For an example plugin written in Ruby, see:
https://github.com/libguestfs/nbdkit/blob/master/plugins/ruby/example.rb
Broadly speaking, Ruby nbdkit plugins work like C ones, so you should read nbdkit-plugin(3) first.
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.
Executablescript
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.
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".
Rubycallbacks
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).
"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.
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)".
Missingcallbacks
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.
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.
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