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.\" ========================================================================
.\"
.IX Title "nbdkit-ruby-plugin 3"
.TH nbdkit-ruby-plugin 3 "2021-01-20" "nbdkit-1.24.1" "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\-ruby\-plugin \- nbdkit ruby plugin
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit ruby /path/to/plugin.rb [arguments...]
.Ve
.SH "WARNING"
.IX Header "WARNING"
The Ruby language is fundamentally broken when it comes to embedding
in a program which uses pthreads.  This means you may see random
\&\*(L"stack overflows\*(R" when using this plugin on some versions of Ruby but
not others.
.PP
For the whole sorry saga, see:
https://redmine.ruby\-lang.org/issues/2294
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-ruby\-plugin\*(C'\fR is an embedded Ruby interpreter for
\&\fBnbdkit\fR\|(1), allowing you to write nbdkit plugins in Ruby.
.SS "If you have been given an nbdkit Ruby plugin"
.IX Subsection "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:
.PP
.Vb 1
\& nbdkit ruby /path/to/ruby.rb
.Ve
.PP
You may have to add further \f(CW\*(C`key=value\*(C'\fR arguments to the command
line.  Read the Ruby script to see if it requires any.
.SH "WRITING A RUBY NBDKIT PLUGIN"
.IX Header "WRITING A RUBY NBDKIT PLUGIN"
For an example plugin written in Ruby, see:
https://github.com/libguestfs/nbdkit/blob/master/plugins/ruby/example.rb
.PP
Broadly speaking, Ruby nbdkit plugins work like C ones, so you should
read \fBnbdkit\-plugin\fR\|(3) first.
.PP
To write a Ruby nbdkit plugin, you create a Ruby file which
contains at least the following required functions:
.PP
.Vb 9
\& def open(readonly)
\&   # see below
\& end
\& def get_size(h)
\&   # see below
\& end
\& def pread(h, count, offset)
\&   # see below
\& 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).  Any other top level statements are run when the script is
loaded, just like ordinary Ruby.
.SS "Executable script"
.IX Subsection "Executable script"
If you want you can make the script executable and include a \*(L"shebang\*(R"
at the top:
.PP
.Vb 1
\& #!/usr/sbin/nbdkit ruby
.Ve
.PP
See also \*(L"Shebang scripts\*(R" in \fBnbdkit\fR\|(1).
.PP
These scripts can also be installed in the \f(CW$plugindir\fR.  See
\&\*(L"\s-1WRITING PLUGINS IN OTHER PROGRAMMING LANGUAGES\*(R"\s0 in \fBnbdkit\-plugin\fR\|(3).
.SS "Methods"
.IX Subsection "Methods"
Your script has access to the \f(CW\*(C`Nbdkit\*(C'\fR module, with the following
singleton methods:
.PP
.Vb 1
\& Nbdkit.set_error(err)
.Ve
.PP
Record \f(CW\*(C`err\*(C'\fR as the reason you are about to raise an
exception. \f(CW\*(C`err\*(C'\fR should either be a class that defines an \f(CW\*(C`Errno\*(C'\fR
constant (all of the subclasses of \f(CW\*(C`SystemCallError\*(C'\fR in module
\&\f(CW\*(C`Errno\*(C'\fR have this property), an object that defines an \f(CW\*(C`errno\*(C'\fR
method with no arguments (all instances of \f(CW\*(C`SystemCallError\*(C'\fR have
this property), or an integer value corresponding to the usual errno
values.
.SS "Exceptions"
.IX Subsection "Exceptions"
Ruby callbacks should throw exceptions to indicate errors. Remember
to use \f(CW\*(C`Nbdkit.set_error\*(C'\fR if you need to control which error is sent
back to the client; if omitted, the client will see an error of \f(CW\*(C`EIO\*(C'\fR.
.SS "Ruby callbacks"
.IX Subsection "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
\&\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
\& def 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 3
\& def open(readonly)
\&   # return handle
\& end
.Ve
.Sp
You can return any non-nil Ruby value as the handle.  It is passed
back in subsequent calls.
.ie n .IP """close""" 4
.el .IP "\f(CWclose\fR" 4
.IX Item "close"
(Optional)
.Sp
.Vb 3
\& def close(h)
\&   # no return value
\& end
.Ve
.ie n .IP """get_size""" 4
.el .IP "\f(CWget_size\fR" 4
.IX Item "get_size"
(Required)
.Sp
.Vb 3
\& def get_size(h)
\&   # return the size of the disk
\& end
.Ve
.ie n .IP """can_write""" 4
.el .IP "\f(CWcan_write\fR" 4
.IX Item "can_write"
(Optional)
.Sp
.Vb 3
\& def can_write(h)
\&   # return a boolean
\& end
.Ve
.ie n .IP """can_flush""" 4
.el .IP "\f(CWcan_flush\fR" 4
.IX Item "can_flush"
(Optional)
.Sp
.Vb 3
\& def can_flush(h)
\&   # return a boolean
\& end
.Ve
.ie n .IP """is_rotational""" 4
.el .IP "\f(CWis_rotational\fR" 4
.IX Item "is_rotational"
(Optional)
.Sp
.Vb 3
\& def is_rotational(h)
\&   # return a boolean
\& end
.Ve
.ie n .IP """can_trim""" 4
.el .IP "\f(CWcan_trim\fR" 4
.IX Item "can_trim"
(Optional)
.Sp
.Vb 3
\& def can_trim(h)
\&   # return a boolean
\& end
.Ve
.ie n .IP """pread""" 4
.el .IP "\f(CWpread\fR" 4
.IX Item "pread"
(Required)
.Sp
.Vb 3
\& def pread(h, count, offset)
\&   # construct a string of length count bytes and return it
\& end
.Ve
.Sp
The body of your \f(CW\*(C`pread\*(C'\fR function should construct a string 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
\&\s-1NBD\s0 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
\&\f(CW\*(C`Nbdkit.set_error\*(C'\fR first.
.ie n .IP """pwrite""" 4
.el .IP "\f(CWpwrite\fR" 4
.IX Item "pwrite"
(Optional)
.Sp
.Vb 4
\& def pwrite(h, buf, offset)
\&   length = buf.length
\&   # 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
\&\s-1NBD\s0 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 \f(CW\*(C`Nbdkit.set_error\*(C'\fR first.
.ie n .IP """flush""" 4
.el .IP "\f(CWflush\fR" 4
.IX Item "flush"
(Optional)
.Sp
.Vb 3
\& def 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.
.Sp
If the flush fails, your function should throw an exception, optionally
using \f(CW\*(C`Nbdkit.set_error\*(C'\fR first.
.ie n .IP """trim""" 4
.el .IP "\f(CWtrim\fR" 4
.IX Item "trim"
(Optional)
.Sp
.Vb 3
\& def trim(h, count, offset)
\&   # no return value
\& end
.Ve
.Sp
The body of your \f(CW\*(C`trim\*(C'\fR function should \*(L"punch a hole\*(R" in the
backing store.  If the trim fails, your function should throw an
exception, optionally using \f(CW\*(C`Nbdkit.set_error\*(C'\fR first.
.ie n .IP """zero""" 4
.el .IP "\f(CWzero\fR" 4
.IX Item "zero"
(Optional)
.Sp
.Vb 2
\& def zero(h, count, offset, may_trim)
\&   # no return value
.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
\&\s-1NBD\s0 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 \f(CW\*(C`Nbdkit.set_error\*(C'\fR first.  In particular, if
you would like to automatically fall back to \f(CW\*(C`pwrite\*(C'\fR (perhaps
because there is nothing to optimize if \f(CW\*(C`may_trim\*(C'\fR is false),
use \f(CW\*(C`Nbdkit.set_error(Errno::EOPNOTSUPP)\*(C'\fR.
.SS "Missing callbacks"
.IX Subsection "Missing callbacks"
.ie n .IP "Missing: ""load"" and ""unload""" 4
.el .IP "Missing: \f(CWload\fR and \f(CWunload\fR" 4
.IX Item "Missing: load and unload"
These are not needed because you can just use ordinary Ruby
constructs.
.ie n .IP "Missing: ""name"", ""version"", ""longname"", ""description"", ""config_help"", ""can_fua"", ""can_cache"", ""cache""" 4
.el .IP "Missing: \f(CWname\fR, \f(CWversion\fR, \f(CWlongname\fR, \f(CWdescription\fR, \f(CWconfig_help\fR, \f(CWcan_fua\fR, \f(CWcan_cache\fR, \f(CWcache\fR" 4
.IX Item "Missing: name, version, longname, description, config_help, can_fua, can_cache, cache"
These are not yet supported.
.SS "Threads"
.IX Subsection "Threads"
The thread model for Ruby callbacks currently cannot be set from Ruby.
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"
.IP "\fI\f(CI$plugindir\fI/nbdkit\-ruby\-plugin.so\fR" 4
.IX Item "$plugindir/nbdkit-ruby-plugin.so"
The plugin.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$plugindir\fR.
.SH "VERSION"
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-ruby\-plugin\*(C'\fR first appeared in nbdkit 1.2.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-plugin\fR\|(3),
\&\fBruby\fR\|(1).
.SH "AUTHORS"
.IX Header "AUTHORS"
Eric Blake
.PP
Richard W.M. Jones
.SH "COPYRIGHT"
.IX Header "COPYRIGHT"
Copyright (C) 2013\-2020 Red Hat Inc.
.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
\&\s-1THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS\s0 ''\s-1AS IS\s0'' \s-1AND
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\s0 (\s-1INCLUDING, BUT NOT
LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES\s0; \s-1LOSS OF
USE, DATA, OR PROFITS\s0; \s-1OR BUSINESS INTERRUPTION\s0) \s-1HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT\s0 (\s-1INCLUDING NEGLIGENCE OR OTHERWISE\s0) \s-1ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.\s0