Provided by: nbdkit-plugin-ruby_1.16.2-1ubuntu3_amd64
nbdkit-ruby-plugin - nbdkit ruby plugin
nbdkit ruby /path/to/plugin.rb [arguments...]
The Ruby language is fundamentally broken when it comes to embedding in a program which uses pthreads. This means you may see random "stack overflows" when using this plugin on some versions of Ruby but not others. For the whole sorry saga, see: https://redmine.ruby-lang.org/issues/2294
"nbdkit-ruby-plugin" is an embedded Ruby interpreter for nbdkit(1), allowing you to write nbdkit plugins in Ruby. If you have been given an nbdkit Ruby plugin Assuming you have a Ruby script which is an nbdkit plugin, you run it like this: nbdkit ruby /path/to/ruby.rb You may have to add further "key=value" arguments to the command line. Read the Ruby script to see if it requires any.
WRITING A RUBY NBDKIT PLUGIN
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. Executable script 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". Ruby callbacks 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)". Missing callbacks 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.
$plugindir/nbdkit-ruby-plugin.so The plugin. Use "nbdkit --dump-config" to find the location of $plugindir.
"nbdkit-ruby-plugin" first appeared in nbdkit 1.2.
nbdkit(1), nbdkit-plugin(3), ruby(1).
Eric Blake Richard W.M. Jones
Copyright (C) 2013-2018 Red Hat Inc.
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