NAME

       nbdkit-ruby-plugin - nbdkit ruby plugin

SYNOPSIS

        nbdkit ruby /path/to/plugin.rb [arguments...]

WARNING

       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

DESCRIPTION

       "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://gitlab.com/nbdkit/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.

FILES

       $plugindir/nbdkit-ruby-plugin.so
           The plugin.

           Use "nbdkit --dump-config" to find the location of $plugindir.

VERSION

       "nbdkit-ruby-plugin" first appeared in nbdkit 1.2.

SEE ALSO

       nbdkit(1), nbdkit-plugin(3), ruby(1).

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

LICENSE

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       permitted provided that the following conditions are met:

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