """config_complete""" 4
.el .IP \f(CWconfig_complete\fR 4
.IX Item "config_complete"
(Optional)
.Sp
There are no arguments or return value.
.ie n .IP """open""" 4
.el .IP \f(CWopen\fR 4
.IX Item "open"
(Required)
.Sp
.Vb 5
\& function open (readonly)
\&     local handle
\&     handle=...
\&     return handle
\& end
.Ve
.Sp
The \f(CW\*(C`readonly\*(C'\fR flag is a boolean.
.Sp
You can return any Lua string or object as the handle.  It is passed
back to subsequent calls.
.ie n .IP """close""" 4
.el .IP \f(CWclose\fR 4
.IX Item "close"
(Optional)
.Sp
.Vb 3
\& function close (h)
\&     \-\- No return value
\& end
.Ve
.Sp
After \f(CW\*(C`close\*(C'\fR 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.
.ie n .IP """get_size""" 4
.el .IP \f(CWget_size\fR 4
.IX Item "get_size"
(Required)
.Sp
.Vb 5
\& function get_size (h)
\&     local size
\&     size= .. the size of the disk ..
\&     return size
\& end
.Ve
.Sp
This returns the size of the disk.
.ie n .IP """can_write""" 4
.el .IP \f(CWcan_write\fR 4
.IX Item "can_write"
(Optional)
.Sp
.Vb 3
\& function can_write (h)
\&     return bool
\& end
.Ve
.Sp
Return a boolean indicating whether the disk is writable.
.ie n .IP """can_flush""" 4
.el .IP \f(CWcan_flush\fR 4
.IX Item "can_flush"
(Optional)
.Sp
.Vb 3
\& function can_flush (h)
\&     return bool
\& end
.Ve
.Sp
Return a boolean indicating whether flush can be performed.
.ie n .IP """is_rotational""" 4
.el .IP \f(CWis_rotational\fR 4
.IX Item "is_rotational"
(Optional)
.Sp
.Vb 3
\& function is_rotational (h)
\&     return bool
\& end
.Ve
.Sp
Return a boolean indicating whether the disk is rotational.
.ie n .IP """can_trim""" 4
.el .IP \f(CWcan_trim\fR 4
.IX Item "can_trim"
(Optional)
.Sp
.Vb 3
\& function can_trim (h)
\&     return bool
\& end
.Ve
.Sp
Return a boolean indicating whether trim/discard can be performed.
.ie n .IP """pread""" 4
.el .IP \f(CWpread\fR 4
.IX Item "pread"
(Required)
.Sp
.Vb 4
\& function pread (h, count, offset)
\&    \-\- Construct a buffer of length count bytes and return it.
\&    return buf
\& end
.Ve
.Sp
The body of your \f(CW\*(C`pread\*(C'\fR function should construct a buffer of length
(at least) \f(CW\*(C`count\*(C'\fR bytes.  You should read \f(CW\*(C`count\*(C'\fR bytes from the
disk starting at \f(CW\*(C`offset\*(C'\fR.
.Sp
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 \f(CW\*(C`error\*(C'\fR.
.ie n .IP """pwrite""" 4
.el .IP \f(CWpwrite\fR 4
.IX Item "pwrite"
(Optional)
.Sp
.Vb 3
\& function pwrite (h, buf, offset)
\&    \-\- No return value
\& end
.Ve
.Sp
The body of your \f(CW\*(C`pwrite\*(C'\fR function should write the \f(CW\*(C`buf\*(C'\fR string to
the disk.  You should write \f(CW\*(C`count\*(C'\fR bytes to the disk starting at
\&\f(CW\*(C`offset\*(C'\fR.
.Sp
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 \f(CW\*(C`error\*(C'\fR.
.ie n .IP """flush""" 4
.el .IP \f(CWflush\fR 4
.IX Item "flush"
(Optional)
.Sp
.Vb 3
\& function flush (h)
\&     \-\- No return value
\& end
.Ve
.Sp
The body of your \f(CW\*(C`flush\*(C'\fR function should do a \fBsync\fR\|(2) or
\&\fBfdatasync\fR\|(2) or equivalent on the backing store.
.ie n .IP """trim""" 4
.el .IP \f(CWtrim\fR 4
.IX Item "trim"
(Optional)
.Sp
.Vb 3
\& function trim (h, count, offset)
\&     \-\- No return value
\& end
.Ve
.Sp
The body of your \f(CW\*(C`trim\*(C'\fR function should "punch a hole" in the backing
store.
.ie n .IP """zero""" 4
.el .IP \f(CWzero\fR 4
.IX Item "zero"
(Optional)
.Sp
.Vb 3
\& function zero (h, count, offset, may_trim)
\&    \-\- No return value
\& end
.Ve
.Sp
The body of your \f(CW\*(C`zero\*(C'\fR function should ensure that \f(CW\*(C`count\*(C'\fR bytes
of the disk, starting at \f(CW\*(C`offset\*(C'\fR, will read back as zero.  If
\&\f(CW\*(C`may_trim\*(C'\fR is true, the operation may be optimized as a trim as long
as subsequent reads see zeroes.
.Sp
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 \f(CW\*(C`error\*(C'\fR.
.SS "Missing callbacks"
.IX Subsection "Missing callbacks"
.ie n .IP "Missing: ""load"", ""unload"", ""name"", ""version"", ""longname"", ""description"", ""config_help"", ""can_zero"", ""can_fua"", ""can_cache"", ""cache""" 4
.el .IP "Missing: \f(CWload\fR, \f(CWunload\fR, \f(CWname\fR, \f(CWversion\fR, \f(CWlongname\fR, \f(CWdescription\fR, \f(CWconfig_help\fR, \f(CWcan_zero\fR, \f(CWcan_fua\fR, \f(CWcan_cache\fR, \f(CWcache\fR" 4
.IX Item "Missing: load, unload, name, version, longname, description, config_help, can_zero, can_fua, can_cache, cache"
These are not yet supported.
.SS Threads
.IX Subsection "Threads"
The thread model for Lua callbacks currently cannot be set from Lua.
It is hard-coded in the C part to
\&\f(CW\*(C`NBDKIT_THREAD_MODEL_SERIALIZE_ALL_REQUESTS\*(C'\fR.  This may change or be
settable in future.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$plugindir\fR\fI/nbdkit\-lua\-plugin.so\fR 4
.el .IP \fR\f(CI$plugindir\fR\fI/nbdkit\-lua\-plugin.so\fR 4
.IX Item "$plugindir/nbdkit-lua-plugin.so"
The plugin.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$plugindir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-lua\-plugin\*(C'\fR first appeared in nbdkit 1.6.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-plugin\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP \(bu 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP \(bu 4
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.
.IP \(bu 4
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.
.PP
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.